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# GDAL FAQ

1. What's this OGR Stuff?

   The gdal/ogr tree holds source for a vector IO library inspired by OpenGIS Simple Features. In theory it is separate from GDAL, but currently they reside in the same source tree and are somewhat entangled. More information can be found at http://ogr.maptools.org. It is my plan to properly fold OGR into GDAL properly at some point in the future. Then GDAL will be a raster and vector library.

2. How do I add support for a new format?

   To some extent this is now covered by the GDAL Driver Implementation Tutorial.

3. Can I get a MS Visual Studio Project file for GDAL?

   The GDAL developers find it more convenient to build with makefiles and the Visual Studio NMAKE utility. Maintaining a parallel set of project files for GDAL is too much work, so there are no project files directly available from the maintainers. Occasionally other users do prepare such project files, and you may be able to get them by asking on the gdal-dev list. However, I would strongly suggest you just use the NMAKE based build system. With debugging enabled you can still debug into GDAL with Visual Studio.

4. Can I build GDAL with MS Visual C++ 2005 Express Edition?

   Yes, you can. It's also possible to use GDAL libraries in applications developed using MS Visual C++ 2005 Express Edition.

   - Download and install Visual C++ 2005 Express Edition. Follow instructions presented on this website:

     http://msdn.microsoft.com/vstudio/express/visualc/download/

   - Download and install Microsoft Platform SDK. Also, follow these instructions carefully without omitting any of steps presented there:

     http://msdn.microsoft.com/vstudio/express/visualc/usingpsdk/

   - Add following two paths to Include files in the Visual C++ IDE settings. Do it the same way as presented in Step 3 from the website above.

         C:\Program Files\Microsoft Platform SDK\Include\atl
         C:\Program Files\Microsoft Platform SDK\Include\mfc

   - Since you will build GDAL from command line using nmake tool, you also need to set or update INCLUDE and LIB environment variables manually. You can do it in two ways:

     1. using the System applet available in the Control Panel
     2. by editing vsvars32.bat script located in

            C:\Program Files\Microsoft Visual Studio 8\Common7\Tools\vsvars32.bat

     These variables should have following values assigned:

         INCLUDE=C:\Program Files\Microsoft Visual Studio 8\VC\Include;
                 C:\Program Files\Microsoft Platform SDK\Include;
                 C:\Program Files\Microsoft Platform SDK\Include\mfc;
                 C:\Program Files\Microsoft Platform SDK\Include\atl;INCLUDE%

         LIB=C:\Program Files\Microsoft Visual Studio 8\VC\Lib;
             C:\Program Files\Microsoft Visual Studio 8\SDK\v2.0\lib;
             C:\Program Files\Microsoft Platform SDK\lib;LIB%

     NOTE: If you have edited system-wide INCLUDE and LIB variables, using System applet, every Console (cmd.exe) will have it properly set. But if you have edited them through vsvars32.bat script, you will need to run this script in the Console before every compilation.

   - Patch atlwin.h header

     At line 1725 add int i; declaration, so it looks as follows:

         ---------------------------------------------------------------
         BOOL SetChainEntry(DWORD dwChainID, CMessageMap* pObject, DWORD dwMsgMapID = 0)
         {
             int i;
             // first search for an existing entry

             for(i = 0; i < m_aChainEntry.GetSize(); i++)
         ---------------------------------------------------------------

   - Patch atlbase.h header

     At line 287, comment AllocStdCallThunk and FreeStdCallThunk functions and add macros replacements:

         ---------------------------------------------------------------
         /***************************************************
         PVOID __stdcall __AllocStdCallThunk(VOID);
         VOID __stdcall __FreeStdCallThunk(PVOID);

         define AllocStdCallThunk() __AllocStdCallThunk()
         define FreeStdCallThunk(p) __FreeStdCallThunk(p)

         pragma comment(lib, "atlthunk.lib")
         ***************************************************/

         /* NEW MACROS */
         define AllocStdCallThunk()
         define FreeStdCallThunk(p)
         ---------------------------------------------------------------

