Re: [OSM-talk] French/Dutch caribbean island Saint Martin (Sint Maarten) not correctly positionned
2009/8/5 Pieren pier...@gmail.com: I'm currently implementing the cadastre support in JOSM for the french part of the island Saint-Martin shared with our Dutch friends (it is a special projection). The island is quite well mapped today, mostly from the hi-res Yahoo imagery I guess: http://www.openstreetmap.org/?lat=18.069lon=-63.0746zoom=13layers=B000FTF The problem is that the data, although they match the Yahoo imagery, seem to be shifted from about 800 meters in north. The French national geographic institut provides files about geodesic reference points and one is marked on the main fortress here: http://geodesie.ign.fr/fiche_geodesie_OM.asp?num_site=9712701X=491000Y=1998000 Here the details of one mark on the ground: http://geodesie.ign.fr/fiche_point_OM.asp?num_site=9712701no_ptg=01 So, the fortress should be at 18.0707416944 lat and -63.0851921944 lon but in OSM it is at (approx.) 18.0705758642 lat and -63.0845414733 lon My guess is that the Yahoo imagery is not correctly georeferenced. I can fix this issue for the French part of the island using the cadastre. But what about the Dutch part of the island ? It is probably the same issue for the neighbourhood, e.g. Scrub Island, Dog Island and Saint Barthelemy. Also how can we inform other mappers that the Yahoo imagery is not correct in this area ? I was thinking before about the miscalibrated imagery such as Yahoo! that became rather popular in osm, and I think josm developers wouldn't mind applying a patch that either hardcodes offsets for the known miscalibrated areas on yahoo! or pulls them from some kind of wiki live. It might be even doable as part of the javascript in the html wms plugin uses to download Yahoo! imagery and should also be possible for potlatch (but this I wouldn't know how to approach). Does that make sense? If the offset is not constant across the whole available area in Yahoo then it's a little more complex but stil doable in JOSM. It would be good to shift all of the island's nodes about the same time such a patch would be applied. Cheers ___ talk mailing list talk@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk
Re: [OSM-talk] French/Dutch caribbean island Saint Martin (Sint Maarten) not correctly positionned
Pieren wrote: I'm currently implementing the cadastre support in JOSM for the french part of the island Saint-Martin shared with our Dutch friends (it is a special projection). The island is quite well mapped today, mostly from the hi-res Yahoo imagery I guess: http://www.openstreetmap.org/?lat=18.069lon=-63.0746zoom=13layers=B000FTF The problem is that the data, although they match the Yahoo imagery, seem to be shifted from about 800 meters in north. The French national geographic institut provides files about geodesic reference points and one is marked on the main fortress here: http://geodesie.ign.fr/fiche_geodesie_OM.asp?num_site=9712701X=491000Y=1998000 Here the details of one mark on the ground: http://geodesie.ign.fr/fiche_point_OM.asp?num_site=9712701no_ptg=01 So, the fortress should be at 18.0707416944 lat and -63.0851921944 lon but in OSM it is at (approx.) 18.0705758642 lat and -63.0845414733 lon I'm not quite clear how you come to that conclusion. While JOSM only displays 4 digits after the comma (ooh, can we change that to 7 someday?), both coordinates lie in a wooded area which may very well be the fortress. And the difference between these two points is 71 metres in total, just 18 metres N-S difference, not 800. The Google images are a lot better than the Yahoo ones, and where Google says the fort is, is close to the first set of coordinates, and that lines up with Yahoo. But in OSM, there is no point to identify the fortress. Furthermore: I downloaded the GPS data for Sint-Maarten and that lines up very nice with the roads of OSM (both in the French as the Dutch part), and the roads line up with the Yahoo imagery. So I don't really see this mismatch you are seeing. Regards, Maarten ___ talk mailing list talk@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk
