Index ¶

To do this, click Extensions > t4su > Edit > FootprintExtruder

A new window will open, asking for the attribute’s name. Select the name of the column where your heights are stored.

If you are not sure which column it is, open your file in an external application and check. Here is an example of a WellKnown-Text file : the attribute we are looking for is named “elevation:double”.

Select whether you would like the geometries to be extruded Upwards (Z direction) or Downwards (-Z direction) and click Okay.

Top Image : The imported geometries are rooftops. Bottom Image : Rooftops have been extruded downwards to create buildings

Warning

If you do not select a specific object, the extrusion will be carried over all geometries. You may also select specific geometries before extruding, in which case only the selected figures will be affected by the extrusion.

Checking the Normal Vector of your Geometries¶

For any simulation to run smoothly, you must make sure that all of your two-dimensional geometries face the correct direction.

To check this, just click Extensions > t4su > View > PlotAllNormals. The following box will appear. Select the correct layer name, and size of the vectors :

Checking Normals Comand Box

Once you click OK, a new layer will be created resembling this screenshot:

Incorectly Positioned Normal Vectors.

As you can notice, one of the geometries is facing the wrong way. Let’s change that :

select the faces you wish to reverse
click on Extensions > t4su > Edit > ReverseFaces

Delete the obsolete PlotAllNormals layer
Repeat Extensions > t4su > View > PlotAllNormals to see if everything worked.

Recreating a Terrain with Contour Lines¶

On many topographical maps, altitude levels are frequently represented by contour lines, or “isohypes” : lines representing equal elevation. We can turn these isolines into a usable digital terrain model. Start by importing your lines.

See also

2D Sky Maps and Vegetation Modelization

This case has an extra step during the import process : click on Options.. and change the Build group(s) of entities line to “No group”.

Next, select everything (Ctrl+A) and click on Draw > Sandbox > From Contours. If when selected, your geometries appear individually bounded by selections boxes, it means you’ve forgotten to import them as No Group. Instead of re-importing your geometries, you can right-click on your selected geometries, then hit Explode.

Correct importation of isolines.

Incorrect importation of isolines.

You should end up with a layer that resembles this:

Simplified TIN Model.

Visibility, including sky view factors, changes on a rough terrain: our sky view is much more restrained at the bottom of a valley than at the top of a hill. Adding precise altitudes to your map will greatly aid in the accuracy of your calculations.

Close-up of three “2D Sky-Maps” on a rough terrain : areas in dips and valleys “see” less of the the sky than the tops of hills.

Projecting Along the Z Axis¶

Projecting along the Z-axis is really helpful in plotting many objects, like trees or people, on a uneven terrain. The concepts is pretty straightforward : we first import or create a layer of points, then project them on our target surfaces. This technique also works for line segments. So first, import your uneven ground layer and the layers to be projected:

Rough Terrain Layer above a Layer Representing Wooded Areas.

Below our ground layer, we can see another set of geometries which correspond in this case to forested areas. That is the layer to perform a `point sample on.

Once you have your points, you can delete the previous layer : we’ll only be needing the points from now on. Hit t4su > Edit > ProjectAlongZAxis.

The “Wooded Area” Faces are Sampled into Points, which are then Projected onto our Terrain Layer.

Once the command box appears, select the layer containing your sampling points. Next, decide which direction to give to the projection : you may choose between upwards and downwards. Since our points were created from a layer below our ground layer, we are going to project upwards. If of course, your point layer is situated above the ground layer, chose downwards projection. This will result in your points “travelling” in the Z direction until they hit a face (in our case, the ground face). We can then use the projected sampling points as locations for more complex geometries.

Example of the Use of Z-Axis Projection

See also

General Tips to Start¶

T4SU offers many features that allow for an automatic selection, modification, creation or visualization of information, for a fast and easy way to treat and analyse our data. Many of these techniques will be used again in examples throughout the manual.

Selecting Geometries¶

One of the many features of T4su is that it will automatically recognize selected objets when running a process over a layer, and will run that process only on the selection. So if you want to run a process on an entire layer, make sure to not have anything selected. The T4su extension includes a very extensive assortment of selection tools, each oriented towards a specific usage. You can find this collection when clicking Extensions > Select. Regardless of the type of selection, you will have the same three post-process after selection choices:

simply selecting the geometries;
automatically deleting the geometries;
moving the geometries to a new layer. The layer will be named [NameOfSelectionProcess]-[Date and Time]

Types of Selection Processes :¶

SelectRoofFaces : this will select the top-most geometries
SelectWallFaces : this will select all the vertical faces.

These two selection features are pretty good at distinguishing one from the other. To test this, I’ve modified my original shapefile, adding a pointed roof and a small dormer on top of the original roof. The selection module still works :

Roof Selection

Wall Selection

SelectGroundLevelFaces : Selects all the faces with a z0 value of 0. When performing this on a building layer, this selects the ground floor. This will also work for ground-level Sampled Faces and Bounding Box with a Z-value of 0.
SelectAllConnectedFaces : used for export towards Salome
Select UnnecessaryEdges : selects Tin edges of coplanar surfaces to simplify a rough terrain by grouping coplanar faces. This is mainly used used directly with the remove geometries option. You may have to play around with the threshold value, as there’s a strong chance some of your faces will disappear. This is because your threshold value is too low, meaning your margin of error is too large when selecting coplanar faces. Conversely, choosing too small a value will hardly reduce your number of faces, if any at all.

