Transcript Proj4js-FOSS4g-09 - OSGeo Server

Proj4js
Coordinate transformations in the browser
Michael Adair
DM Solutions Group
Proj4js
A JavaScript library to transform point
coordinates from one coordinate system to
another, including datum transformations
The problem to solve
representing 3D information in a 2D medium
Same issue for paper maps and web maps on
computer screen.
Mathematics defines: Coordinate Reference Systems
(CRS)
Coordinate Reference Systems (CRS)
• consist of a map projection + datum
• projections define 3D surface to 2D plane
transformation
• datums define the size and shape of the
earth
• geographic data is collected/stored using any one
CRS
Map Projections
• ‘project’ a 3D surface to 2D
• Applicable for defined geographic areas
• attempt to minimize distortions in area, distance, scale,
etc.
Datums
• Datums define the size and shape of the earth
• The earth is not a sphere, almost an ellipsoid
Examples
Examples
Examples
Examples
Back to the problem
• A map can only use one CRS
• Need to combine data stored in different
CRS’s
• transformation is required between
coordinate systems
Solutions for web mapping
• Vector data consists of a series of points representing
points, line and polygon geometries (along with attributes)
• Coordinates in various CRS
On the server side:
• PROJ.4, CS-MAP, GeoTools
• Returns a raster already transformed (in general)
• Raster images must be processed on the server
• Proj4js is for points, not rasters
It is desirable to manipulate individual features in the browser
Solutions for web mapping
• convert between CRS in the Client (web
browser) with Proj4js!
• Allows client to maintain individual
features for selection, highlighting, etc.
About Proj4js
• A JavaScript library
• Port of PROJ.4 and GCTPC (C code)
• ~50k compressed; less if not all projections
needed
• Includes some datum transformations
• Dynamic loading of CRS parameters (or not)
• Dynamic loading of projection code (or not)
• Easy to use with OpenLayers
Project infrastructure
• Originally developed in MapBuilder
• Now part of the OSGeo MetaCRS project
http://wiki.osgeo.org/wiki/MetaCRS
• Wiki and Trac http://trac.osgeo.org/proj4js/
• SVN: http://svn.osgeo.org/metacrs/proj4js/
• Mailing lists:
http://lists.osgeo.org/mailman/listinfo/MetaCRS
How to use it
<script src="lib/proj4js-combined.js"></script>
<script src="lib/defs/EPSG42304.js"></script >
…
var source = new Proj4js.Proj(‘EPSG:4236’);
var dest = new Proj4js.Proj(‘EPSG:42304’);
var p = new Proj4js.Point(-76.0,45.0);
Proj4js.transform(source, dest, p);
….
• p.x and p.y are now EPSG:42304 easting and northing in
meters
Demo
• http://localhost:8080/mapbuilder/examples/projDe
mo/index.html
• http://localhost/oltrunk/examples/graticule.html
• http://localhost/proj4js/test
Demo (2)
• Equal area maps:
http://www.equal-area-maps.com/mollweide.php
• KML:
http://bbs.keyhole.com/ubb/download.php?Number=35858
Questions?
Thanks for your interest!
• http://trac.osgeo.org/proj4js/
• http://proj4js.org/
Mike Adair
[email protected]
with graphics from Peter H. Dana, The Geographer's Craft Project, Department of Geography, The University of Colorado at Boulder
http://www.colorado.edu/geography/gcraft/notes/mapproj/mapproj.html