FAQ Foliage Heat Overlay
First and foremost, the height of the foliage is required for shade, sky view and vegetation calculations in the Heat DPRA Module. It is not the tree trunks that we are specifically interested in. One of the options to determine foliage height is to use the tree locations, combined with the height of the tree and a crown factor parameter, to determined the foliage height of cells near the tree trunks. This leads to circular tree foliage, which is only a rough approximation. For more information on this model, we would also like to refer to Foliage Height Calculation model.
Since the foliage of a tree is often not circular and uniform, two additional options allow users to directly supply the foliage height without supplying the tree locations themselves and their crown factors;
- Areas with a foliage height parameter.
- GeoTIFF with cell specifying the height of foliage per cell, with optional no data when foliage is not present.
The following section contains questions and answers.
From option 1: How I can adjust functions per tree, and is it possible to include the trees in the analysis, and not to filter them from the data just like buildings.
Simply open the Function Table, which can be accessed through the Building Popup in the ribbon bar. By default this table does not yet show the trees, since you have not yet adjusted them. Add the functions you would like to adjust and optionally filter on environmental attributes to show the relevant Heat-2019 Overlay attributes: Crown Factor, Bowen Ratio and Vegetation Fraction.
From option 2: I want to know exactly which data is expected, since the point data that I have does not seem to work.
What is expected for this step:
- Each geometry should be an area describing exactly the foliage of the tree, not the tree trunks. This allows users to define more precisely where the foliage is situated.
- Each area should contain a value indicating the height at which the foliage is situated. This height should be relative to the terrain height.
If you have a file that only contain tree locations as points, you should instead import these points as tree buildings and not import them as foliage areas. The imported trees will then use their height and their function's (or if supplied: building's) CROWN FACTOR to determine the foliage area.
For option 3, I would also like to know what this data should look like. For example, is this a grid with height values, where there are only trees? So no individual trees need to be identified?
The grid data provided should present, per cell, the height of the foliage relative to the terrain height. A foliage height of 0 or lower is handled as no data and therefor ignored.
Now that we have a point data set in which each tree is known as a loose point, to which the tree height and crown surface parameters are given at this time. How can I currently see the crown areas, which are calculated in m2, compared to the tree crown factor?'
As explained above, the point data set if trees as loose points is not what we look for, we only use the data describing the relative height of the foliage for each grid cell. This foliage height is acquired by any of the three selectable options. In order to see the foliage areas, select the result type: Foliage Areas in the Result Overlay step in the Heat Overlay Wizard.
From the sentence "Tree height multiplied by" tree crown factor "judge whether the surrounding cells are in the shade." I cannot figure out what exactly you calculate with this. Is this the crown diameter / cross section, radius, perimeter, area?
Only for foliage option 1 the Tree functions are identified that have a tree crown factor. In order to obtain a foliage height for this Tree function, the height of this tree is multiplied with the crown factor, which leads to a foliage radius. In case a tree is 10 meters tall and their crown factor is 0.25, their foliage radius is calculated as 10 * 0.25 = 2.5 meters.
The Heat model will apply a calculation step in which the foliage height will be set based on nearby trees and their foliage radius. The foliage height set will be the height of the tree. There is one exception: if the cell is a building which is taller than the foliage height, there will be no foliage on this cell.
Does the tree then have 1 fixed height value, or does it calculate in the case of a GeoTIFF with several height values to calculate a kind of sphere shape?
Only for option one: Tree functions this question is relevant. The tree has a single height value. If you as a user want more freedom in the definition of the foliage height, you can import the foliage height directly as areas or as GeoTIFF data.
Do you start from a surface file (I see that the few BGT trees are currently defined together as a polygon)?
The BGT defines trees in two forms, points and areas. The areas are used when large forests are defined as areas with a function or purpose. Points are used when individual trees are situated on other function areas such as grass. These are two separate data layers in the BGT data set.
It really depends on the owner (often Municipality) of the local BGT data if the trees are drawn as points. Trees inside gardens are often not defined in the BGT data, since the Municipality does not own these.
What exactly do you mean by foliage? Is that the height of the trees relative to the ground level?
Yes, the height of the tree 's foliage relative to the terrain height a.k.a. ground level.
What exactly does the foliage crown factor mean? What is calculated with that? I assume the crown diameter?
The crown factor is a means to calculate the foliage diameter from the height of a tree. Depending on the type of tree, this crown factor will be different. Recently, several tree generalizations have been added to the set of functions with a distinct model.
How are trees currently included in the model?
Trees are used for 2 purposes:
- When foliage option 1 is selected, trees are used for determining the foliage height or cells with the tree or surrounding the tree. Cells within the crown diameter are given the same foliage height as the cell containing the tree and its foliage height. The foliage height is used for shade and sky view calculations.
- Trees are building-functions and therefore have a vegetation fraction and bowen factor. Also cells that are marked having a foliage always receive a vegetation fraction of 1 and a bowen factor of 0.4;
What is the effect of 1 tree compared to a group of trees (or forest) in the model? Is a distinction made in this?
Note that this answer is only relevant for selection option 1: Tree functions.
When a group of trees or a forest is defined as an area, we can only make a generalization. This means that the polygon is expected to be fully saturated with trees, even on the borders. The foliage height is currently calculated as an average for this polygon and set as an attribute for this tree-forest-building. Similar to a single tree, neighboring cells within the calculated crown radius of the forest-building are given the foliage height, vegetation fraction and bowen ratio of that forest. It then depends on the grid size if a single tree has relatively more influence then a forest. However, since the general grid cell size is often 1 or 2 meters, the relative influence between a single tree and a forest will not be that different.
I now have a point file for all trees in a test area, with a tree height (m) and a crown area (m2) for each tree. Can I also use the height and crown area from my file in the model? I have tried a few things but it seems that you assume that there is a fixed foliage crown factor for every tree, so that is the same for every tree? I would therefore like to use my own calculated crowns, because a fixed factor is out of the question. Is that possible?
Yes, you can use your own crown height when you have this data as areas or GeoTIFF data. The options to these answers are explained above. However, one point to mention is that although the Input layer Tree Crown Factor can be added, it is not relevant when a user uses areas or GeoTIFFs to specify the foliage heights.