Original Terrain

Deleting Unnecessary Edges with Too Low a Threshold will Puncture the Terrain

Result of Selecting a Correct Threshold for Unnecessary Edges

SelectGroupsAccordingToVolumeProperties : Whether the 3D geometry is closed (ie, hermetic, bounded by faces on all sides), or open (missing a face, or with window or door openings).

An interesting feature is that these types of selections will bypass geometry Groups made in sketchUp.

Batch Processing¶

Batch Processing allows you to automate a number of processes available in t4su, rather than working with point-and-click.

You first need to plot any points of interest, through a sampling method, or by importing a layer or geo-localized points. Using predefined data for your calculations allows for a more precise result than using your mouse for each individual point, and may be time-saving depending on the number of points you have.

See also

Once you click Extensions > Misc > BatchProcess, the following box will appear :

Select your layer containing your data points. These will be at the center of your disks (if you work in 2D) or spheres (for 3D Isovists or Sky Mapping), or the origin of your rays (in the case of Ray Casting).

Running Times and Geometrical Precision¶

Most modules in T4SU allow you to choose your precision. There are general guidelines for choosing the correct. First, we can notice that the precision “p” follows the equation:

What is the concrete impact of augmenting precision ? Here is a first concrete example using 3D Sky Map:

3D Skymap (p=4)

3D Skymap (p=16)

3D Skymap (p=64)

3D Skymap (p=256)

3D Skymap (p=1024)

3D Skymap (p=4096)

3D Skymap (p=16384)

For n=1, the most basic form is a four-sided pyramid. For n=2, the number of faces is 4²= 16. For n=3, the number of faces is 4³= 64 and so on. To calculate running times, each type of 3D SkyView has been repeated ten times, leaving the Ruby Console window on to show the elapsed time after each run. Although computation times will change according to the hardware used, here is a general idea of how long each type of 3D Sky View will take; As you can see, the last construction took 19 minutes (1150 seconds) to finish!

Time (in seconds) taken for each 3D SkyView construction depending on its number of faces

Precision is also important when constructing isovists. As with Sky Views, we have the a choice of 4:sup:`n precision. In this case, we are subject to 4, 16, 64 (...) rays to build our isovist. Too few rays, and the negative space will not be entirely mapped out :

2D isovist built with 4 Rays, also shown.

2D isovist built with 16 rays, also shown

In the pictures above (n=1,2 respectively), because the precision is too low, parts of the isovist traverse the buildings. You can verify this by clicking View > Face Style > X-ray. When moving to a precision of 256 (n=3), we can still notice a small error:

2D Isovist Constituted of 256 Rays.

Close-Up of the 256-Ray Isovist.

Moving to 1024 or 4096 rays correct this problem: they are almost identical, but the first one takes approximately 0.3 seconds whilst the second one take 1.3 seconds.

2D Isovist (1024)

2D Isovist (4096)

In this case, the isovist with a precision of 1024 rays has the best precision to calculation time ratio.

See also

What is an isovist ?

Using Buffers as Visual Information¶

T4su allows you to create buffers around points: a polygon enclosing a point at a fixed distance. You can access it by clicking Extensions > t4su > Edit > Buffer.

The buffer’s form can be either Round or Mitre, meaning square.

Round 2D Buffers

Mitre 2D Buffers

The buffer’s command box also allows you to set your buffer in 2D or 3D space. If you have selected a mitre buffer, the 3D result will be a cube, the size of the sides being your set buffer size. If you’ve selected a round buffer, your set buffer size will represent the length of the radius of a sphere.

Cubic Buffer

Spheric Buffer

Warning

These buffers are regular geometries in your field. If they are left apparent while running a module that requires ray casting, your buffers will be taken into account and will interfere with the calculations.

Basic Example of Data Visualization¶

T4su allows for an easy and rapid way of determining the range of building heights.

First, click Extensions > t4su > View > ColorFaces

The following box will appear :

Select your color scheme. You will notice that all colors appear twice, one of each containing a “-” in front (see picture above). Selecting a color scheme without “-” will allocate darker colors to smaller buildings and lighter colors to taller buildings. The “-” will produce the opposite : darker colors for higher buildings, lighter colors for smaller buildings.
Select the correct attribute amongst the list. Here we have selected building heights (elevation:double) but you could choose another criteria ( as long as it is float or integer).
Select the number of classes you wish to create. This is entirely up to you, but keep in mind that above 5 classes, the difference in color may be difficult to distinguish. If you do, we recommend you use “fire”, “ice” or “GreenRed” which allows for a change of hue instead of a change in lightness / darkness. Be careful to select a color scheme that corresponds to the nature of your data.

Here is an example of the end result :

Sampling Methods¶

T4su offers a four ways to sample your geometries. Sampling is very useful whenever a phenomena may be better studied with spatial discretization. whether they be flat surfaces or lines. They can be found by clicking Extensions > t4su > Edit :

Sampling the Bounding box¶

As with lines and geometries, the “ground floor” of your SketchUp can also be sampled into points or faces.

See also

Calculating the Sky View Factor

Start by clicking Extensions > t4su > Edit > SampleBBoxArea.

