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I'm currently working on a project wherein I am trying to assess the feasibility of creating a DSM from a DEM by adding building footprints to it (LiDAR manipulation is something I'd like to avoid for now). I have a DEM in ESRI Grid format, and another ESRI Grid raster that I created from a building footprint polygon shapefile (set the z-value to 20m for all buildings, then used Polygon to Raster conversion). Both rasters have the same projection and cell size. I would like to take the 20m values for the buildings, and add them on top of the elevation values present in the DEM (vs. simply replacing them). I will be using the resulting DSM for flow path modeling, so I need a continuous raster surface.

I thought to use Raster Calculator to combine the two, but I'm unsure if I am using the correct syntax. Simply trying to add DEM + Footprint Raster didn't work, and doesn't even create a new raster.

This was my syntax

"BldFootClp" + "CityDEMClip"

This is what my layers look like. DEM and Building Footprint Raster

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    From your description, it seems like the adding should have worked. Are both rasters in the same CRS? What does the building footprints raster look like? Do the cells between buildings have a 0 value? – JoshC Mar 21 '18 at 23:47
  • Could you please add the exact syntax you used in the raster calculator? – Aaron Mar 22 '18 at 1:55
  • @JoshC - The rasters are both NAD_1983_HARN_Transverse_Mercator, but the cells between buildings don't have a zero value. Should I somehow reclassify the building footprint raster to have a zero and a 20? – KMatGIS Mar 28 '18 at 20:15
  • What value(s) is stored for the non-building cells in your second layer? – NewAtGis Mar 28 '18 at 20:21
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As some of the comments have been getting at, I would presume that this is an issue of your non-building spaces having a "no data" value. Depending on how your building footprints raster was derived, this can be both intentional and useful to restrict your values in this way, but it's clearly not what you want.

Here's a building footprint raster I created with "no data" values between buildings. I understand this is a screenshot from QGIS, but the underlying issue is common to both programs, and I have done this in both. If I was at my work computer, I'd have an Arc screenshot for you.

no data

When I attempt to add it to the underlying DEM raster, the results are effectively "clipped" to the building shapes.

aftercalc

So, to answer the question in your comment, yes. You'll need to reclassify the nodata values as 0 in your footprints raster, which you can do using either the Raster Calculator or the Reclassify tool. I prefer using the calculator.

On Arc's help site regarding the raster calculator, one of the examples they give is actually just what we want:

Con(IsNull("footprints_raster"), 0, "footprints_raster")

This will convert all the null values to 0, while leaving the non-null values alone. The result should be a continuous raster, which you can then add to your DEM.

The output of the reclassification, in a different color ramp to show that the spaces have a defined value:

reclassed raster

With the newly reclassified raster, try the footprints_raster + DEM operation again. The output should be more useful to you:

added rasters

Let me know if this works for you. If not, I can try to re-work it in Arc when I get a chance.

  • I used reclassify and it worked like a charm. This should work for my flow path modeling beautifully. – KMatGIS Apr 3 '18 at 23:16

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