How do I swap the scanning order? for example north to south changed 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. Use the grib_set command to swap the scanning order for the latitudes:

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


There is also the key "swapScanningX" to do the same task for longitudes.

Note: This only works for regular lat/lon grids and Gaussian grids. It is not to be used for Lambert, Polar stereographic, Mercator etc.

Check the resultant file to ensure the data is shown correctly e.g. by plotting it

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