Benchmark Wageningen analysis

The Wageningen and Tygron-generated PET overlays are shown below:

PET daytime map for August 2, 2013 12 UTC with the meteorology of the reference station Herwijnen from the DPRA Heat stress report
PET daytime map for August 2, 2013 12 UTC with the weather data from the reference station Herwijnen as calculated by the Tygron Platform

A few differences in the PET results can be explained by differences in input data:

Foliage height source.
Wind calculation for areas with a lot of foliage.
The spatial variation of vegetation, especially in gardens.
Slight differences in geometry data versions (buildings, parking lots, and water present/not present).

Foliage and the effect on wind

The calculated wind speed differs in areas with a large number of trees, as you can see in the images below (Tygron wind map is on the right):

Wind speed August 2, 2013 12 UTC
Wind speed August 2, 2013 12 UTC as calculated by the Tygron Platform

The foliage input data might be a cause for these differences. See the next section for a possible explanation and possible improvements.

Foliage input generation

The Wageningen PET Test Case contains a tree height polygon data source. Per polygon, the maximum height is stored and the polygon itself represents the crown of the tree. This results in generally too much foliage height, since the decline of the trees towards the edges of the crown is not taken into account. Higher foliage height generally causes excessive shade and overestimates wind reduction.

To address this, we created an approximate foliage height GeoTIFF by rasterizing the tree height polygons and subtracting the surface and terrain heights.

Overview of the foliage height in Wageningen
Zoomed in on the foliage height in Wageningen, east-south-east corner of the overview

However, this GeoTIFF may introduce excessive height variations among trees, overestimating the frontal foliage surface. More research has to be done to find out what effect this variation of foliage height has on wind reduction.

Vegetation fraction spatial variation

The vegetation fraction in the PET Wageningen result has been calculated using the RGB and Infra-red imagery data. In the Tygron engine the vegetation fraction is determined by the buildings , their function and terrains in case no buildings are present.