Instantiation¶

Geo::Proj4->new($string|%options)

The object defines the target projection, but that's easier said than done: projections have different parameter needs. The parameters which can (or need to) be used are listed with "cs2cs -lP". The manual page of "cs2cs" explains how the configuration works.

Two ways are provided to define the projection. Either, use a list of %options, which are pairs of parameters, or pass one string which contains all parameters at once. You must supply a "proj" parameter.

In case of an OPTION list: WARNING: Specify boolean parameters (e.g. the south parameter to the UTM projection) with a matching value of undef.

example:

 my $proj = Geo::Proj4->new(proj => "merc",
    ellps => "clrk66", lon_0 => -96 )
       or die Geo::Proj4->error;
 my $proj = Geo::Proj4->new("+proj=merc +ellps=clrk66 +lon_0=096")
    or die Geo::Proj4->error;
 my $proj = Geo::Proj4->new(init => "epsg:$epsg");
    

Accessors¶

$obj->datum()

Tries to return a datum name for this projection.

$obj->dump()

Write the definition in extended form to stdout. This output cannot be caught, because it is done on stdio level, below the reach of PerlIO.

Geo::Proj4->error()

Returns a dualvar (see Scalar::Util) containing the error number and error string of the last reported error.

example:

 my $proj = Geo::Proj4->new(...);
 unless(defined $proj)
 {   my $error = Geo::Proj4->error;
     warn "error-code: ".$error+0;
     warn "error-string: $error\n";
 }
    

$obj->hasInverse()

Returns whether the reverse function for the projection exists. Some projections are one-way.

$obj->isGeocentric()

Returns true when the source projection is using a geocentric coordinate system; i.e. uses x-y coordinates.

$obj->isGeodesic()

Returns true when the source projection is using a geodetic coordinate system; i.e. uses lat long coordinates. Same as isLatlong()

$obj->isLatlong()

Returns true when the source projection is using a geodetic coordinate system; i.e. uses lat long coordinates. Same as isGeodesic().

$obj->normalized()

Returns a string which is produced by the library based on the data extracted from the initiation parameters. This string may be more explicit than the passed values, and could be used for debugging.

$obj->projection()

Returns the projection type.

Converters¶

$obj->forward($latitude, $longitude)

Perform a forward projection from $latitude and $longitude (LL) to the cartographic projection (XY) represented by the Geo::Proj4 instance.

WARNING: for historic reasons, latitude and longitude are assumed to be in (floating point) degrees, although the library expects rads. See forwardRad(). A latitude south of the Equator and longitude west of the Prime Meridian given with negative values.

Returned are two values, usually X and Y in meters, or whatever units are relevant to the given projection. When the destination projection also than the order of parameters will be returned as LONG,LAT (not lat,long!)

On error, "forward" will return undef for both values.

example:

 my ($x, $y) = $proj->forward($lat, $lon);
 my ($long2, $lat2) = $proj->forward($lat, $lon);
    

$obj->forwardRad($latitude, $longitude)

Perform a forward projection from $latitude and $longitude (LL) to the cartographic projection (XY) represented by the Geo::Proj4 instance. This function reflects to library function "forward()", expecting radians, not degrees.

$obj->inverse(($x,$y) | ($lat,$long))

Perform an inverse projection from the (cartographic) projection represented by this Geo::Proj4 object, back into latitude and longitude values.

WARNING: for historic reasons, latitude and longitude are assumed to be in (floating point) degrees, although the library expects rads. See inverseRad().

On error, "inverse" will return undef for both values.

example:

  if($proj->hasInverse)
  {  my ($lat, $lon) = $proj->inverse($x, $y);
     ...
  }
    

$obj->inverseRad(($x,$y) | ($lat|$long))

Perform an inverse projection from the (cartographic) projection represented by this Geo::Proj4 object, back into latitude and longitude values. Latitude and longitude are assumed to be in radians. See inverse().

