Simple PROJ

This is a ruby extension library for map projection conversion using PROJ. Note that this library is not a verbatim wrapper for all functions in PROJ.

gem install simple-proj

• PROJ version 6 or 7

• Conversion of latitude and longitude to map projection coordinates (forward transformation)
• Conversion of map projection coordinates to latitude and longitude (inverse transformation)
• Transformation a map projection coordinate to another map projection coordinate

PROJ.new(PROJ_STRING)  ### transform defined by proj-string
PROJ.new(CRS1, CRS2)   ### transform from CRS1 to CRS2
PROJ.new(CRS2)         ### CRS1 is assumed to be "+proj=latlong +type=crs"

The arguments should be String objects one of

• a proj-string,
• a WKT string,
• an object code (like 'EPSG:4326', 'urn:ogc:def:crs:EPSG::4326', 'urn:ogc:def:coordinateOperation:EPSG::1671'),
• a OGC URN combining references for compound coordinate reference systems (e.g 'urn:ogc:def:crs,crs:EPSG::2393,crs:EPSG::5717' or custom abbreviated syntax 'EPSG:2393+5717'),
• a OGC URN combining references for concatenated operations (e.g. 'urn:ogc:def:coordinateOperation,coordinateOperation:EPSG::3895, coordinateOperation:EPSG::1618')

Forward transformation.

PROJ#transform(x1, y1, z1=nil)  =>  x2, y2[, z2]
PROJ#transform_forward(x1, y1, z1=nil)  =>  x2, y2[, z2]   ### alias of #transform

Inverse transformation.

PROJ#transform_inverse(x1, y1, z1=nil)  =>  x2, y2[, z2]

These are special methods provided to avoid converting between latitude and longitude units between degrees and radians.

PROJ#forward(lon1, lat1, z1=nil)        =>  x2, y2[, z2]
PROJ#inverse(x1, y1, z1=nil)            =>  lon2, lat2[, z2]

The units of input parameters for PROJ#forward are degrees. Internally, these units are converted to radians and passed to proj_trans(). Similarly, the units of output parameters for PROJ#inverse are degrees.

proj = PROJ.new("+proj=webmerc")

x, y = proj.forward(135, 35)
p [x, y]     ### => [15028131.257091932, 4163881.144064294]

lon, lat = proj.inverse(x, y)
p [lon, lat] ### => [135.0, 35.000000000000014]

Note: This method should not be used for the PROJ object initialized with the proj-string that do not expect geodetic coordinates in radians for the source CRS. In such case, use #transform_forward and #transform_inverse instead of #forward and #inverse. This is an example for invalid usage.

pj = PROJ.new("+proj=unitconvert +xy_in=deg +xy_out=rad")

p pj.forward(135, 35)    ### does not work as expected
# => [0.041123351671205656, 0.0106616096925348]

p pj.transform(135, 35)  ### works as expected
# => [2.356194490192345, 0.6108652381980153]

For EPSG:4326 (in PROJ), the geographic coordinates are in the order of latitude and longitude. Then, the transformation from EPSG:4326 requires the arguments in the order of latitude and longitude.

#########################################
# EPSG:4326 <-> EPSG:3857
#########################################

proj = PROJ.new("EPSG:4326", "EPSG:3857")

x, y = proj.transform(35, 135)

p [x, y]     ### => [15028131.257091932, 4163881.144064294]

lat, lon = proj.transform_inverse(x, y)

p [lat, lon] ### => [35.00000000000001, 135.0]