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Diffstat (limited to 'docs/source/faq.rst')
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1 files changed, 20 insertions, 20 deletions
diff --git a/docs/source/faq.rst b/docs/source/faq.rst index f72ec132..21fe158b 100644 --- a/docs/source/faq.rst +++ b/docs/source/faq.rst @@ -17,7 +17,7 @@ Where can I find the list of projections and their arguments? :doc:`Here<../operations/projections/index>`. -Additinoally, the ``proj`` command itself can report the list of projections +Additionally, the :program:`proj` command itself can report the list of projections using the ``-lp`` option, the list of ellipsoids with the ``-le`` option, the list of units with the ``-lu`` option, and the list of built-in datums with the ``-ld`` option. @@ -25,7 +25,7 @@ the ``-ld`` option. How do I do datum shifts between NAD27 and NAD83? -------------------------------------------------------------------------------- -Using the ``cs2cs`` application. The following example demonstrates using the +Using the :program:`cs2cs` application. The following example demonstrates using the default shift parameters for NAD27 to NAD83: :: @@ -46,23 +46,23 @@ How do I build/configure PROJ to support datum shifting? -------------------------------------------------------------------------------- After downloading and unpacking the PROJ source, also download and unpack the -set of datum shift files. See :ref:`download` for instructions how to fetch -and install these files +set of datum shift files. See :ref:`download` for instructions how to fetch +and install these files. A default build and install on Unix will normally build knowledge of the directory where the grid shift files are installed into the PROJ library (usually ``/usr/local/share/proj``). On Windows the library is normally built thinking that C:\PROJ\NAD is the installed directory for the grid shift files. -If the built in concept of the PROJ data directory is incorrect, the ``PROJ_LIB`` -environment can be defined with the correct directory. +If the built-in concept of the PROJ data directory is incorrect, the :envvar:`PROJ_LIB` +environment variable can be defined with the correct directory. How do I debug problems with NAD27/NAD83 datum shifting? -------------------------------------------------------------------------------- -1. Verify that you have the binary files (eg. ``/usr/local/share/proj/conus``) +1. Verify that you have the binary files (e.g. ``/usr/local/share/proj/conus``) installed on your system. If not, see the previous question. 2. Try a datum shifting operation in relative isolation, such as with the :program:`cs2cs` - command listed above. Do you get reasonable results? If not it is likely + command listed above. Do you get reasonable results? If not, it is likely that the grid shift files aren't being found. Perhaps you need to define :envvar:`PROJ_LIB`? 3. The :program:`cs2cs` command and the underlying :c:func:`pj_transform()` API know how to do a @@ -91,16 +91,16 @@ How do I debug problems with NAD27/NAD83 datum shifting? ``PROJ_DEBUG`` support is not yet very mature in the PROJ library. 5. The :option:`-v` flag to :program:`cs2cs` can be useful in establishing more detail on what - parameters being used internally for a coordinate system. This will include + parameters are used internally for a coordinate system. This will include expanding the definition of ``+datum`` clause. How do I use EPSG coordinate system codes with PROJ? -------------------------------------------------------------------------------- There is somewhat imperfect translation between 2D geographic and projected -coordinate system codes and PROJ descriptions of the coordinate system +coordinate system codes and PROJ descriptions of the coordinate systems available in the ``epsg`` definition file that normally lives in the ``proj/data`` -directory. If installed (it is installed by default on Unix), it is possible +directory. If installed (as is the default on Unix), it is possible to use EPSG numbers like this: :: @@ -120,10 +120,10 @@ to use EPSG numbers like this: # # +proj=latlong +datum=NAD27 +ellps=clrk66 +nadgrids=conus,ntv1_can.dat -The ``proj/data/epsg`` file can be browsed and searched in a text editor for -coordinate systems. There are known to be problems with some coordinate -systems, and any coordinate systems with odd axes, a non-greenwich prime -meridian or other quirkiness are unlikely to work properly. Caveat Emptor! +The ``proj/data/epsg`` file can be browsed and searched for coordinate systems +using a text editor. There are known to be problems with some coordinate +systems. In particular, any coordinate system with odd axes, a non-greenwich prime +meridian, or other quirky characteristics is unlikely to work properly. Caveat Emptor! How do I use 3 parameter and 7 parameter datum shifting -------------------------------------------------------------------------------- @@ -144,7 +144,7 @@ formatting of numbers, altering the role of commas and periods in numbers, then PROJ 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 :envvar:`LC_NUMERIC` environment variable to C. +default numeric locale by setting the :envvar:`LC_NUMERIC` environment variable to ``C``. :: @@ -160,7 +160,7 @@ default numeric locale by setting the :envvar:`LC_NUMERIC` environment variable Changing Ellipsoid / Why can't I convert from WGS84 to Google Earth / Virtual Globe Mercator? ---------------------------------------------------------------------------------------------- -The coordinate system definition for Google Earth, and Virtual Globe Mercator +The coordinate system definition for Google Earth and Virtual Globe Mercator is as follows, which uses a sphere as the earth model for the Mercator projection. @@ -177,8 +177,8 @@ But, if you do something like: +to +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +no_defs -to convert between WGS84 and mercator on the sphere there will be substantial -shifts in the Y mercator coordinates. This is because internally :program:`cs2cs` is +to convert between WGS84 and Mercator on the sphere there will be substantial +shifts in the Y Mercator coordinates. This is because internally :program:`cs2cs` is having to adjust the lat/long coordinates from being on the sphere to being on the WGS84 datum which has a quite differently shaped ellipsoid. @@ -200,7 +200,7 @@ Note the strategic addition of ``+nadgrids=@null`` to the spherical projection definition. Similar issues apply with many other datasets distributed with projections -based on a spherical earth model - such as many NASA datasets. This coordinate +based on a spherical earth model, such as many NASA datasets. This coordinate system is now known by the EPSG code 3857 and has in the past been known as EPSG:3785 and EPSG:900913. When using this coordinate system with GDAL/OGR it is helpful to include the +wktext so the exact PROJ string will be preserved |