Sample BBox Command Box

Set the X sampling distance : that is the direction of the red line in Sketchup. The “(less than [...m])” gives you an information on how large the bounding box will be.
Set the Y sampling distance : that is the length of the sample in the direction of the green line.
You can set the z0 value. You may want to change this for several reasons: if you are studying at which intersections a driver would be directly blinded by the incoming sunlight, it would be best to set the z-value at eye-level when sitting down. If you are looking at how much direct sunlight hits the ground, setting a z0 value to 0 or 0.1 is more appropriate.
You can select your unit length, which should not change throughout your project, and be in concordance with your original SketchUp model.
The “Sample into” is very important:
- If you select SketchUp::ConstructionPoint, points will be created. In this case, X and Y distances represent the distance between points (every Xmeters in the “red direction”, every Y meters in the “green direction”).
- If you select SketchUp::Face, rectangles or squares will be created. In this case, X and Y distances represent the length of the sides of the sampling area.
Finally, selecting indoor or outdoor will sample either the area inside of your geometries (indoor) exclusively, or use the building faces as a limit to sample only the outside area (outdoor). If you choose both, the bounding box will be evenly discretized, ignoring any limits set by your buildings/geometries.

See also

Sun View Factors

Up-facing view with sampling distances X and Y at 1m

Creating Point and Face Samples of your Buildings¶

As we have seen in previous posts, we can sample a line or your bounding box into points or faces.

See also

Creating Partial 2D Isovists

Sampling the Bounding box

We can perform the same type of task on your geometry faces. Here is how :

Click Extensions > t4su > Edit > SampleFacesAreas

You will be asked to fill in the following command box:

SampleFaces Command Box

Select the layer to sample.
Select the sampling distance : unlike SampleBBoxArea, you only have one sampling distance : you can only create squares, or points separated by an equal distance in both (X and Y) directions.
Select your unit length
You can also select the distance to face. Similar to the z0 value, the distance is calculated as the normal to the surface. Setting a small distance (eg:0.1) is helpful in avoiding pesky overlapping sketchup geometries.
Again, you can choose to create points or faces. This really depends on what information you wish to visualize over the discretized space.

Here are two examples of the difference between Sample Points and Sample Faces, both with a 5.0m precision and 0,1m distance from the original geometries.

Result of Face Sampling

Result of Point Smapling

Sampling A Path¶

As We’ve seen, sampling methods will grid your your area of study evenly through point or line sampling, whethere they be your bounding box, or your geometry faces. You can also perform this operation on line segments : the command box for SamplePathway is comparatively very simple :

See also

Using Buffers as Visual Information

“SamplePathway” Command Box

Select the layer containing your edge geometry.
Set the sampling distance. Curvilinear is a fancy way to say we are leaving cartesian coordinates: the distance taken into account only follows the line segment, not the X/Y coordinates. If a line is 10m long and is sampled every 3m, three points will be made along the line at the 3m, 6m, and 9m marks.
Always keep the same unit length throughout of your project.

You now have a new Layer called SamplePathWay_[original_file_name], containing points spread along your line.

Line Segment Sampled every 5m.

See also

Creating Partial 2D Isovists

Viewing Direct Solar Irradiation along a Path

Triangulating Geometries with Gmsh¶

Unlike other sampling methods which result in point samples, or quadrangular (square or rectangle) tessellations, Gmsh allows for a complete subdivision of your building geometries into triangles ; it creates smaller triangles where shapes are more variable, and larger ones on flatter surfaces.

See also

Triangulating Geometries with Gmsh

The first step of the process is to download Gmsh. Keep in mind where you decompress the .zip file ; you will need to copy/paste that location further on.

Next, click on Extensions > t4su > Edit > GmshTriangulator

The command box that follows allows you to connect Sketchup to Gmsh :

Gmsh Command Box (1)

Select the correct layer. In this example, we’ve previously selected a building in our layer, the triangulation will therefore be performed only on that specific geometry. If nothing is selected, the triangulation will be operated over the entirety of the layer.
In the text box below, paste the location of wherever you extracted the Gmsh zipfile, and add \gmsh.exe to the end.
The triangulation will be saved in a temp file, you can decide upon its location in the third text box.
Finally and most importantly, select the default size for your triangles, though some will be much smaller depending on the actual shape of your building. You may be tempted to create very small triangles for better precision, but keep in mind this will have a great impact on computation times, especially once you use them to analyse Sun View Factors for example.

The next window will tell you the number of triangles that were created. After clicking OK, A new window will appear that is very much like the one that appeared when you imported your geometry into Sketchup. Minimum and maximum x,y,z values express the recalibration of the coordinate system.

Gmsh Command Box (2)

Once you click OK, a new layer will be added, named as a series of numbers. Within it, you may find a copy of your original buildings whose faces have been triangulated.

Building Triangulated with Gmsh.

Drawing Sun Paths¶

Drawing Paths of Points¶

Tools4SketchUp offers very different possibilities than the built-in Shadow feature or other plugins available.

The first step in your solar analysis should be to set the location of your project, as well as setting the time-period for your analysis. This will be automatically translated into the appropriate Sun Paths.

First, click on Extensions > t4su > Sun views > SunPath :

The next window allows you to enter the necessary information for the creation of sun paths :

Set the correct latitude.
Set the start date and time
Set the end date and time

It is sometimes practical to select dates that are of special interest for solar paths such as the solstices : the 21st of June and 21st of December represent the highest and lowest paths of the year, respectively.

The daily sun path is a continuous line in the sky, which is here represented by a series of points. You can therefore decide on the interval of time (in minutes) at which the position of the sun is saved.
If your start and end dates are not on the same day, you will have a series of sun paths. If you don’t wish to trace a path for every single day, you can change this parameter : setting “2” will trace one path every two days. Setting “30” will trace a path roughly every month.

