How to exclude missing values from raster layer?

Question

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)

Answer 1

Two options are available

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

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

   ("my_rasterA@1" > -999) * "my_rasterA@1"

2. 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.

Answer 2

If you are only creating a map you can hide these values in QGIS by going to your layer properties --> transparency and then selecting the values you want to hide.

You can also use gdalwarp to replace the missing values as such:

gdalwarp -srcnodata -999 -dstnodata 0 source.tif output.tif

Answer 3

1. GeoTIFF with NODATA value set to your liking. If set correctly, QGIS should ignore them in colormaps.
2. eq( [input]@1, -999, 0 ) NOTE: this is a command for RasterCalc plugin (available via plugin installer, also needs pyparsing), this is not for embeded Raster Calculator.
3. Why not ASCII GRID for doing change you need? Raster\Convertion\Translate, choose output filename with type Arc/Info ASCII GRID You can also try: gdal2xyz