calculation of vvf, bvf and svf to the south

[Anouk02]

Hello,

I want to derive vegetation and building view factors as well as the sky view factor to the south.

In order to do so I composed with the "intermediate" output such as "svfSaveg,svfSveg, svfS,...".
I used the following equations for the computations:
VVF= (1- SVFveg)+(1-SVFaveg)
BVF=(1- SVFbuilt)-(1-SVFaveg)
SVF_south=SVF_built, South-(1-SVF_veg, south)

I obtained negative values for the svf to the south.

To understand these unlikely values I computed the vertical SVF with the same formulation as the SVF_south in order to compare it with the vertical SVF that qgis gives us. The two layers computed are very similar; the only differences are in areas with trees.

I'm wondering if there is an explanation for these variations and maybe for the negative values.
Does it come from the equation? Do you know if it's the right way to calculate these factors?
Or do you know if something else might be the cause of these difficulties?

Thanks in advance for your help!

[biglimp]

Hm, I wonder if you have the svfaveg and svfveg considered correctly. One of them are inverted (I think it is the svfaveg one). You should have a look in Lindberg and Grimmond 2011 and see how the equations are set up there . I dont have it in my head at the moment.

Lindberg, F. and Grimmond, C. (2011) The influence of vegetation and building morphology on shadow patterns and mean radiant temperature in urban areas: model development and evaluation. Theoretical and Applied Climatology 105:3, 311-323.