Sun Path Command Box

Once your sun paths have been traced and placed in a Layer, as well as triangulated your geometries, you will be ready to run analyses such as Sun View Factor.

See also

Sun Path Result for the 21st of September

Setting Sun Paths from September 21st to Ocober 21st every two days, plotting every 10 minutes.

Resulting Solar Paths from September 21st to October 21st.

Resulting Solar Paths from the 21st of June to the 21st of December, every 90 days, with 5-minute intervals.

Geode Sun Paths¶

Once you’ve drawn your Sun Paths, you may realize your calculation times for Sun View Factors are too much to handle. There is a way to drastically reduce time, but it come at the price of precision : Geode Sun Paths.

Reaching the Geode Sun Path

Geode Sun Path Command Box

The “sun points”, representing the location of the sun at a specific time, travel over a sky vault. By intersecting our sun path points with the sky vault, we can find which facets of the sky could replace our sun path. geodsunpath-isoca-resulthigh-nb-patches Geode Sun Path with very high resolution : as each face is allocated to only one point, calculation times may stay similar but precision will be lost

Geode Sun Path : each face covering multiple points, reducing calculation time

You can use the Geode Sun Path for processes such as the Sun View Factor. In that case, the center of each face is taken into account : the bigger the tessellation and the further away their center is from the original sun path, the less precises the model become.

Here is an exaggerated view of the problem (the model has been designed to maximize the discrepancy) :

Gedoe Sun Path result : 83.4 seconds.

Point Sun Path : 346 seconds

Specific Sun Paths¶

Now that we’ve seen how to create sun paths, you may want to study solar radiation during specific time periods.

See also

Running Times and Geometrical Precision

You can, for example, decide to only plot for specific hours. Here is an example for plotting the sun’s position at noon once a month:

The input goes as such :

Set the start date at 21/06 (summer solstice)
set your start time at 12:00 (noon)
set the end date to 21/12 (winter solstice)
leave the end time at 23:59
Set the time slice per day to 1440 (24hours)
Set the time slice between dates to 30.

Position of the Sun at Noon every 21st, from June to December at 24.7° latitude.

You may also want to have a more precise idea of the course o the sun through the entire year. Here is the appropriate input :

Set the start date at 21/06 (summer solstice)
Set your start time at 00:00
Set the end date to 21/12 (winter solstice)
Leave the end time at 23:59
Set the time slice per day to 10 minutes
Set the time slice between dates to 30.

Daily Sun Paths every 21st, from June until December, 10-minute intervals.

Ray Casting and Isovists¶

The Science Behind : Isovists¶

A Short History¶

Although isovists are considered to have been first set in theory by Benedikt in 1979 [1] (a first mention of them was made by the same author in 1977 [2]), their first appearance in scientific litterature was a paper by Tandy [3], itself based on writings from on AC Hardy [4]. This concept sprouted from JJ Gibson’s work [5] on “optic arrays”. Benedikt and Burnham [6] give a good explanation of the difference between optic rays and isovists :

::: Imagine a rectangle (representing a room) and the space within it. Draw a straight line between two points on the rectangle; draw another, and another, until the space of the rectangle is effectively filled with criss-crossing lines between all pairs of points. Now chose an point within the rectangle and you find that a large number of lines pass through that point, one of which is shared with another different set of lines passing through a point nearby. If we consider these lines to represent light rays (i.e., photon streams) of varying wavelength and intensity scattered and bounced by the edges of the rectagle (i.e., the walls of the room), then you have an optic array - a set of rays that pass through the point of interest. But where does the otic ray start and end? Is a ray that happens to pass through our point the same ray that it was before it bounced off the wall? If it is, then almost every ray in the room qualifies as belonging to our optic array, as it would eventually pass through our point. Clearly this isn’t going to work. The solution lies in concidering a ray only after its “last bounce”, that is, in concidering only the set of lines – rays – joining our point to the nearest light-scattering surface in every direction. Now, the isovist is simply this optic array, with wavelength and intensity information omitted.

In other words,

::: To every bearing (omega, phi) from a point of observation x in the world there corresponds a distance l, from the point to the nearest surface such that we have a unique ordered set

Several equations derived from isovists and isovist fields (that is, when caculating the isovist for each point in space), mainly from it’s perimeter and area, which an be discretized. We can produce a number of morphological indicators from isovists, including :

Mean Depth
Compactness fields
Occlusivity, occlusivity fields
Visible Perimeter Fields

Isovist Moments can be derived into three types :

M1 represents represents the derivation from the mean of the perimeter’s distance to x. M2 represented the derivation from the variance of the perimeter’s distance to x. M3 represents the derivation from the skewness of the perimeter’s distance to x. This last formulae can be used to find areas which can see well but

[1]	ML Benedikt, 1979, “To Take Hold of Space : Isovists and Isovist Fields”, Environment and Planning B, vol 6, pp. 46 - 65

[2]	ML Benedikt, 1977, “Path-dependence and position-dependence in isovist fields”, research report for the Council of Advanced Transportation Studies, University of Texas Austin, Austin, Texas.

