|Version 16 (modified by mloskot, 6 years ago)|
Coordinate Systems and Projections
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?
Is the GDAL library thread-safe?
No. GDAL is not completely thread safe.
However for GDAL 1.3.0 much work has been done on making some common scenarios thread safe. In particular for the situation where many threads are reading from GDAL datasets at once should work as long as no two threads access the same GDALDataset object at the same time. However, in this scenario, no threads can be writing to GDAL while others are reading or chaos may ensue.
Also, while the GDAL core infrastructure is now thread-safe for this specific case, only a few drivers have been vetted to be thread safe.
It is intended that work will continue on improving GDAL's thread safety in future versions.
Does GDAL work in different international numeric locales?
No. GDAL makes extensive use of sprintf() and atof() internally to translate numeric values. If a locale is in effect that modifies formatting of numbers, altering the role of commas and periods in numbers, then PROJ.4 will not work. This problem is common in some European locales.
On Unix-like platforms, this problem can be avoided by forcing the use of the default numeric locale by setting the LC_NUMERIC environment variable to C, e.g.
$ export LC_NUMERIC=C $ gdalinfo abc.tif
How do I debug GDAL?
Various helpful debugging information will be produced by GDAL and OGR if the CPL_DEBUG environment variable is set to the value ON. Review the documentation for the CPLDebug() function for more information on built-in debugging messages.
On Unix operating systems GDAL can be built with the CFG environment variable set to debug to enable debugger support with the -g compiler switch.
On Windows edit the nmake.opt and ensure /Zi appears in the OPTFLAGS variable.
How should I deallocate resources acquainted from GDAL on Windows?
The safest way to release resources allocated and returned (with ownership transfered to caller) from GDAL library is to use dedicated deallocator function. Deallocators promise to release resources on the right module side, without crossing modules boundaries what usually causes memory access violation errors.
- Example of correct resource deallocation:
OGRDataSource* poDS = NULL; // OGRDataSource aquisition made on side of the GDAL module poDS = OGRSFDriverRegistrar::Open( "point.shp", FALSE ); // ... // Properly resource release using deallocator function OGRDataSource::DestroyDataSource( poDS );
* Example of incorrect resource deallocation:
OGRDataSource* poDS = NULL; // OGRDataSource aquisition made on side of the GDAL module poDS = OGRSFDriverRegistrar::Open( "point.shp", FALSE ); // ... // Deallocation across modules boundaries. // Here, the deallocation crosses GDAL DLL library and client's module (ie. executable module) delete poDS;
More detailed explanation of the problem can be found in the Allocating and freeing memory across module boundaries article.