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Quick start

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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 +Geodetic transformation.

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A proj-strings holds the parameters of a given coordinate transformation, e.g.

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+proj=merc +lat_ts=56.5 +ellps=GRS80
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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 Mercator projection 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.

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By using the above projection definition as parameters for the command line +utility proj we can convert the geodetic coordinates to projected space:

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$ proj +proj=merc +lat_ts=56.5 +ellps=GRS80
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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:

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$ echo 55.2 12.2 | proj +proj=merc +lat_ts=56.5 +ellps=GRS80
+3399483.80      752085.60
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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:

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$ 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
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Notice the +to parameter that separates the source and destination +projection definitions.

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If you happen to know the EPSG identifiers for the two coordinates reference +systems you are transforming between you can use those with cs2cs:

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$ echo 56 12 | cs2cs +init=epsg:4326 +to +init=epsg:25832
+231950.54      1920310.71 0.00
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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

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