Hi. I am a bachelor in cartographer and surveying. My MAIN interest in OpenStreetMap is not about maps. It is not about "free data". It is about the effect of us, the community, as a big potential crowd of data collectors on the quality of geodata within OpenStreetMap database. I have this philosophy that once the community gets big and active enough, OpenStreetMap data quality will outrank that of professional companies.
There is no such thing as an exact location. Even the best surveying equipment will still only be able to make an estimate! GPS and surveying is about errors[1], about knowing error vectors and about using them to determine the best estimates. In GPS there is so much to take into account: Atmospherical conditions, Enhancing GPS with ground stations is called differential GPS[2] and is used to get a better estimate of the errors in order to get higher accuracy and a better estimate of the location you are trying to determine. Surveying is a skill. It is a profession that requires mathematical knowledge and algorithms of the highest degree. OpenStreetMap introduces something new (not now, but in the future): If a lot of people are collecting location data, these people will form a "wise crowd"[3]. They introduce errors but since the error vectors will not be homogeneous, will be able to be used to determine the best estimate too. Maybe even better then professional surveying equipment can. surveying problems involve the solution of large meshes of triangles, with hundreds or even thousands of observations. Complex triangulation problems involving real-world observations with errors require the solution of large systems of simultaneous equations to generate solutions.[4] Hope I shed some light. Kind regards, Milo van der Linden [1] http://www.kowoma.de/en/gps/errors.htm [2] http://en.wikipedia.org/wiki/Differential_GPS [3] http://en.wikipedia.org/wiki/The_Wisdom_of_Crowds [4] http://www.survequip.com/surveying-triangulation/ 2011/1/26 <si...@mungewell.org> > > > > > I was under the impression that you still need those reference > > stations for doing surveys, because of those atmospheric conditions > > you mention. > > > > A reference station is really just a receiver that doesn't move and is > well surveyed in. > > You can use a reference station if you have one close (say less than > 20km), use a virtual reference network (normally a bit pricey) or use a > second unit (aka 'base') as the reference station. > > When you are using a pair of receivers your system is really just giving > you the deltas in position, which is why you would place the base above a > survey marker in the pavement/sidewalk. > > You can even get pretty good results using just an L1 receiver, say 5cm > with good antenna/reception but will have a much more limited baseline > length. > > Simon. > > > > _______________________________________________ > talk mailing list > talk@openstreetmap.org > http://lists.openstreetmap.org/listinfo/talk >
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