https://github.com/mbloch/mapshaper-proj

A JavaScript port of the Proj.4 map projection library.
https://github.com/mbloch/mapshaper-proj

datum epsg proj4 projection

Last synced: 4 days ago
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A JavaScript port of the Proj.4 map projection library.

• Host: GitHub
• URL: https://github.com/mbloch/mapshaper-proj
• Owner: mbloch
• License: other
• Created: 2016-06-27T13:46:27.000Z (over 8 years ago)
• Default Branch: master
• Last Pushed: 2024-11-19T19:58:38.000Z (about 1 month ago)
• Last Synced: 2024-12-23T08:09:42.421Z (11 days ago)
• Topics: datum, epsg, proj4, projection
• Language: JavaScript
• Size: 2.03 MB
• Stars: 38
• Watchers: 6
• Forks: 5
• Open Issues: 7
• Metadata Files:
  • Readme: README.md
  • Changelog: CHANGELOG.md
  • License: LICENSE.md

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README

        
# mapshaper-proj

This software is a JavaScript port of v4.9.3 of the [Proj.4](http://proj4.org/) map projection library. Although it was created to be used by [mapshaper](https://github.com/mbloch/mapshaper), all are welcome to use and improve it.

Why create another Proj.4 port, when there is already [proj4js](https://github.com/proj4js/proj4js)? Unlike proj4js, this port is a very literal translation to JavaScript, so staying up-to-date with future changes to Proj.4 should be relatively simple. Also, this software's output is more consistent with Proj.4's output.

##### Missing features

* No support for datum transformations using grid files.

* About 1/5 of Proj.4's 130-odd projections have not been ported.

## Command line programs: mproj and mcs2cs

These are workalike versions of the `proj` and `cs2cs` programs from Proj.4.

To install the current versions system-wide, run `npm install -g mproj`.

## Node API

The software can be used in either the Proj.4 style or the proj4js style.

##### proj4js style

```

var proj = require('mproj');

# create functions for translating between two coordinate systems

# returned object has forward() and inverse() functions

var obj = proj(, );

# WGS84 is assumed if a source definition is not given

var obj = proj();

# a shortcut for translating a single point

#  is an [x, y] array or an object with x and y properties

var xy = proj([,] dest definition, );

```

See the [proj4js.org website](http://proj4js.org/) for more detailed help.

##### Proj.4 style

```

var proj = require('mproj');

# create a projection object from a proj4 string

var P = proj.pj_init();

# project and inverse-project geographical coordinates

var xy = proj.pj_fwd({lam: , phi: }, P);

var lp = proj.pj_inv({x: , y: }, P);

# transform arrays of coordinates from one coordinate system to another

proj.pj_transform(, , , [, ]);

```

##### Error handling

The x and y coordinates of unprojectable points are set to `Infinity`. Other errors cause an Error to be thrown containing an appropriate message.

## Building

mapshaper-proj uses the (old-fashioned) technique of concatenating source files and wrapping them in a function to create a  module with a shared scope. This method allows for global variables that are invisible outside of the program, making porting from the original C much simpler than if we were to make each source file a separate module.

Running `build` creates a build containing all supported projections in the `dist/` directory.

Running `build merc,lcc,aea` creates a build containing only the listed projections. You can customize the comma-separated list to include only the projections that you want. Run `mproj -l` to see a list of all supported projections.