Jon, Thanks for your detailed response. It appears I'm dealing with at least two different sets of map data that disagree somewhat.
We have an operational map database that has been used for more than 10 years. The heritage is unknown, though I think the original source was the U.S. government. In the case of the southernmost point of that island, our database agrees with the spatialreference.org map. Clearly our old database and spatialreference.org show this point to be at 49.709 degrees latitude, while the openstreetmap.org database shows it at 49.701 (rounded to 3 decimal places). The difference is about .9 km. I tend to agree with your second statement "the map data they are using puts the island at a different position to OSM". I wonder which map data is more correct? By the way, my purpose is to update our old database with more up-to-date roads, cities, airports, coastlines, etc. I noticed problems when substituted OSM coastlines and ended up with boundaries of coastal cities extending over water. It appears I may have to migrate to new data all at once, not in parts. But the question remains - am I improving our maps, or am I going to be less accurate? Brad -----Original Message----- From: Jon Burgess [mailto:[email protected]] Sent: Monday, September 13, 2010 6:10 PM To: Simpson Brad-C-Lockheed Cc: mapnik-users; Robert Coup Subject: Re: [Mapnik-users] Projection problems On Tue, 2010-09-14 at 00:54 +0100, Jon Burgess wrote: > > The coordinates are -2.2227, 49.709 degrees corresponding to > > about -247437,6395975 in the spherical Mercator projection. > I > > extracted the polygon in shoreline_300 and found the > > southernmost point. Its coordinates were -247169,6394645. > > > > > > I think the answer is no, the SR-ORG:6 does not match the OSM > spherical > mercator projection. The (-2.2227, 49.709) is not at the southern tip > of > the island: Looking again at the http://spatialreference.org/ref/sr-org/6/ site, it looks like this is the correct projection, the input/output co-ordinates match exactly to the ones computed by cs2cs. It looks like the issue is that the co-ordinates being displayed do not align well with the red marker on the SR.org map. Alternatively the map data they are using puts the island at a different position to OSM. Jon _______________________________________________ Mapnik-users mailing list [email protected] https://lists.berlios.de/mailman/listinfo/mapnik-users
