Coordinate transformations are defined by, what in PROJ terminology is known as, "proj-strings". A proj-string describes any transformation regardless of how simple or complicated it might be. The simplest case is projection of geodetic coordinates. This section focuses on the simpler cases and introduces the basic anatomy of the proj-string. The complex cases are discussed in :doc:`transformation`.
A proj-strings holds the parameters of a given coordinate transformation, e.g.
+proj=merc +lat_ts=56.5 +ellps=GRS80
I.e. a proj-string consists of a projection specifier, +proj, a number of
parameters that applies to the projection and, if needed, a description of a
datum shift. In the example above geodetic coordinates are transformed to
projected space with the :doc:`Mercator projection<../operations/projections/merc>` with
the latitude of true scale at 56.5 degrees north on the GRS80 ellipsoid. Every
projection in PROJ is identified by a shorthand such as merc in the above
example.
By using the above projection definition as parameters for the command line
utility proj we can convert the geodetic coordinates to projected space:
$ proj +proj=merc +lat_ts=56.5 +ellps=GRS80
If called as above proj will be in interactive mode, letting you type the
input data manually and getting a response presented on screen. proj
works as any UNIX filter though, which means that you can also pipe data to
the utility, for instance by using the echo command:
$ echo 55.2 12.2 | proj +proj=merc +lat_ts=56.5 +ellps=GRS80 3399483.80 752085.60
PROJ also comes bundled with the cs2cs utility which is used to transform
from one coordinate reference system to another. Say we want to convert
the above Mercator coordinates to UTM, we can do that with cs2cs:
$ echo 3399483.80 752085.60 | cs2cs +proj=merc +lat_ts=56.5 +ellps=GRS80 +to +proj=utm +zone=32 6103992.36 1924052.47 0.00
Notice the +to parameter that separates the source and destination
projection definitions.
If you happen to know the EPSG identifiers for the two coordinates reference
systems you are transforming between you can use those with cs2cs:
$ echo 56 12 | cs2cs +init=epsg:4326 +to +init=epsg:25832 231950.54 1920310.71 0.00
In the above example we transform geodetic coordinates in the WGS84 reference frame to UTM zone 32N coordinates in the ETRS89 reference frame. UTM coordinates