[GIS] Clipping raster by mask layer polygon produces Nodata values in QGIS

clipdemgdalqgisraster

I have a raster .tif Digital Elevation Model I am trying to clip to an irregular polygon using the GDAL\Clip raster by mask layer. I ticked 'Crop the extent of target dataset to extent of cutline'.

However, the output raster has the grid cells outside the mask layer cutline converted to 'Nodata' (apart from being annoying, it also retains the large file size of the source raster). I tried setting the destination nodata value to 'None', but this resulted in values of zero instead. (Please note that I do not know how to use the Commander/Python.) Any advice please?

processing.runalg("gdalogr:cliprasterbymasklayer","Input Raster", "Clip layer.shp",
                  "",False,True,False,5,4,75,6,1,False,0,False,"","Output raster")

Best Answer

This is normal behaviour, since raster data are matrices defined by extent (rectangle by default) and cell size- resolution (each cell have same size) or number of columns and rows in extent.

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[GIS] Mask pixel values in Raster image QGIS

I think the proper way to extract only the vegetation/cropland cover is to do the following:

Create a land use/cover map by classifying the image first using supervised/unsupervised classification technique of your choice;
Convert the final land use/cover map from raster to vector
Select the class that represents the vegetation/cropland cover and save it to a new shapefile.
If you want the output as raster image to keep only the vegetation/cropland cover and mask other classes, you can use the following expression in QGIS Raster Calculator:
```
("ImageName@1" = PixelValueOfCrop) *"ImageName@1"
```

[GIS] Extracting raster by mask outputs raster with extent beyond the mask layer’s in QGIS

The raster is clipped exactly to the shapefile boundaries.

But remember, a raster is a kind of matrix. It's composed by n rows and m columns, so the extent of it is rectangular.

The black area are pixels without values, so you only have valid pixels in the extent of the shapefile.

If you want to hide null pixels, check this answer How to display NoData of a raster in QGIS.

What have you done is clip out some values. The result is the desired but the meaning is different:

What have you done:

In this case, NULL pixels are defined by a value equals to 255. You can clip out those pixels setting a max of 254 and clipping out the remains values:

Sure work, even you can set two boundaries to remove some values of the render (min and max). But QGIS has a dedicated section to remove NULL or No Data values. Gives you more control about how to handle these values:

Best Answer

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[GIS] Mask pixel values in Raster image QGIS

[GIS] Extracting raster by mask outputs raster with extent beyond the mask layer’s in QGIS

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