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This is a discussion on Some concerns about calcpos sub within the MapPoint Desktop Discussion forums, part of the Map Forums category; i am new to this forum and I noticed that there is a calpos routine going around . Having looked ...
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| Some concerns about calcpos sub |
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| Calcpos Concerns
Hi all, I have used the calcpos routine to calculate hundreds of thousands of map pin positions. I then use the results to recreate the pins later. The accuracy I have found to be within just a few metres so, even though you may be justified in your observation, in practice and using MP2004 this routine works exceptionally well. Cheers Chris
__________________ VB6 and Mappoint Programmer specialising in Vehicle Telemtry and monitoring Systems |
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I can confirm that it works well. MP will be using WGS84 for its distance calculations, and I suspect this results in very small errors. (you are correct that if you use the wrong geoid for plotting then errors of 100m or so are probable). About 12 months ago, I had to write some code that performed extensive geometry calculations. I foget the details now, but I did have to make the decision "spherical Earth" or "elliptical Earth". When I got as far as estimating the differences it was a matter of metres within distances of 100km+. Richard
__________________ Winwaed Software Technology LLC http://www.winwaed.com See http://www.mapping-tools.com for MapPoint Tools See the Geoweb Guru for online mapping |
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Thanks for all your comments. I am still amazed about the accuracy. May be MP is actually using the same assumption to calculate distances from lat long which would explain why it's working so well. I will check on a few points and let you know. |
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Hi again, I have checked. Actually MP is using the spherical assumption to calculate distances (to make sure just get distances for points at a same longitude and realize that they are proportional to the angle in radian). The radius seems to be 6378200m .This explains why calcpos actually works well. However, as a consequence distance calculations have a certain error. It's about 15 m by tenth of degrees for mid latitudes which I guess is bearable for most needs (around 0.27%). It would be interesting to know what microsoft has to say about this -not much advertised- approximation in the calculation of distances... |
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