Tree coverage ratio calculation

Egi.K · May 12, 2022, 2:01pm

Hi all,

I am trying to calculate the effect of trees in outdoor comfort calculations. I want to include different % of tree coverage. I can easily change the ratio of trees in “Dragonfly assign model UWG properties”. However, I would also like to physically add the trees in Rhino as their position in the urban area makes a difference in the values of UTCI. I am trying to figure out how does the above mentioned parameter measures the tree coverage when you input them as “context shade” in the model. I am trying with a very simple model in an area of 20x20 m and with only one building and one tree input. When it comes to the calculation of the building density is the same as the parameter shows in the report. However, when I try for the tree coverage what the “Dragonfly assign model UWG properties” gives is not the same as if I manually calculate the horizontal surface of the tree divided per total area.
I input the trees with the Pando tools.
Would someone please be able to let me know how does the program calculate this specific parameter, or where can I find documents related to this?

Thank you

chris · May 13, 2022, 7:07pm

Hey @Egi.K ,

It’s hard to know what exactly you are referring to here without any screenshots or a minimal sample file that recreates what you are describing.

What I can tell you is that, as long as you set the uwg_is_veg_ property to be True when you create the DF Context Shade, then the ratio of tree coverage to ground area will be correctly computed for the UWG input.

Here is the source code that does this automatic tree coverage calculation:

github.com

ladybug-tools/dragonfly-uwg/blob/645a8a2ff96201ab23d012e194276723bb4c170b/dragonfly_uwg/properties/model.py#L382-L391

    
      
          def _autocalcualted_tree_coverage(self, ground_area):
              """Autocalculate the tree coverage from the model context shades."""
              veg_shds = []
              for shd in self.host.context_shades:
                  if shd.properties.uwg.is_vegetation:
                      veg_shds.extend(shd.geometry)
              tree_area = self.compute_horizontal_area(veg_shds)
              if ground_area <= 0:
                  return 0
              return tree_area / ground_area if tree_area / ground_area <= 1 else 1

And note that the UWG exepcts the tree coverage to be computed with just the exposed ground area (discounting the building footprint area):

github.com

ladybug-tools/dragonfly-uwg/blob/645a8a2ff96201ab23d012e194276723bb4c170b/dragonfly_uwg/properties/model.py#L125-L128

    
      
          @property
          def exposed_ground_area(self):
              """Get the area of the terrain exposed to the outdoor air."""
              return self.terrain.horizontal_area - self.host.footprint_area

Egi.K · May 17, 2022, 9:18am

Hi @chris,
sorry for taking so long to reply, I was trying to figure out what does it exaclty takes as horizontal area when I input context shade as vegetation. I will add here the file. I have considered a study area of 20x20m and one building with dimensions 5x10 m, I also add a tree in a conical shape and try to calculate manualy the tree coverage to compare it to what the component brings out.

I am affraid, I do not totally understand how the component calculates the tree coverage. As you said in the previous message it does not consider the area where the building is so then we remain with 400-500 = 350 m2 for the area. The radius of the tree I input is 2.8 m so the horizontal surface of the tree would be 24,62 m2. So in this case the tree coverage with my calculations would be around 0.07% while the components shows 0.14%. So it more or less doubles the actual horizontal area of the tree and the same happens when changing the crown shape.

That is why I wonder if this doubling happens due to the shade which comes at the ground, which is probably bigger then the actual horizontal surface of the crown?

I am trying to understand this to figure out how many trees I can add for a certain % of tree coverage.

Thank you,
Egi
How does UWG calculate tree coverage.3dm (45.0 KB)
How does UWG calculate tree coverage.gh (46.2 KB)