[Qgis-user] Edit raster (DEM) values with polygon

[Richard McDonnell]

Hi,
I would be open to correction on this, (and I have not tested it) but to my mind it’s a simple raster calculator exercise, assuming the “Burn raster” has the same resolution and pixel/grid alignment as the DEM.
I would suggest, as you have the new elevations (Burn Raster), that you create a depth/difference raster based on the “Burn raster” by first subtracting the DEM from it.
This should then give you an elevation change raster, which you should be able to add to the DEM.
The QGIS Raster Calculator can be used for this, you can also use the Processing toolbox to carry out the steps one after the other without generating the intermediate datasets.
Regards,

Richard.

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From: Qgis-user <qgis-user-bounces at lists.osgeo.org> On Behalf Of Doug Johnston
Sent: 26 March 2019 20:21
To: qgis-user at lists.osgeo.org
Subject: [Qgis-user] Edit raster (DEM) values with polygon

Hello all.

I am attempting to edit (change) the values for all pixels in a DEM in region of the DEM.

The real-world problem is I want to burn in a proposed flood wall/dike into an existing DEM.

I have a polygon with the desired elevation field, and I also have a raster with the desired elevation values (rasterized from the polygon).

I have looked at rasterize tool, however cannot find the correct way burn the overlying polygon into the dem (referencing an attribute field), or if it is even possible.

I would also think raster calculator would do it with the two rasters, but I do not have the syntax down:

pseudo-code: if dike-dem is_not "no_data" |> use dike-dem; else use original-DEM.

Here is a screen shot of the two rasters.

All help is appreciated; thanks in advance.

Doug Johnston

OPW - Ag féachaint don am atá le teacht - Ag caomhnú ón am atá thart
OPW - Looking to the future - Caring for the past

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