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FAQ - Coordinate Systems and Projections
- What are Well Known Text projections, and how do I use them?
- Can I reproject rasters with GDAL?
- Why doesn't GDAL automatically choose the datum transformation?
What are Well Known Text projections, and how do I use them?
OpenGIS Well Known Text is a textual format for defining coordinate systems. It is loosely based on the EPSG coordinate systems model. While GDAL itself just passes these definitions around as text strings, there is also an OGRSpatialReference class in gdal/ogr for manipulating them and a linkage to PROJ.4 for transforming between coordinate systems. The OGRSpatialReference, and PROJ.4 linkaged (but not PROJ.4 itself) is linked into the GDAL shared library by default. More documentation on WKT and OGRSpatialReference can be found in the OGR Projections Tutorial.
Can I reproject rasters with GDAL?
Yes, you can use the gdalwarp utility program or programmatically use the GDALWarpOperation class described in the GDAL Warp API Tutorial.
Why doesn't GDAL automatically choose the datum transformation?
There is no such thing as an accurate "default" set of datum transformation parameters for a datum. OGR uses (NADCON) by default, which is the most accurate available for North America, but in the general case (worldwide) this is very hard to determine and there normally is no such thing as a default. The transformation that should be used depends on the exact area covered, accuracy required etc. In other words, user beware and do your homework. 1, 2
