Le 08/01/2011 02:34, Mike N. a écrit :
I'm not familiar with how survey marks are used - are there multiple
marks at the same lat/lon but different ele?
Not always but very often, the French IGN made several geodesic points
on the same object to alow multiple mesurement.[1]
The typical case is the cross among a bell tower : one point is on the
top or center of the cross, one other is on the bottom of the cross.
So they exactyl have the same lat/lon but a dirrerent elevation [2]
A mistake came also after the creation of water towers.
On the IGN's survey marks, they are indications on the object carrying
it: church, chemney, water tower... A little tool was made to find
it.[3] A lot of objects have been created from those indications, mainly
water tower. But those objects had no elevation tag for it was unknonw.
Keepright (and other) indicate those point has dupes. And several have
been merged. [4]
Those points, in OpenStreetMap, are very precious. They are used for
cheking the quality of aeral imagery and other stuff...
[1]
http://geodesie.openstreetmap.fr/cgi-bin/index.py?zoom=12&lat=48.05932&lon=1.91779&layers=B0T&ch=1100,1200
[2] http://www.openstreetmap.org/browse/node/670053466
http://www.openstreetmap.org/browse/node/670053469
[3]
http://osmose.openstreetmap.fr/map/cgi-bin/index.py?zoom=10&lat=48.09885&lon=2.04795&layers=B000000FFFFFFFFFFFFT&item=7010
[4] http://www.openstreetmap.org/browse/node/670053455
http://www.openstreetmap.org/browse/node/527901535
--
FrViPofm
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