How do I swap (reverse) 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

    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).

Note: This only works for regular lat/lon grids and regular Gaussian grids. It is not to be used for reduced grids, 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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