Mapping Hacks

Add Relief to Your Topographic Maps #72

Chapter 6, Mapping on Your Desktop

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345

HACK

binaries from http://gdal.maptools.org/. The output from gdalinfo looks like

this:

$ gdalinfo sf_north.dem

Driver: USGSDEM/USGS Optional ASCII DEM

Size is 1109, 1395

Coordinate System is:

PROJCS["UTM Zone 10, Northern Hemisphere",

GEOGCS["NAD27",

DATUM["North_American_Datum_1927",

SPHEROID["Clarke 1866",6378206.4,294.978698213898,

AUTHORITY["EPSG","7008"]],

TOWGS84[-3,142,183,0,0,0,0],

AUTHORITY["EPSG","6267"]],

PRIMEM["Greenwich",0,

AUTHORITY["EPSG","8901"]],

UNIT["degree",0.0174532925199433,

AUTHORITY["EPSG","9108"]],

PROJECTION["Transverse_Mercator"],

PARAMETER["latitude_of_origin",0],

PARAMETER["central_meridian",-123],

That’s quite a bit of metadata, but it tells us a few things we need to know in

order to create a new location in GRASS. As usual, a lot of the USGS’s freely

available topographic data is stored in UTM coordinates, referenced to the

NAD27 geodetic datum. (NAD27 has since been superseded by NAD83, but

the USGS still has most of its data kicking about in NAD27.)

If you’re outside the Bay Area, you might try getting National

Elevation Data (NED) and other base maps from the USGS

Seamless Data Distribution System

[Hack #67]. Obviously, the

smaller the resolution you can get, the better. For areas out-

side the U.S., you might try NASA’s Shuttle Radar

Topography Mission (SRTM) data, available from