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Script for guessing parameters of cartographic projection

Project description

guessproj is a Python script that calculates unknown parameters of cartographic projection or coordinate system from coordinates of identical points in some known coordinate system and in unknown one. You should know projection type, though. The script can also determine parameters of transformation between two datums.

The script uses pyproj and scipy internally. The method of least squares is used, so the more points you have, the better accuracy will be achieved.

Supported Python versions

Python 2.6+ and 3.3+ are supported. The script is written in pure Python itself but depends on some packages that are not.


The best way to install guessproj is using pip:

pip install guessproj

Be aware that guessproj has some binary dependencies that you need to install before trying to install guessproj. These are GDAL and PROJ.4. If you are a GIS specialist you probably already have these libraries. You also need Python bindings for them (GDAL bindings and pyproj) as well as NumPy and SciPy packages. The pip tool will try to install these packages automatically but on most systems you’ll need to install them in a platform-specific way.

Note that GDAL is an optional dependency but may become required in future versions of guessproj.

Instead of using pip, you can download the source archive, unpack it and run

python install

in the unpacked directory. You need setuptools to do this.

Also, you can use guessproj without installation. Just download the file and run it like any Python script:

python --help

You still need dependencies to be installed, of course.

Input data format

The input data for the script is a text file that contains lines of space separated values, each line representing a point.

Input data format for 2D points is as follows:

x1 y1 x2 y2 point name

Input data format for 3D points is as follows:

x1 y1 z1 x2 y2 z2 point name

Point name may contain spaces but should not start with a number. If the line starts with #, it will be ignored.

The coordinates in known coordinate system go first, followed by coordinates in unknown one. 3D points are mostly useful if there exists a datum transformation between coordinate systems.

Obviously, the number of point coordinate values must be not less than the number of unknown parameters to determine.

Both decimal format and degrees, minutes and seconds format (XXXdXX'XX.XXX") are supported for coordinate values. You can use comma as well as period to separate fractional part.

Command line syntax

Command line syntax used to run the script is similar to that of cs2cs utility which comes with PROJ.4 library. You should specify parameters of known coordinate system in PROJ.4 format (referred as projstring here), known and unknown parameters of the unknown system, and a path to input file. Exact parameters of various coordinate systems you can find in PROJ.4 documentation.

The unknown parameters are specified among all others in the projstring, the only difference is using ~ symbol instead of =. The numeric value that follows the ~ symbol is an initial approximation of the parameter value. For +towgs84 parameter, you can specify ~ before any (or all) of comma-separated values. The combination =~ is the same as ~.


guessproj +proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs +to \
  +proj=tmerc +ellps=krass +lat_0=0 +lon_0~44 +x_0=300000 +y_0~-4.7e6 \
  +towgs84=23.57,-140.95,-79.8,0,-0.35,-0.79,-0.22 points.txt

All that goes before +to argument is a projstring for the known system (if omitted, WGS84 longitude/latitude is used by default). All the rest parameters starting with + are the projstring for the unknown system, where initial approximations of the unknown parameters are marked with ~. In this example, parameters +lon_0 and +y_0 are unknown. The last argument is a name of input text file containing point coordinates.

The script can evaluate numeric parameters only, so you should specify at least +proj and +ellps. It’s worth mentioning, also, that some parameters have the same (or nearly the same) effect, so it’s a bad idea, for example, to specify both +lat_0 and +y_0 as unknown for Transverse Mercator projection. The system of equations will be ill-determined in that case.


Any additional program options (which are not part of projstring syntax) start with - or --.

Option -h or --help prints a short command line reference and exits the program.

Option --encoding=ENCODING_NAME specifies the encoding of input file (--encoding=utf-8 by default).

Option --proj or --proj4 forces output of resulting projstring only, suppressing table of residual errors.

Option --wkt forces output of projection parameters in OGC WKT format (GDAL Python bindings required).

Option --esri forces output of projection parameters in Esri WKT format (experimental, GDAL Python bindings required).

Option --pretty forces pretty WKT formatting when used with --wkt or --esri.


The default output of the program is a projstring in which approximated values of parameters are replaced with the exact values found by the script, and a list of residual errors for each point. Other forms of output can be specified using program options.

If GDAL bindings are installed, the projstring will be formatted so as to be represented in a normalized form.


To run unit tests with Python 2.7 or 3.3+, execute in source directory:

python -m unittest discover test

In Python 2.6, you should install unittest2 package and use:

PYTHONPATH=. unit2 discover test

You can also run scripts from test/ directory directly.

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