>> But an even bigger error can be caused by using different projections. I >> forget which one OSM uses, but using different projections can move a given >> point 20m quite easily, and a survey marker may well be on a different one.
> Can we get a definitive answer on this? I think the terminology is a little confused. The projection is the way that a the 3D shape is projected onto our 2D representation. It shouldn't "move" a given point on an elipsoid, but the 2D representation of it may be quite different. That really shouldn't effect what we are doing here. Perhaps we are talking about the conversion from AGD to GDA. Last time I looked there were still some maps of some areas using the old datum, which can result in this kind of separation. Just separated enough to make you think you know what you are doing, and at the same time get very lost. However, I very much doubt that is the case here. I think the survey mark is using the GDA. As to how accurate the location of the survey marks is, I have no idea. I think it would depend on the technology available at the time the mark was last surveyed. Lets face it, once we are talking about 2m errors, the sources are numerous, from parallax on the photo, to surveying errors, to GPS errors. Ian. _______________________________________________ Talk-au mailing list [email protected] http://lists.openstreetmap.org/listinfo/talk-au
