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How do I swap (reverse) the scanning order? for example changing north-to-south to south-to-north

Step-by-step guide

Say you have a GRIB file with scanning order of south-to-north (for latitudes). This means the key "jScansPositively" is 1. You want to change this to north-to-south i.e. jScansPositively should become 0

1. Ensure that the grid geometry of your GRIB message specifies both the latitudes: 

   latitudeOfFirstGridPointInDegrees and latitudeOfLastGridPointInDegrees

2. Check what scanning modes you have in your input file:

   Code Block
   % grib_ls -p jScansPositively,iScansPositively input.grib

   
   

3. Use the grib_set command to swap the scanning order for the latitudes:

   Code Block
   % grib_set -s swapScanningY=1 input.grib output.grib

   This changes the order in which the data is stored in the GRIB message

There is also the key "swapScanningX" to do the same task for longitudes (east-to-west changed to west-to-east).

Here is a way of visualising the different scanning orders:

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