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Lots of things have been imported into OSM from IGM as "trig points" But some of these "trig points" don't have any evidence on the ground. For instance, node 670525588 says "Cimetiere : Mur de cloture : Face Ouest : Boulon scellé - Point constaté détruit en 2002" so should it be included, even if no visible trace of it exists, if it is used to survey other things that do exist? Also, some of these "trig points" are very small and you would have to look very hard to find them - I think "Boulon scellé" means bolthead? Isn't it mainly useful to have them as landmarks? |
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These survey points are very usefull for evaluating and fixing the position of imagery support, or positioning natural peaks. These points are (or was) mainly attached to a building or construction that is visible on imagery, or to a natural peak. You're pointing a points that is marked as destroyed (« Point constaté détruit »). Ok, this point doesn't exist anymore on the ground, but we know that this position is on the west wall of the cimetery (« Mur de cloture : Face Ouest »), so the position of this point is still usefull. We can check that the cimetery visible on an imagery is well positioned in regard of this survey point. We have some discussions in the french community for storing this points in a separate database. The main motivation is that these points should not be moved. But technically, it easier to have them in the main database for now, so that they are loaded automatically. 2
Discussion on french mailing list: http://lists.openstreetmap.org/pipermail/talk-fr/2012-September/047449.html
(26 Sep '12, 11:04)
Vincent de P... ♦
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Isn't it OSM policy that everything in the database should have a visible trace on the ground? I'm not convinced it's useful to have these points in the database unless they're on a structure of their own like this - http://upload.wikimedia.org/wikipedia/commons/2/2f/Trig_Point_near_Wootton_Wawen.jpg For sake of reference, the wall itself is already in there, and it's not at a summit. 1
Are the administrative boundaries visible on the ground? Sorry, but I don't understand the purpose of your last sentence. You agree that the coordinates of the survey point are still relevant although the survey point doesn't exist on the ground, because the support structure is still there (concrete, wall, building)?
(26 Sep '12, 16:28)
NicolasDumoulin
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Hi Tenietz, the bolt youre pointing at is a old but secure point for local use of levveling-instruments. All the levels of the bolts in the area are listed and marked. Its not important to tag them since theyre located in local buildings or whatever is available to make a sturdy point. The real trig point (Governmental triangle points) are ment for theodolites to survey the surroundings and fi visible on topographic maps. But the system has probatly been abandoned with new systems in use with GPS instead. Greetz 1
omg. He is speaking about this : http://geodesie.ign.fr/index.php?page=points_geodesiques They are used in OSM to accurately reposition external images like Bing. If the point is physically away but the support still there (church, wall, building, peak, etc), it is still present in many documents (cadastre, maps, images) and still can be used to reposition accurately external sources.
(04 Oct '12, 12:08)
Pieren
Thanks & greetz
(04 Oct '12, 13:09)
Hendrikklaas
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