[3]	Tandy, 1966

[4]	University of Newcastle, unpublished

[5]	Lynch 1976

[6]	Gallagher, 1972

[7]	1966, The Ecological Approach to Visual Perception, Psychology Press Classic Editions

[8]	Perceiving architectural space

Ray Casting : an Introduction to Visibility Studies¶

Ray Casting involves choosing a point in your urban space, from which rays will spread at equal distance from each other. The ray stops when it hits either an obstacle or its set limit. It is the simplest and quickest way to assess visibility, but doesn’t give as much information as isovists or hedgehog. First off, click on Extensions > t4su > Visibility studies > RayCasting. A command box will appear, giving you the following possibilities:

Ray Casting Command Box

Choose the number of rays. This will affect the results’ precision, but taking the maximum isn’t necessarily better : if you select a small visible range, you may not need too many rays.
The visual range is the maximum length of the rays ; any obstacle after said range will not be picked up during the process
Set the height (z0 value). If you’re using ray casting for visibility purposes, keep a z-value around eye-level.
Make sure the units you are working with are correct.
Finally, you can choose to cast rays in a 2D or 3D space. This is entirely up to the type of information you want to collect. If you are only interested in street visibility and do not need information on the impact of building heights for example, a 2D Ray Casting is much more legible and should suffice.

Once you have click OK, you can click anywhere on your map to cast your rays, as many times as you need (you do not need to repeat the procedure to plot a second RayCast).

2D Ray Casting with 256 rays at a distance of 40m

2D Ray Casting with 1024 rays at a distance of 150m

If you are not interested in the rays themselves, but rather the point of impact they share with the facades of your buildings, check out CloudOfPoints.

What is an isovist ?¶

Isovists represent the entirety of the visible plain (in 2D) or volume (in 3D) at a specific point in space, bounded by the built environment. Because isovists are built upon a point, the associated form changes as it is displaced, just as our notion of urban space changes as we walk through it. They are frequently used in many fields of visibility studies : wireless network design, landscape management and analysis, pedestrian access or security.

Isovist at a Point Close to the Cathedral.

Isovist at an Other Point in Space.

So how are isovists built? Originating from our point of origin (view point), rays are cast into the open space, up to a certain arbitrary distance (view limit). The resulting points of impact between each ray and its barriers (either obstacles or the view limit) are joined to form the isovist’s edge. The rays are then replaced by an isovist area.

Isovists can also be used to study a space’s convexity, that is, a region for which every pair of points in said region can be linked by a straight line also within the region. Here is a simple example of convex and non-convex spaces:

In a Convex Space, Isovists have the Same Shape no Matter the Location of the View Point

In a Non-Convex (Concave) Space, more than one Isovist is Needed to Map the Totality of the Area.

We can notice the link between ray casting and isovist formation when playing with its precision.

See also

Creating Partial 2D Isovists¶

Partial isovists are isovists that are constrained by a certain aperture. You can automate their process using BatchProcess, or set your positions and orientation manually by clicking Extensions > t4su > Visibility Studies > PIsovist 2D. If you’re using the manual method, you will be redirected to the Partial Isovist Command Box.

First, import a linear geometry (ie: made of lines or multilines) into your Layers, or draw one yourself using the Line Tool.

See also

Next, you’ll want to sample your pathway, then use a Batch Process.

See also

Sampling A Path

3D Sky Maps and Building Facades

Select PIsovist2D (which stands for Partial 2D Isovist) in the Process to apply and select the correct layer (starting with SamplePathway[...] if you’ve created your points through sampling). Finally, select Sketchup::ConstructionPoint as Geometry type.

If don’t want to use a Batch Process or would rather select the points manually,you can do so too. Simply click to set the point of origin and drag to set the orientation of your partial isovists.

Whether you’re plotting manually or automatically, you will have to fill in this command box :

Select the aperture of your circular sectors (ie: how wide you wish the isovist to be)
Select the number of rays : a higher number will give a more precise outline, but a lower number will greatly accelerate computation time. For example, setting the aperture to 30 and the number of rays to 60 will result in a ray every 0.5 degrees.
Select ray length. Some trial-and-error may be needed to adjust this parameter : too short, they may not cover enough ground. Too long will result in confusing overlapping geometries and longer computation times.
Setting the z0 value allows you to decide at which height the partial isovists are calculated. By default, they are at 1m60, an average person’s eye-height.
You may also change the appearance of the isovists : transparency (from 0 to 1, 1 being the most opaque) as well as the color.
When you are ready, click OK

What do we see ?

The partial isovists are constrained by the extruded building geometries. Their profile mimics our visibility as we advance along the path we have decided upon. We can notice the gradual enlargement of the isovist as we advance towards an plaza or an intersection.

3D Isovists and Hedgehogs¶

Like 2D Isovists, 3D Isovists and hedgehogs use ray casting to map the surrounding urban space. Unlike it’s 2D counterpart though, 3D Isovists will only output assembled rays that effectively touch a surface.

Isovist Command Box

When creating these 3D objects, you will have to decide whether to draw “tetrahedra or spots” (or both). By choosing tetrahedra, the cast rays are drawn, set in a group and shown. Spots on the other hand, draw out the surfaces visible from our point of interest (i.e., the faces’ areas touched by the tetrahedra).

3D Isovist, Spots Only

3D Isovist, Spots and Tetrahedra

The surfaces are created by a minimum of three points, meaning if only two rays reach a surface, that surface will not be taken into account in the Isovist/Hedgehog rendering. You will also get the following error:

The difference between a hedgehog and a 3D isovist lies in whether or not these batches of three rays form individual tetrahedra/spots, or are assembled into one. Here’s an illustration of a Hedgehog result: compare it with the Isovist result above (resolution was slightly lowered).

Hedgehog Result, Spots Only.

As with many other features, it is possible to run a batchprocess of 3D isovists. In this last example, the cathedral building wall faces were selected and sampled into points. The following result allows us to know which building facades have a potential view of any side of the cathedral.

