CDSM extraction from DSM

[AlexKDo]

Hello. I would like to calculate the outdoor thermal comfort for a specific area in Athens, Greece. I have the DSM and DEM files, but not the CDSM. In the DSM file that I have, the trees and buildings are mixed. Is there a way that I can somehow create a CDSM file from that? Thank you in advance

[biglimp]

Yes, but you need a vector polygon footprint layer of all buildings. If you dont have that you could probably get one from Open Street Map. Then you du the following:

1. Subtract your DEM from your DSM to get object above ground
2. Buffer your building footprint with e.g. 2 meters
3. Add an attribute with zero values to your buffered layer
4. Burn these buffered polygons with zero value into your raster created in 1 and voila, you now have a CDSM.

However, you also need to clean your original DSM, i.e. remove trees. This can be done by creating an “inverted building footprint” and then do the process above again. A final step includes adding your DEM since your DSM should be meters above sea level.

I recommend QGIS for doing all these steps and let us know if you are having problems.

[AlexKDo]

Hello.. Thank you for your answer. I did all the steps you told me and i have my CDSM.

I iam trying to do the opposite, but somehow it seems impossible. I tried many different ways. but step 1 for DSM cleaning, after i do all the steps doesnt change a bit. U thing it has to do with the attribute table?

[biglimp]

Could be. Also check your CRS of your inverted polgonlayer.

[AlexKDo]

CRS are correct. So in my inverted polygonlayer, should I add an attribute field with zero values again? In the first try I added an attribute field like this.

In the invered polygonlayer, i created a new polygon in the same area, without the buildings and I added 0 value only in the new polygon - the other ones in the dame filed remain NULL. I also used the landuse polygonlayer - clipped it with inverted buffer buildings shp- did the same procedure as your example but still nothing changed in the above ground tiff..
Attribute table for this one is here:

[AlexKDo]

I finally did it. I created a new project and did everything again and somehow it happened! Thank you

[AlexKDo]

Alo again.. So I created CDSM and separated trees from DSM and I now have 2 files.
CDSM looks likes this.. I buffered the buildings and then I tried to make it better using the DSM:

But I am trying to run eg SVF and this is the result I get. 1 one only with DSM and 2 with DSM & CDSM.
1.

Also.. in my try to create Daily Shadow pattern, the only DSM output was ok but then again..

You think it might be a solution to create a CDSM with Tree Generator, using my clean DSM file? I am experimenting with a small square with not that muvh vegetation around.

[AlexKDo]

sorry, CDSM without DSM background looks like this

[biglimp]

All pixels tjat are not trees in CDSM should be 0, not NoData or very small numbers which happens some time when you have a sperarate DEM and DSM.

[AlexKDo]

I did it that way.. You think when I added the new file+ DEM file something changed? Because everything is 0, except from trees but it doesnt look like this when I did SVF and Daily Shadow. I painted 0 with blue so that you can see

[AlexKDo]

So apparently 0= no data value. How did this happen?

[biglimp]

Attach your dsm and cdsm and I can have a look.

[AlexKDo]

I created a respiratory, I am really new here so if there is any problem tell me please. Thank you
https://github.com/AlexKDo/project-files.git

[AlexKDo]

Problem solved. I didnt have to add my created CDSM to the DEM file. thank you for your help!