Mapping Hacks

322

|

Chapter 6, Mapping on Your Desktop

#68 Convert Geospatial Data Between Different Formats

HACK

read and write a variety of formats; run it without any options to see which

ones your version supports.

Clipping and Warping Raster Data

gdal_translate has some other tricks up its sleeve. For example, if we want to

extract just the portion of the digital elevation model that covers the Pre-

sidio, a national park in the northwest corner of San Francisco, we can use

the “projection window” parameter of gdal_translate to clip out that bit and

save it to a separate file. (Note that, as usual, longitude comes before lati-

tude because it’s the x-coordinate.)

$ gdal_translate -projwin -122.4862 37.8116 -122.4462 37.7860 sf_dem.tif

presidio.tif

Input file size is 570, 416

Computed -srcwin 111 4 144 92 from projected window.

0...10...20...30...40...50...60...70...80...90...100 - done.

If you don’t specify an output format with -of, gdal_trans-

late assumes you want a GeoTIFF by default.

Another common problem you find when working with geodata is that

you’re using one coordinate system or projection in your GIS—say, Univer-

sal Transverse Mercator—while the data you have is in another projection

or coordinate system, such as latitude and longitude. Unfortunately, the sit-

uation is a bit like apples and oranges, so the new data needs to be con-

verted to your existing projection. This process is referred to