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Cassini Support in geo_helper.py - Nick [entries|archive|friends|userinfo]

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Cassini Support in geo_helper.py [Dec. 12th, 2006|06:21 pm]
With our work on old maps, we've come across quite a few pre-National Grid maps. These are mainly of Scotland, and as we do have a few of these for places needing coverage, we're going to need to work with them.

These maps are projected with a Cassini-Soldner projection, which was quite popular in the 19th century. It's apparently easier to work with than the more modern grid systems (eg Mercator), but is only suitable for areas with a larger north-south distance than an east-west one (handily like the UK).

In order to work with them, we're going to need to be able to turn positions on an Cassini based map into national grid co-ordinates. Andy has come up with a very nifty JavaScript converter and demo app. Since it's not always easy to work in JavaScript, I've updated my python geo_helper.py to support the OSGB flavour of the Cassini Projection. (I used equations from here, and checked it gave the same answers as Andy's code). So, we now have a command line way to convert to and from Cassini co-ordinates :)

In other news, we've been busy adding a few more bits of Scotland to npemap. If you look on the front page, you'll see we now have coverage of a fair bit of the mid-east coast. Quite a bit more to go though....