Building Facades with Direct View of the Cathedral

Sky Maps¶

Sky Maps are exactly what they’re named after : maps of the visible sky. Sky Maps work in a similar fashion as Ray Casting : as rays are sent from a point of origin, they either reach the sky vault or they don’t : Sky maps only take into account the rays that do reach the sky. They can be projected onto a dome, or reprojected onto a disc for a 3D or 2 visualization.

3D Sky Maps¶

See also

Ray Casting : an Introduction to Visibility Studies

What’s a 3D Sky Map ?¶

Let’s imagine for starters that we’re at a point on a completely flat surface, without anything blocking our view. Our result would be something like this :

Of course in an urban setting, some of our rays would be blocked, and the associated dome faces would not appear :

How do I create 3D Sky Maps?¶

You can place 3D Isovists through the point-and-click method by clicking t4su > Sky views > 3D Sky Map, or through a BatchProcess if you already have your locations set out.

See also

What Can I Use Them For?¶

On their own and at a first glance, theses domes help you assess the openness of an area, the local skyline and roughly estimate the sky view factor. But they hold much more interesting information when used, for example, to assess solar radiation. As we’ve seen, we have methods to calculate the Sun View Factor after having drawn Sun Paths. What happens when we reproduce this procedure on our 3D Sky Maps ? To try this out, first create your domes, then click t4su > Edit > AutomaticReverseFaces because half of your dome’s faces are facing inwards. Then proceed to draw your Sun Paths and calculate your Sun View Factor over the layer containing your 3D Sky Maps. Finally, use ColorFaces to depict the differences in energy received by each face :

See also

Sun View Factors

Running Times and Geometrical Precision

3D Sky Maps Created by Point-And-Click, Colored by Direct Solar Irradiation Value

We can see a lot more information at particular points in space than when doing a regular `SunViewFactor over a tesselated area:

We can see what parts of the sky contribute most (and least) to our direct solar irradiation. Naturally, North-facing faces receive the least sunlight, and Southern faces at a roughly 45° vertical angle receive the most. We can thus find which sections of the sky contribute most to the points’ solar energy gains.

3D Sky Maps and Building Facades¶

3D Sky Maps and Building Facades both start from a half-dome, at the center of which is the point of interest. Rays start from the center point, pass through the center of each dome face and continue on, either hitting a facade or not.

_images/skymap-3d-buildingfacade-result.png

A combination of SkyMap 3d (clear blue) and BuildingFacade (grey) at the same point in space

See also

Whilst Building Facades will only show the dome faces who’s rays have hit an obstacle, 3D Sky Maps will only show those that have not : They show complementary information. So when is one or the other more appropriate?

Sky maps are much more useful in showing which parts of the sky are visible at a certain point. They hold an advantage over 2D Sky Maps,in that the sky is not projected over a 2D surface: they are not deformed by any necessary spherical projection.

See also

Calculating the Sky View Factor

Sky-Map 3D over an intersection

The BuildingFacade module works great for street-level visibility : it highlights the skyline, the openings in the urban space represented by “dips” in the dome formation, allowing you to assess the relative confinement of urban space.

Typical BuildingFacade profile for a narrow street

Once built, the dome faces also hold an attribute, “dist_to_facade”(float). This feature expresses the distance between the point of interest and the obstacle represented by the dome face.

Ruby console showing attributes when using PickUpEntity

You can view it by clicking View > ColorFaces and choosing the dist_to_facade attribute.

_images/buildingfacades-colorfaces-box.png

ColorFaces Command Box

Originally, the faces all turn inwards: the results may be more visible if the faces where reverted. This can be easily achieved by selecting Edit > AutomaticReverseFaces, and selecting your BuildingFacade layer. Sometimes, intersecting faces are not automatically reversed. For an optimal result, render your building layer invisible before reversing your BuildingFacade layer.

2D Sky Maps and Vegetation Modelization¶

As we have seen, we can easily cover large areas by multiplying a single object, such as a person to make a crowd or a tree to make a forest. There are more than 1400 trees to choose fromin the 3D Warehouse, you really have a large choice. So are all SketchUp trees the same? What types are there and how do they affect the results of our visibility studies? Let’s take two concrete examples.

First Tree : the FaceMe Tree¶

If you’re familiar with Sketchup, you may know about the faceMe technique. Basically, a 2D Object is set to automatically turn to always face your camera, no matter which way you look at it (unless you look from above).

Second Tree : the Carboard Cutout¶

The Cardboard Cutout also uses the faceMe method for it’s vertical face, but also has an added horizontal face.

Comparison :¶

Let’s now compare the impact these two different trees would have on our visibility studies. For starters, are the sky maps the same? If they’re not, what else would that change? To find out, import both types of trees into different layers, and place them at identical spots. To make sure we compare the same points in space, we can create a path, sample it and project the sampling points over our terrain. We can then launch a 2D Sky Map Batchprocess over our sampled path twice ; once for each tree layer type.

Sky view Factor with Tree 1.

Sky view Factor with Tree 2.

Comparison of Both Sky View Factors.

Clearly, trees composed of only a vertical face allow for a much greater sky view factor. In this particular case, a greater sky view also means greater direct solar radiation.

See also

Sampling A Path

Conclusion:¶

Choosing the correct type of tree is very important : they’re part of every urban setting and need to be taken into account when analysing urban energy balances. Vegetation’s correct integration in such simulations is still at its premise, mainly because their modelization induces calculating variable opacity over many complex shapes. Comparatively, this SketchUp method may not be as precise as other microclimatic models, but it’s definitely much faster. In any case, it’s vital to know what type of geometries we’re working with and anticipate the types of problems that could be encountered, whether it be a 2D, 2D+ or 3D tree model.

Sunny Sky Maps¶

We can project given sun paths onto our 2d sky map. This allows us to quickly understand buildings’ impact on solar availability at a point in space, over different periods of time.

First, create the sun paths you wish to have on your sky view graph.

See also

The Science Behind : Solar Radiation

In thisexample, we will be using the sun paths of Winter and Summer solstices, as well as an equinox, at a latitude of 47°. This allows us to see the range over an entire year.

Sun Paths during the Solstices in Nantes, France.

Next, click on Extensions > Sun views > SunnySkyMap2D :

Reaching SunnySkyMap

In the following command box, first select the layer containing your sun path:

SunnySkyMap Command Box

Enter the number of rays used for the calculation, ie: its precision in finding the contours of the surrounding buildings
Set the visual range : this is the size of your graph. This depends your project and your personal preferences : the amount of space you have available to plot the graph, how far out you would like to zoom and still read it...
Select the height at which you would like to calculate your sky view factor (z0). If you would like to visualize projections at ground floor, 0.0 or 0.1 are an acceptable inputs.
Select your unit of length. Be sure to always keep the same units throughout the entirety of the project!
Next, select the type of projection you would like to use. If you’re not sure which one fits your needs best, check this pagefor more ample information.
Finally, you can change the color of the building projections.

Once you hit OK, you can start clicking anywhere on your project to insert an graph at that location.

From left to right : Stereographic, Orthogonal and Isoaire (Equal-Area) projections at the same location.

Reading such graphs are relatively simple. The disc represents a 360-view of the sky: the ground floor is projected at the circumference, whereas the center of the disk is the point in the sky directly above it (what happens in between depends on your type of projection). The filled-in parts of the graph represent parts of the sky that are blocked off by obstacles.

If a point of the Sun Path is contained in the filled in areas of the graph, it means that at that specific time and place the point you are studying is in the shade. Conversely, a sun path point that is not superimposed over a filled in area of the graph means there is direct sunlight at that specific time and place.

View Factors¶

The Science Behind : Sky View Factors¶

A Short History¶

One of the earliest SVF calculations were made with the help of digital cameras mounted with Fisheye lens. The masks were then delineated manually and the SVF is calculated with Steyn’s method [1]. Other raster-based calculations were later developped, using pixel counting methods [2] [3]. To overcome this somewhat manual and time-consuming method [4]

See also

Sky View Factor and Radiation¶

A Sky View Factor (SVFs) represents the ratio at a point in space between the visible sky and a hemisphere centered over the analyzed location (Oke 1981) :

At a point where the SVF = O, the entire sky is blocked from view by obstacles. If there is an obstacle blocking parts of the sky view, the SVF will be greater than 0.

for SVF = 0 :
- there is no short-wave reflection
- there is no long-wave nocturnal interference

According to Oke [5], cited by many papers including Hammerle et al. [6], the energy budget (in terms of flux) can in this case be considered ideal

Where Q_* is the total energy balance, Q_Gis the energy stored in soil and/or buildings, Q_His the sensible heat (air temperature) and Q_Eis the latent heat (evaporated water).

for SVF > 0 :
- incoming day-time short-wave reflection increases during the day
- outgoing night-time long-wave radiation is reduced
- incoming night-time long-wave radiation is increased
- altered soil heat flux

Note

For more information on th different types of radiation, see Night-time Radiation

Because of its influence on energy balances, the SVF holds an importance in various fields of climatology (urban [7], forest [8], human biometeorology [9]...) but its most widespread use concerns the heat island phenomena.

Sky View Factor and Heat Ilsand Phenomena¶

One of the earliest studies demonstrating the link between sky view factor and local urban temperatures was made by Yamashita et al. [10] (1985).

Footnotes

[1]	Steyn, D. G. (1980). The calculation of view factors from fisheye‐lens photographs: Research note. Atmosphere-Ocean, 18, 254–258. https://doi.org/10.1080/07055900.1980.9649091

[2]	MMatzarakis, A., Rutz, F., & Mayer, H. (2007). Modelling radiation fluxes in simple and complex environments: Basics of the RayMan model. International Journal of Biometeorology, 51, 323–334. https://doi.org/10.1007/s00484-006-0061-8

[3]	Rzepa & Gromk 2006, Gal et al 2007

[4]	Hämmerle, Gal, Unger, Matzarakis 2001,

[5]	Oke, T. R. (1982). The energetic basis of the urban heat island. Quarterly Journal of the Royal Meteorological Society, 108(455), 1–24. https://doi.org/10.1002/qj.49710845502

[6]	Hämmerle, Gal, Unger, Matzarakis, 2011, Comparison of Models Calculating the Sky View Factor used for Urban Climate Investigations, Theoretical and Applied Climatology, Oct. 2011, Vol. 105, n. 3, pp. 521-527

[7]	Watson, I. D., & Johnson, G. T. (1987). Graphical estimation of sky view-factors in urban environments. Journal of Climatology, 7(2), 193–197. https://doi.org/10.1002/joc.3370070210

[8]	Holmer et al., 2001

[9]	Matzarakis, 2001

[10]	Yamashita, 1985, On Relationships Between Heat Ilsand and Sky View Factor in the Cities of Tama River Basin, Japan. Atmopheric Environment Vol. 20, n°4, pp. 681-685 1986 Pergamon Press Ltd.

