Cartographic projection¶
The foundation of PROJ is the large number of projections available in the library. This section is devoted to the generic parameters that can be used on any projection in the PROJ library.
Below is a list of PROJ parameters which can be applied to most coordinate system definitions. This table does not attempt to describe the parameters particular to particular projection types. These can be found on the pages documenting the individual projections.
Parameter
Description
+a
Semimajor radius of the ellipsoid axis
+axis
Axis orientation
+b
Semiminor radius of the ellipsoid axis
+ellps
Ellipsoid name (see
proj -le
)+k
Scaling factor (deprecated)
+k_0
Scaling factor
+lat_0
Latitude of origin
+lon_0
Central meridian
+lon_wrap
Center longitude to use for wrapping (see below)
+no_defs
Don’t use the /usr/share/proj/proj_def.dat defaults file
+over
Allow longitude output outside -180 to 180 range, disables wrapping (see below)
+pm
Alternate prime meridian (typically a city name, see below)
+proj
Projection name (see
proj -l
)+units
meters, US survey feet, etc.
+vunits
vertical units.
+x_0
False easting
+y_0
False northing
In the sections below most of the parameters are explained in details.
Units¶
Horizontal units can be specified using the +units
keyword with a symbolic
name for a unit (ie. us-ft
). Alternatively the translation to meters can be
specified with the +to_meter
keyword (ie. 0.304800609601219 for US feet). The
-lu
argument to cs2cs
or proj
can be used to list symbolic unit names.
The default unit for projected coordinates is the meter.
A few special projections deviate from this behaviour, most notably the
latlong pseudo-projection that returns degrees.
Vertical (Z) units can be specified using the +vunits
keyword with a
symbolic name for a unit (ie. us-ft
). Alternatively the translation to
meters can be specified with the +vto_meter
keyword (ie. 0.304800609601219
for US feet). The -lu
argument to cs2cs
or proj
can be used to list
symbolic unit names. If no vertical units are specified, the vertical units will
default to be the same as the horizontal coordinates.
Note
proj
do not handle vertical units at all and hence the +vto_meter
argument will be ignored.
Scaling of output units can be done by applying the +k_0
argument. The
returned coordinates are scaled by the value assigned with the +k_0
parameter.
False Easting/Northing¶
Virtually all coordinate systems allow for the presence of a false easting
(+x_0
) and northing (+y_0
). Note that these values are always expressed in
meters even if the coordinate system is some other units. Some coordinate
systems (such as UTM) have implicit false easting and northing values.
Longitude Wrapping¶
By default PROJ wraps output longitudes in the range -180 to 180. The +over
switch can be used to disable the default wrapping which is done at a low level
in pj_inv()
. This is particularly useful with projections like the
equidistant cylindrical
where it would be desirable for X values past -20000000 (roughly) to continue
past -180 instead of wrapping to +180.
The +lon_wrap
option can be used to provide an alternative means of doing
longitude wrapping within pj_transform()
. The argument to this option is a
center longitude. So +lon_wrap=180
means wrap longitudes in the range 0 to
360. Note that +over
does not disable +lon_wrap
.
Prime Meridian¶
A prime meridian may be declared indicating the offset between the prime meridian of the declared coordinate system and that of greenwich. A prime meridian is clared using the “pm” parameter, and may be assigned a symbolic name, or the longitude of the alternative prime meridian relative to greenwich.
Currently prime meridian declarations are only utilized by the
pj_transform()
API call, not the pj_inv()
and pj_fwd()
calls.
Consequently the user utility cs2cs
does honour prime meridians but the
proj
user utility ignores them.
The following predeclared prime meridian names are supported. These can be
listed using with cs2cs -lm
.
Meridian
Longitude
greenwich
0dE
lisbon
9d07’54.862”W
paris
2d20’14.025”E
bogota
74d04’51.3”E
madrid
3d41’16.48”W
rome
12d27’8.4”E
bern
7d26’22.5”E
jakarta
106d48’27.79”E
ferro
17d40’W
brussels
4d22’4.71”E
stockholm
18d3’29.8”E
athens
23d42’58.815”E
oslo
10d43’22.5”E
Example of use. The location long=0
, lat=0
in the greenwich based lat/long
coordinates is translated to lat/long coordinates with Madrid as the prime
meridian.
cs2cs +proj=latlong +datum=WGS84 +to +proj=latlong +datum=WGS84 +pm=madrid
0 0 <i>(input)</i>
3d41'16.48"E 0dN 0.000 <i>(output)</i>
Axis orientation¶
Starting in PROJ 4.8.0, the +axis argument can be used to control the axis orientation of the coordinate system. The default orientation is “easting, northing, up” but directions can be flipped, or axes flipped using combinations of the axes in the +axis switch. The values are:
“e” - Easting
“w” - Westing
“n” - Northing
“s” - Southing
“u” - Up
“d” - Down
They can be combined in +axis in forms like:
+axis=enu
- the default easting, northing, elevation.+axis=neu
- northing, easting, up - useful for “lat/long” geographic coordinates, or south orientated transverse mercator.+axis=wnu
- westing, northing, up - some planetary coordinate systems have “west positive” coordinate systems
Note
The +axis
argument does not work with the proj
command line
utility.