On Wed, Sep 15, 2010 at 09:14:45AM -0600, Simpson Brad-C-Lockheed wrote: > Dane, > > What is the source of the mapping data used on the spatialreference.org > website? I would have assumed openstreetmap to be a likely source, except > the spatialreference coordinates match our 10+ year old map data and differ > from openstreetmap. > > This is easy to confirm when zooming into the southernmost point of Alderney > Island (English Channel) using spatialreference.org. Both the EPSG:4326 and > SR-ORG:6 pages show the location to be -2.223W, 49.709N. This agrees with > our old map database. The SR-ORG:6 page also displays the spherical Mercator > coordinates as -247437, 6395975. > > From what I understand, openstreetmap uses shoreline_300 coastline data for > low resolution maps, and processed_p for high resolution maps. I've plotted > both together and they essentially match, with processed_p being much more > detailed. Both are spherical Mercator. The southernmost point of Alderney > Island is easiest to extract from shoreline_300. Its coordinates (according > to shoreline_300) is -247169, 6394645. The difference is about .9 km. How > do we explain this discrepancy?
The map used in the SR.org website is just a simple vmap0-based map. vmap0 is extremely coarse (1:M scale) data; it is not designed for accuracy, and the map itself is not considered an important part of the application, and we are not attempting to provide a map as anything other than a vauge reference to the area of the world. If you imagine that the map was not zoomable, then you probably have a better understanding of the level of detail we are currently interested in presenting through the SR.org reference map. -- Chris > Brad > > -----Original Message----- > From: Simpson Brad-C-Lockheed > Sent: Tuesday, September 14, 2010 10:58 AM > To: Jon Burgess > Cc: mapnik-users; Robert Coup; Dane Springmeyer; Simpson Brad-C-Lockheed > Subject: RE: [Mapnik-users] Projection problems > > 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 > > -- Christopher Schmidt Web Developer _______________________________________________ Mapnik-users mailing list [email protected] https://lists.berlios.de/mailman/listinfo/mapnik-users
