Hi, I posted this on the ENVI forum, but have just realised that the HDF-EOS commands are part of standard IDL, so hopefully someone here can help me out! I have been trying to modify a geofield in a MODIS HDF-EOS file with the following code: (infile is a string with the location of the .hdf) nswath = eos_sw_inqswath(infile, swath_list) swath_name = strsplit(swath_list, ',', /extract) fid = eos_sw_open(infile, /rdwr) swath_id = eos_sw_attach(fid, swath_name[0]) geofield_count = eos_sw_inqgeofields(swath_id, geofield_list, geofield_ranks, geofield_types) geofield_names = strsplit(geofield_list, ',', /extract) name=geofield_names[0] print,name result = eos_sw_detach(swath_id) swath_id = eos_sw_attach(fid, swath_name[0]) status = EOS_SW_DEFDIM(swath_id, "GeoTrack", 2000) status = EOS_SW_DEFDIM(swath_id, "GeoXTrack", 2200) status = EOS_SW_WRITEGEOMETA(swath_id,name, "GeoTrack,GeoXtrack", 5) print,status result = eos_sw_detach(swath_id) swath_id = eos_sw_attach(fid, swath_name[0]) geofield_count = eos_sw_inqgeofields(swath_id, geofield_list, geofield_ranks, geofield_types) print,geofield_list result=eos_sw_close(fid) However, I am having a bit of a problem. Rather than overwriting the previous geofield the EOS_SW_WRITEGEOMETA command seems to be creating an entirely new one with the same name. I compared geofield_list pre- and post- calling this command and before I have this output: Latitude, Longitude After calling eos_sw_writegeometa I get this: Latitude, Longitude, Latitude Could someone point me in the direction of what I've done wrong here? I'm guessing I have overlooked something simple! I'm also not sure about the detach and attach commands, I've tried various combinations of positions and numbers of calls, but they don't seem to make much difference. I've left them in the above code, though. Thanks, Simon.