Srtmtag

srtmtag -h

srtmtag [-t <taglist>] <srtmfile> [<extentfile>]
modify srtm-files by add/subtract multiples of 10000m to store attributes
-t <taglist>           manage layers 1(urban), 2(wood), 3(water), 5(?), 6(?) -t 123
                       4(original), no tags: inspect srtm file only
<srtmfile>             3601x3601 srtm1 or 1201x1201 srtm3 file
<extentfile>           3601x3601 png image with tag colours
                       no file: delete tags in srtmfile if in taglist