   - Building GDAL

     - Open console windows (Start -> Run -> cmd.exe -> OK)

     - If you have edited vsvars32.bat script, you need to run it using full path:

           C:\> "C:\\Program Files\\Microsoft Visual Studio 8\\Common7\\Tools\\vsvars32.bat"
           Setting environment for using Microsoft Visual Studio 2005 x86 tools

     - Go do GDAL sources root directory, for example:

           C:\> cd work\gdal

     - Run nmake to compile

           C:\work\gdal> nmake /f makefile.vc

     - If no errors occur, after a few minutes you should see GDAL libraries in C:\work\gdal.

   Now, you can use these libraries in your applications developed using Visual C++ 2005 Express Edition.

5. Can I build GDAL with Cygwin or MingW?

   GDAL should build with Cygwin using the Unix-like style build methodology. It is also possible to build with MingW though there are some complications. The following might work:

       ./configure --prefix=$PATH_TO_MINGW_ROOT --host=mingw32
       	--without-libtool --without-python $YOUR_CONFIG_OPTIONS

   Using external win32 libraries will often be problematic with either of these environments - at the least requiring some manual hacking of the GDALmake.opt file.

6. Can I build GDAL with Borland C or other C compilers?

   These are not supported compilers for GDAL; however, GDAL is mostly pretty generic, so if you are willing to take on the onerous task of building an appropriate makefile / project file it should be possible. You will find most portability issues in the gdal/port/cpl_port.h file, and you will need to prepare a gdal/port/cpl_config.h file appropriate to your platform. Using cpl_config.h.vc as a guide may be useful.

7. What exactly was the license terms for GDAL?

   The following terms are the same as X windows is released under, and is generally known as the "MIT License". It is intended to give you permission to do whatever you want with the GDAL source, including building proprietary commercial software, without further permission from me, or requirement to distribute your source code. A few portions of GDAL under under slightly different terms. For instance the libpng, libjpeg, libtiff, and libgeotiff license terms may vary slightly though I don't think any of them differ in any significant way. Some external libraries which can be optionally used by GDAL are under radically different licenses.

        Copyright (c) 2000, Frank Warmerdam

        Permission is hereby granted, free of charge, to any person obtaining a
        copy of this software and associated documentation files (the "Software"),
        to deal in the Software without restriction, including without limitation
        the rights to use, copy, modify, merge, publish, distribute, sublicense,
        and/or sell copies of the Software, and to permit persons to whom the
        Software is furnished to do so, subject to the following conditions:

        The above copyright notice and this permission notice shall be included
        in all copies or substantial portions of the Software.

        THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
        OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
        FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
        THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
        LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
        FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
        DEALINGS IN THE SOFTWARE.

8. 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.

9. Can I reproject rasters with GDAL?

   Yes, you can use the gdalwarp utility program or programmatically use the GDALWarpOperation class described in the Warp API Tutorial.

10. Why won't gdalwarp or gdal_merge write to most formats?

    GDAL supports many raster formats for reading, but significantly less formats for writing. Of the ones supported for writing most are only supported in create copy mode. Essentially this means they have to be written sequentially from a provided input copy of the image to be written. Programs like gdal_merge.py or gdalwarp that write chunks of imagery non-sequentially cannot easily write to these sequential write formats. Generally speaking formats that are compressed, such as PNG, JPEG and GIF are sequential write. Also some formats require information such as the coordinate system and color table to be known at creation time and so these are also sequential write formats.

    When you encounter this problem it is generally a good idea to first write the result to GeoTIFF format, and then translate to the desired target format.

    To determine which formats support which capabilities, use the --formats switch with pretty much any GDAL utility. Each driver will include either rw (read-only), rw (read or sequential write) or rw+ (read, sequential write or random write).

11. 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.

12. 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

13. 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.

14. 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 here: Allocating and freeing memory across module boundaries.

$Id: faq.dox,v 1.12 2006/10/17 22:30:22 mloskot Exp $

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