$obj->transform($to, $point|ARRAY-of-$points)

Translate the $points into the projecten of $to. Each point is specified as two or three values in an ARRAY. In case of latlong source or destination projections, coordinates are translated into radians and/or back. Both input and output values are always in X-Y/LongLat order. See transformRad()

example:

 my $from  = Geo::Proj4->new("+proj=latlong +datum=NAD83");
 my $to    = Geo::Proj4->new("+proj=utm +zone=10 +datum=WGS84");
 my $point = [ 1.12, 3.25 ];  # See Geo::Point
 my $pr_point = $from->transform($to, $point);
 my $pr    = $from->transform($to, [ $point1, $point2 ]);
 my $pr_point1 = $pr->[0];
 my $pr_point2 = $pr->[1];
    

$obj->transformRad($to, $point|ARRAY-of-$points)

Translate the $points into the projecten of $to. Each point is specified as two or three values in an ARRAY. In case of latlong source or destination projections, coordinates are expected to be in radians. Both input and output values are always in X-Y/LongLat order. See transform()

Library introspection¶

Geo::Proj4->datumInfo($label)

Returns a hash with information about the specified datum. With listDatums(), all defined LABELS can be found.

Geo::Proj4->ellipsoidInfo($label)

Returns a hash with information about the specified ellipsis. With listEllipsoids(), all defined LABELS can be found.

$obj->libVersion()

Geo::Proj4->libVersion()

Returns the version of the proj4 library

Geo::Proj4->listDatums()

Returns a list with all defined datum labels.

example:

 foreach my $id (Geo::Proj4->listDatums)
 {   my $def = Geo::Proj4->datum($id);
     print "$id = $def->{ellips_id}\n";
 }
    

Geo::Proj4->listEllipsoids()

Returns a list with all defined ellips labels.

example:

 foreach my $id (Geo::Proj4->listEllipsoids)
 {   my $def = Geo::Proj4->ellipsoid($id);
     print "$id = $def->{name}\n";
 }
    

Geo::Proj4->listTypes()

Returns a list with all defined projection types.

example:

 foreach my $id (Geo::Proj4->listTypes)
 {   my $def = Geo::Proj4->type($id);
     print "$id = $def->{description}\n";
 }
    

Geo::Proj4->listUnits()

Returns a list with all defined unit labels.

example:

 foreach my $id (Geo::Proj4->listUnits)
 {   my $def = Geo::Proj4->unit($id);
     print "$id = $def->{name}\n";
 }
    

Geo::Proj4->typeInfo($label)

Returns a hash with information about the specified projection type. With listTypes(), all defined LABELS can be found.

Geo::Proj4->unitInfo($label)

Returns a hash with information about the specified unit. With listUnits(), all defined LABELS can be found.

Install¶

Geo::Proj4 uses XS to wrap the PROJ.4 cartographic projections library. You will need to have the PROJ.4 library installed in order to build and use this module. You can get source code and binaries for the PROJ.4 library from its home page at <http://www.remotesensing.org/proj/>.

Projections¶

Covering all the possible projections and their arguments in PROJ.4 is well beyond the scope of this document. However, the cs2cs(1) utility that ships with PROJ.4 will list the projections it knows about by running cs2cs -lp, the ellipsoid models it knows with the -le parameter, the units it knows about with -lu, and the geodetic datums it knows with -ld. Read cs2cs(1) for more details.

Alternately, you can read the PROJ.4 documentation, which can be found on the project's homepage. There are links to PDFs, text documentation, a FAQ, and more.

Bugs¶

One common source of errors is that latitude and longitude are swapped: some projection systems use lat-long, other use x-y which is a swapped order. Especially the forward() and inverse() cause this problem, always flipping the coordinate order. The transform() method is much easier: input and output in x-y/long-lat order.

Also be warned that the values must have the right sign. Make sure you give negative values for south latitude and west longitude. For calculating projections, this is more important than on maps.