[OSM-talk] French/Dutch caribbean island Saint Martin (Sint Maarten) not correctly positionned

[andrzej zaborowski]

2009/8/5 Pieren <pieren3 at 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=18.069&lon=-63.0746&zoom=13&layers=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=9712701&X=491000&Y=1998000
> Here the details of one mark on the ground:
> http://geodesie.ign.fr/fiche_point_OM.asp?num_site=9712701&no_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