[GIS] How to exclude missing values from raster layer

qgisraster

In my raster layers the value -999 means "missing". Those points completely mess up any colormap in qgis because qgis treats them as normal values and includes them in the classification.

I would like to ask

1) is there any raster format that allows for missing values ? (I mostly use geotiff)

2) how can I replace -999 with for example 0 in every raster-cell? QGIS raster-calculator does not seem to do this. I did not find anything like an inline if-function like if(val=-999,0,val)

Best Answer

Two options are available

You can either set values to zero '999 -> 0' or remove them entirely from the raster '999 -> nodata'.

Set to zero '999 -> 0' with the raster calculator (Raster > Raster Calculator):

("my_rasterA@1" > -999) * "my_rasterA@1"
Set to NA '999 -> nodata', use Translate (Raster > Conversion > Translate) Save the output from the previous step with the raster calculator and use it as the input for this step.

gdal_translate -a_nodata 0 -of GTiff "C:/Users/Desktop/my_rasterA.tif" "C:/Users/Desktop/my_output.tif"

*** note: when using translate be sure to add the -a_nodata 0 in the little editing window at the bottom & click ok while you are still in editing mode.

Related Solutions

[GIS] Raster diff: how to check if images have identical values

Try converting your rasters to numpy arrays and then check to see if they have the same shape and elements with array_equal. If they are the same, the result should be True:

ArcGIS:

import arcpy, numpy

raster1 = r'C:\path\to\raster.tif'
raster2 = r'C:\path\to\raster.tif'

r1 = arcpy.RasterToNumPyArray(raster1)
r2 = arcpy.RasterToNumPyArray(raster2)

d = numpy.array_equal(r1,r2)

if d == False:
    print "They differ"

else:
    print "They are the same"

GDAL:

import numpy
from osgeo import gdal        
    
raster1 = r'C:\path\to\raster.tif'
raster2 = r'C:\path\to\raster.tif'

ds1 = gdal.Open(raster1)
ds2 = gdal.Open(raster2)

r1 = numpy.array(ds1.ReadAsArray())
r2 = numpy.array(ds2.ReadAsArray())

d = numpy.array_equal(r1,r2)

if d == False:
    print "They differ"

else:
    print "They are the same"

[GIS] Cell grid not aligning in QGIS

When you reproject in QGIS, it is using something called gdalwarp under the hood. Try explicitly setting the -te (target extent) and -tr (target resolution) options explicitly to exactly match the raster you want to align to. These values are implicit if unspecified. To coerce conformity, you may also need to use the -tap (target aligned pixels) option, but depending on the alignment of your comparison (unprojected) raster, you may also have to warp that one in the same projection but with -tap on (ie warp both using -tap).

To do this, set up your warp command in the usual way and then click the yellow pencil button by the text box at the bottom of the dialog. You can now edit the gdalwarp command following the documentation.

Best Answer

Two options are available

Related Solutions

[GIS] Raster diff: how to check if images have identical values

[GIS] Cell grid not aligning in QGIS

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