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I have a grid of data that is in a gnomonic projection, centered on a lat/long point (WGS-84). The grid has a 1km spacing. Given this I can easily obtain the x,y coordinates/distances from centre point where the latitude and longitude is known.

What is the best way to obtain the lat/long for any arbitrary datum in this grid (ie, how can I create a grid of lat/long pairs)?

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You can set up a custom projection in proj4 for a gnomonic projection on every point of the world:

+proj=gnom +lat_0=51 +lon_0=7 +x_0=0 +y_0=0 +R=6371000 +towgs84=0,0,0,0,0,0,0

You can reproject Natural Earth world boundary data to this CRS using QGIS, and create a regular vector grid on it:

enter image description here

After densifying and reprojecting to Pseudo Mercator, the grid gets bended:

enter image description here

You can as well create a WGS84 degree grid, and reproject it to gnomonic:

enter image description here

For single coordinates, you can use GDAL cs2cs to convert the data from and to WGS84:

cs2cs +init=epsg:4326 +to +proj=gnom +lat_0=51 +lon_0=7 +x_0=0 +y_0=0 +R=6371000 +towgs84=0,0,0,0,0,0,0  -f %%f in.txt >out.txt

cs2cs +proj=gnom +lat_0=51 +lon_0=7 +x_0=0 +y_0=0 +R=6371000 +towgs84=0,0,0,0,0,0,0 +to +init=epsg:4326 -f %%f in.txt >out.txt

Just make sure your data is on the same hemisphere as your origin. The projection is not valid worldwide.

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