Sky and Sun View Factors

10. Sky View Factors

10.1. 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 10×10 grid, with z=0.1m.

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

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”
https://f-origin.hypotheses.org/wp-content/blogs.dir/3847/files/2017/05/bbox-5x5-for-svf-box1.png

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.
https://f-origin.hypotheses.org/wp-content/blogs.dir/3847/files/2017/05/bbox-5x5-for-svf-box2.png

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.

Select Extensions > t4su > View > ColorFaces.

https://f-origin.hypotheses.org/wp-content/blogs.dir/3847/files/2017/05/view-svf-box.png
  • 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 :

https://f-origin.hypotheses.org/wp-content/blogs.dir/3847/files/2017/05/skyview-10x10-result.png

Sky View Factor over Cathedral Sector, Nantes

https://f-origin.hypotheses.org/wp-content/blogs.dir/3847/files/2017/05/skyview-10x10-result-box.png

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.

 

10.2. 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.

Clicking on Extensions > t4su > Sun Views >SunViewFactor will enable the following command box :

https://f-origin.hypotheses.org/wp-content/blogs.dir/3847/files/2017/05/sunviewfactor-box.png
  • Select the two corresponding layers : your SunPath layer and your geometry layer.
  • You can choose to alerate the results slightly by choosing a specific type of sky (from “pure” to “cloudy”).

Once that is done, you will find that your geometries now have extra attributes:

  • directSolarIrradiance:Float,
    represents the cumulative theoretical radiative energy received by the geometry from the sun at each point, which depends on their  covisibility and the angle of the sun’s rays.*
  • nbHitsTotal:Int,
    the number rays between the geometry and the SunPath points
  • nbMinTotal:Float,
    the number of rays between the geometry and SunPath, multiplied by the frequency of the plot of the Sun Path (eg: for a plot every 5 minutes if nbHitsTotal=2, nbMinTotal=10)
  • ratioTotal:Float

You can then use ColorFaces to see the geographical variations of each of these new attributes. Below, we’ve drawn two different maps of the same phenomena : The number of minutes a defined space will receive direct sunlight on two opposing dates : 21st of June and 21st of December, the longest and shortest days of the year respectively.

https://f-origin.hypotheses.org/wp-content/blogs.dir/3847/files/2017/05/numminwinter.png

Number of Minutes of Sunlight for the 12st of December

https://f-origin.hypotheses.org/wp-content/blogs.dir/3847/files/2017/05/numminsummer.png

Number of Minutes of Sunlight for the 21st of June


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