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Some concerns about calcpos sub

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 ...

  1. #1
    lucci56 is offline Junior Member White Belt
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    Some concerns about calcpos sub

    i am new to this forum and I noticed that there is a calpos routine going around . Having looked at the code, I have some concern. It seems that the program is assuming the earth is spherical , ie constant radius. MP is clear about actually using a WGS84 ellipsoid. The error then can easily reach several hundred of meters if using a spherical assumption. Anybody has concerns about that or I am actually wrong? Has anybody checked how MP actually computes distances between 2 points? Thanks.

  2. #2
    Eric Frost's Avatar
    Eric Frost is offline Administrator Black Belt
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    I have heard people talk about this as a potential problem but in practice it has been OK.

    Also, take a look at MapPoint 2006, it now has lat/lon as properties you can use and it might take care of any issues?

    Eric
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  3. #3
    Chris_Taverner is offline Junior Member White Belt
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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

  4. #4
    Winwaed's Avatar
    Winwaed is offline Mapping-Tools.com Black Belt
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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
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  5. #5
    lucci56 is offline Junior Member White Belt
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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.

  6. #6
    lucci56 is offline Junior Member White Belt
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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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