Long + Lat Domains

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Permalink Reply by Timothy Logan on November 10, 2015 at 8:30pm

Generally a file, whether it's an HGT, GeoTiff, or IMG file, spans a 1°x1° patch of land.  The name of the file typically indicates the lower left corner, so a file named N46W122.hgt would cover all elevation points from N46-47 latitude (south to north) and W122-121 longitude (west to east).

The domain you feed it needs to be within that domain.  Unless you're trying to get a really large part of land, you would typically not need the entire 1°x1° tile and would choose a subset of it.  Something like a latitude domain of 46.126 To 46.282, and a longitude domain of -121.619 To -121.511 (note western longitudes are negative numbers, southern latitudes are also negative numbers).  That would grab all of the elevation points within essentially a 2D domain and return a topography based on that.

HGT files don't have any header information that stores the location that it represents, only the name of the file and the spec which specifies it's a 1°x1° chunk of land indicates this.  GeoTiff and IMG files both contain header information that tells you the size of the tile, typically slightly larger than 1°x1°, but you can know what the upper/lower ends are.  The latest version of Elk that I released will report what this min/max domain is for GeoTiff and IMG files, but will just try to parse it out of HGT files according to their name.

-Tim