View Factors reduce the information given by sky maps and sunny sky maps to a single numerical value, allowing for a more traditional cartography of a discretisized space. Sky View Factors represent the ratio of visible sky at a point, which we could find the Sky View Factor alternatively by attributing to a Sky Map Disc’s center the ratio between the disc’s area and the area representing visual obstacles. Similarily, Sun View Factors can be found by comparing the number of points of a sun path not blocked by obstacles with the total amount of points in the sun path.

Calculating the Sky View Factor¶

The Sky View Factor (SVF) is used in the evaluation of the impact of urban geometry on the micro-climate, specifically temperature and the Urban Heat Island phenomenon. Sky View Factors help in the comprehension of an area’s potential direct sunlight in a given amount of time. The SVF indicates the ratio between :

the radiation received / emitted by a surface from/to the sky

and

the theoretical total hemispheric radiating environment.

Direct “visibility” between the sky and a plane means radiation can escape the urban setting into the atmosphere, whereas radiation between said plane and another plane keeps radiation trapped. Higher SVFs therefore indicate spaces that would theoretically cool down (emit more radiation towards the sky) than a space with low SVF.

Let’s see how we assess the Sky View Factor in an urban setting. First off, follow the instructions to create a grid layout of the urban space. For this example, we’re using a 10x10 grid, with z=0.1m.

Next, select the Sky View Factor in the BatchProcessing options.

See also

Sampling the Bounding box

Fill in the next two command boxes accordingly. In the first box:

Select SkyViewFactor as process
Select the layer containing your sampled bounding box
Select “Sketchup:Face”

The second command allows you to control the SVF parameters:

Select the number of rays used to calculated the SVF, ie: its precision. Be careful though, this process is repeated for each sample and will impact your computing time dramatically
Set the z0 value to 0.1, meaning we will check the SVF at ground level.
Select your unit length
Finally, select the name under which the values are stored. We recommend you keep the default name “svf” to keep from any confusion.

Once that is done, you can check your new feature by selecting Extensions > View > PickUpEntity to view individual squares, or by going to Extensions > View > PrintAttributeValues ** and by selecting **svf:Float in the following command box to show the values of every sample.

But there is a much better way of viewing and presenting the data, much like what we’ve done when viewing building heights.

See also

visualizing-bilding-heights

Select Extensions > t4su > View > ColorFaces.

In the next command box, select the color range you wish to use.
In the drop-down menu next to “select attribute’s name”, find and select “svf:Float”
Select the number of classes you want for your map.
Hit OK

Your grid should be colored to resemble something like this :

Sky View Factor over Cathedral Sector, Nantes

Associated Legend of the SVF Map.

Unless you’ve taken a color range with a “-” in front, lighter values express areas with a high SVF, meaning areas which “view” much of the sky. Darker areas, mostly in small streets and inside building blocks, do not “see” much sky directly : most of the radiation is captured and recaptured by adjacent walls.

The Science Behind : Solar Radiation¶

Initial Definitions :¶

Solar Energy is the amount of energy sent by the sun, meaning the maximum energy received by the Earth, without taking into account climatic conditions (clouds), date or time of day which impact the angle of the surface relative to the sun.

Solar Irradiance (or insolation) is the energy received by a given area of the Earth’s surface. It’s measured in Watts per square meter. As the angle between Sun and surfaces’ normal grows, the solar energy is spread over a larger surface, reducing solar radiance. Solar Irradiation is Solar Irradiance intergrated over time.

Solar Angle and Irradiance¶

Both place and time come into play when calculating the the solar angle. Place concerns where we are : latitude and longitude :

There are two conjoined earthly movement to be taken into account. - The rotation of the Earth on itself :

The rotation of the Earth around the Sun :

Finally, we must keep in mind that the Earth’s rotation axe is inclined at 23°27 from the ecliptic plane. This tilt is at the origin of our yearly seasons : we consider the Earth tilted at +23°27 during summer (Souther hemisphere) and -23°27 during winter, the tilt beeing equal to 0 during the equinoxes.

Time concerns The day of the year and the time of day. All of these factors need to be taken into account before any more local analyses.

For Solar irradiation, we need to factor in time :

Atmorsphere’s Interference¶

Radiation reaches the Earth’s surface in a more or less direct way, depending on the atmospheric interference. Water, dust and pullutants floating in the air will refract rays, dispersing sunlight in multiple directions. The higher the interference, the more diffused the sunlight will be when reaching the surface. The unit of measurement to describe the amount of cloud cover is called an Okta, where 0 Okta is a completely clear sky and 8 Okta is a completely cloudy sky.

Night-time Radiation¶

Any radiation received by the ground (or buildings) will start releassing heat the moment the air surrounding it turns colder than the soil, typically during night-time.

The radiation emitted has changed to long-wave radiation. In a given urban model, for there to be energy loss (ie, heat loss), the radiation needs to escape back to the atmosphere. This implies a covisibility between the surfaces and the sky vault. More information on Sky View Factor and Radiation

Local weather will greatly affect solar radiation, especially the presence of clouds. As the sun passes through them, the light gets reflected and defused.

Sun View Factors¶

Once you’ve created a Sun Path, you can use it to calculate the Sun View Factor, which works a little like the Sky View Factor : instead of checking covisibilities between geometry faces and the sky dome, it will check the covisibility between a geometry and each point of your Sun Path.

See also