Raster – Using Map Algebra in Grass GIS to Merge Two Rastras with NoData Value

grassmap-algebrarasterraster-calculator

I have a slope raster and a road raster. The black pixels in the road raster are NoData values and the pink pixels have a value of 1. They don't cover exactly the same area.
enter image description here

I want to create a new raster based on the slope raster, except for pixels where there is a road pixel. If there is a road pixel, I want to assign a value of zero to the slope raster.

I am not quite sure how the formula needs to look like.
I tried the following, put it didn't give me the expected results:

r.mapcalc "output = ((Slope+(Road*null())))"

Any ideas?

Best Answer

Use this r.mapcalc expression:

r.mapcalc "output = not(if(Road))*Slope"

because:

if(Road)is equal to 1 if Road not zero, 0 otherwise.
not(if(Road))is equal to 0 if Road not zero, 1 otherwise.

Related Solutions

[GIS] Change NODATA pixel value in raster to arbitrary value

You can use the r.null.to function via GRASS to transform all NULL value cells into a user-defined value, which in your case is 800:

r.null.to

Note: You will have to create a GRASS Mapset before you can use r.null.to as it is not available in the Processing Toolbox. Once you created a mapset, use the GRASS option to add a raster layer, select Open GRASS Tools and search the Module list:

GRASS Tools

Module list

[GIS] Merge raster layers with NoData cells

One option to achieve this is using gdal_merge.py. But unfortunately, this tool stops my automated script with a "Python.exe has stopped working" error, as others complained here, here and here.

Another option, which actually works for me, is using SAGA's tool Mosaic raster layers, which doesn't generate any errors:

#prepare QGIS environment
import sys
from qgis.core import *
from PyQt4.QtGui import *
app = QApplication([])
QgsApplication.setPrefixPath("E:\\QGIS\\apps\\qgis", True)
QgsApplication.initQgis()

#prepare processing framework 
sys.path.append('E:\\QGIS\\apps\\qgis\\python\\plugins')
from processing.core.Processing import Processing
Processing.initialize()
import processing
from processing.tools import *

#set variables
algorithm = "saga:mosaickrasterlayers"
inputMain = path + "Flood_main.tif;"
input2D = path + "Flood_cvasi2D.tif"
addInput = None
dataType = 7
interpolation = 0
overlapAreas = 1
blendDistance = 10
match = 0
fit = 0
extent = "612550.33,660022.33,679329.28,703773.28"
result = path + "Flood.tif"

#run algorithm
processing.runalg(algorithm,inputMain + input2D, addInput, dataType, interpolation,
                  overlapAreas, blendDistance, match, fit, extent, result)

#exit QGIS
QgsApplication.exitQgis()
QApplication.exit()

QGIS version: 2.14.3-Essen

Best Answer

Related Solutions

[GIS] Change NODATA pixel value in raster to arbitrary value

[GIS] Merge raster layers with NoData cells

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