# How to calculate average of pixel values around a coordinate?

I have tried to figure this one out but so far without much success. I need to calculate the average of the pixel values within a distance from a certain coordinate. To be able to script this I need to be able to give a coordinate and a radius. Does anyone have any ideas?

Do you just need this for a list of coordinates or do you want a lowpass filtered image? Different solutions I think. If I had to do this in GMT I would

1. Convert my image to red, green, blue grids via grdmix
2. Filter each component grid with a boxcar filter of desired diameter
3. Either convert the filtered grids back to an image via grdmix or sample the three grids at your desired coordinates.

There may be simpler non-GMT solutions depending on the initial question.

I need to do this for a single coordinate and a given radius. The image(es) has just one single band and I need to return the average value for the pixels within the radius for further processing. I can sample one single pixel with grdtrack but instead of a single pixel I need to either get the values for all pixels within the given radius or the average of the pixels within the radius.

I assume we are talking very local calculations here (not average over a 4000 km circle). Maybe this scheme would work:

1. Given your lon,lat, radius, use grdcut -Slon/lat/radius+n to extract a grid which only has values inside the circle and NaN outside.
2. Use grdinfo -L2 on that grid to report the mean value

May not be fast but may not matter to you.

Yes, the calculations are local 0.5 to 1 km or so. I tried grdcut but it is unusably slow. I have ~100 grd files I need to sample in a specific location but grdcut is taking 10+ minutes per file when using -Slon/lat/radius+n

Probably you can then do a rectangular crude grdcut first to only get a tiny rectangular grid that is ensured to contain the circle, then run the slow part on that small grid. E.g., compute your w/e/s/n for the rectangular subset using Lon and lat +/- radius (in degrees if geographic files) plus 10% padding to be sure.