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lucci56 · #1 (**permalink**) 07-17-2006

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.

**Eric Frost** · #2 (**permalink**) 07-17-2006

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

Chris_Taverner · #3 (**permalink**) 07-18-2006

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

**Winwaed** · #4 (**permalink**) 07-18-2006

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

lucci56 · #5 (**permalink**) 07-18-2006

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.

lucci56 · #6 (**permalink**) 07-18-2006

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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Some concerns about calcpos sub

Some concerns about calcpos sub

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