srs
v1.2.0
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Spatial reference library for node
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Node-Srs
This module tries to detect projections, also known as "spatial reference systems". It works similiarly to gdalsrsinfo.
node-srs
supports parsing a variety of textual representations of projections, like the formats known as OGC WKT
, ESRI WKT
, OGC CRS URN
, or proj4
. It supports shapefiles and GeoJSON. Shapefiles optionally come with a separate .prj and inside the .prj
the text is usually in the ESRI WKT
format. GeoJSON optionally contains a crs
property that declares projection as OGC CRS URN
.
Detecting projections is important for applications like TileMill, which - through Mapnik - needs the proj4
representation of a projection to create coordinate transformations for re-projecting vector or raster data on the fly.
node-srs
includes a variety of hacks to determine if your projection looks like web mercator
(epsg:3857) or wgs84
(epsg:4326) and if so returns the canonical representations of these projections (according to @springmeyer). This ensures applications like TileMill can avoid unneeded projection. It is common for data out in the wild in web mercator projection to store slightly different projection strings based on the software that created the files. node-srs
ensures a consistent final proj4
representation is returned for all of the variety of representations of web mercator. This is critical because even mercator to mercator transformations are expensive if proj4 is asked to do this for large datasets.
node-srs
does not support looking for, or detecting, projection information in formats like GeoTIFF, PostGIS, or SQLite. Rather for those formats you would need to extract the projection information yourself and then pass it to node-srs
.
API
srs.parse(string)
Parse a string of projection specification data and return an object describing
the detected projection. If the SRS cannot be parsed, throws a TypeError
describing the issue.
Example
Detect a proj4 literal string as spherical mercator:
> var srs = require('srs');
> srs.parse('+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs')
{ proj4: '+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over',
srid: 3857,
auth: 'EPSG',
pretty_wkt: 'PROJCS["WGS 84 / Pseudo-Mercator",\n GEOGCS["WGS 84",\n DATUM["WGS_1984",\n SPHEROID["WGS 84",6378137,298.257223563,\n AUTHORITY["EPSG","7030"]],\n AUTHORITY["EPSG","6326"]],\n PRIMEM["Greenwich",0,\n AUTHORITY["EPSG","8901"]],\n UNIT["degree",0.0174532925199433,\n AUTHORITY["EPSG","9122"]],\n AUTHORITY["EPSG","4326"]],\n UNIT["metre",1,\n AUTHORITY["EPSG","9001"]],\n PROJECTION["Mercator_1SP"],\n PARAMETER["central_meridian",0],\n PARAMETER["scale_factor",1],\n PARAMETER["false_easting",0],\n PARAMETER["false_northing",0],\n EXTENSION["PROJ4","+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs"],\n AUTHORITY["EPSG","3857"],\n AXIS["X",EAST],\n AXIS["Y",NORTH]]',
esri: false,
name: 'Google Maps Global Mercator',
valid: true,
is_geographic: false,
input: '+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs' }
Detect a WKT string as WGS84:
> srs.parse('GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137,298.257223563]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]]')
{ input: 'GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137,298.257223563]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]]',
proj4: '+proj=longlat +ellps=WGS84 +no_defs',
srid: 4326,
auth: 'EPSG',
pretty_wkt: 'GEOGCS["GCS_WGS_1984",\n DATUM["D_WGS_1984",\n SPHEROID["WGS_1984",6378137,298.257223563]],\n PRIMEM["Greenwich",0],\n UNIT["Degree",0.017453292519943295],\n AUTHORITY["EPSG","4326"]]',
esri: false,
name: 'GCS_WGS_1984',
valid: true,
is_geographic: true }
Installation
npm install
Test
npm test
License
BSD, see LICENSE.txt