srs

Node-Srs

Linux:

Windows:

This module tries its best to understand your 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 is ESRI WKT that lives inside the .prj files that come with shapefiles and usually OGC CRS URN that optionally lives inside the crs property inside of GeoJSON files. TileMill - through Mapnik - needs the proj4 representation to create coordinate transformations between two projections in order to reproject vector or raster data on the fly.

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.

node-srs works overtime to try 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 uneeded reprojection. It is common for data out in the wild in web mercator projection to store slighly different projection strings in WKT 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.

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 }

Depends

• Node >= 0.6.13 (development headers)
• No other required dependencies (though can optionally depend on external libgdal)

Installation

npm install srs

From source:

git clone git://github.com/mapbox/node-srs.git
cd node-srs
npm install

Against external libgdal (avoids compiling internal copy of libosr)

npm install --shared_gdal

Test

npm test

License

BSD, see LICENSE.txt

Changelog

0.4.0

• Now builds on FreeBSD
• srs.jsonCrs added for detecting projection inside JSON object representing GeoJSON. Pass the result to srs.parse
• srs.parse no longer supports being passed a filepath to a GeoJSON.