Geographic Projection Web Services redux
Tuesday, January 9th, 2007A couple of years ago I was playing around with Twisted Python and developed a simple web service for reprojecting data (this is no longer live). It was merely a glorified wrapper around the spatial reference tools in GDAL, but a few folks found it useful and I still receive an email about it here and there.
Lately, I’ve been playing around with Django, mostly as an effort to find a less punishing framework than Zope/Plone. I’m not very much of a web person however, but I’ve been following some of the recent developments like OpenLayers, GeoJSON, and, of course, the Google. One of the more common and somewhat insane things that I see as part of my daily routine hanging out on #gdal on IRC is somebody showing up asking about projection math and how they can implement it in JavaScript. The Open Source GIS world already has a world-class projections library in Proj.4, but for some reason it makes sense to a few folks to try and re-invent this wheel. I’ve always thought it was kind of silly, and any attempt to do so would probably result in a miserable development effort that retreads the misery that Gerald and Frank endured making Proj.4 in the first place.
JSON
JSON, or JavaScript Object Notation, is rapidly becoming the transport of choice for all of the AJAXian wunderkinds out there. The greatest advantage of JSON is that the parsing overhead of it for JavaScript is zero, and it can practically be eval’d in directly to the client. With these two things in mind, I set about to play around with a JSON-emitting webservice that projects geographic data (points).
The projector lives at http://projection.hobu.biz/json . It takes in these parameters:
| Parameter | Value | Description |
| x | -93.0 | The x coordinate (longitude) |
| y | 42.0 | The y coordinate (latitude) |
| inref | EPSG:4326 | The input spatial reference of the point |
| outref | EPSG:26915 | The output spatial reference of the point |
| function | myfunction | The name of the JavaScript function to wrap the output in |
| id | 3 | An id to give the data |
Here’s an example that projects a point in Iowa from EPSG:4326 to EPSG:26915 (decimal degrees WGS 84 to UTM Zone 15):
Using the service with EPSG codes would probably be the most common way to work, but because the service builds on the spatial referencing library in GDAL, there are many more options. For example, we could specify our input projection in Proj.4 format instead:
Or even get really nuts and specify our output projection in OGC WKT:
Finally, we can output a coordinate in a spatial reference that doesn’t have an EPSG code, like USGS Albers:
Python usage
You aren’t limited to using GET requests either (although you could probably construct one easily with urllib). For example, with jsonrpclib for Python, you can request projection of points much like you would with XMLRPC:
>>> import jsonrpclib>>> rpc = jsonrpclib.ServerProxy('http://projection.hobu.biz/json')>>> rpc.project('-93.0', '42.0', 'EPSG:4326', 'EPSG:26195')>>>{u'id': 2, u'result': {u'features': {u'center': [500000.0, 4649776.2247029999, 0.0], u’title’: u’title’, u’spatialCoordinates’: [[500000.0, 4649776.2247029999, 0.0]], u’srs’: u’EPSG:26915′, u’geometryType’: u’point’, u’id’: u’1′}}}
If you find it useful, let me know.
