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
- Ensure that the grid geometry of your GRIB message specifies both the latitudes:
latitudeOfFirstGridPointInDegrees and latitudeOfLastGridPointInDegrees
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.
Here is a way of visualising the different scanning orders:
