# [mkgmap-dev] help needed

From GerdP gpetermann_muenchen at hotmail.com on Fri Aug 1 11:16:14 BST 2014

```Hi all,

I think my problem is solved.
JOSM doesn't display the so called great circle which is the shortest line
between two
points on a spere. I am not sure what exactly JOSM displays, but obviously
I can't use it to verify the calculations.

Thanks for the hints!

Gerd

GerdP wrote
> Hi Bill,
>
>> I tried to confirm a couple of things, and ran out of time, but thought
>> the ideas might be of use to you.
>> 1) Are you sure that both the algorithm and JOSM are drawing the same
>> line between LAX and JFK.  For example, one could be using a great
>> circle route, and the other using a direct heading.  This would result
>> in a different distance between A and the line.
>
> I think JOSM does it right.
>
>> 2) Have you tried using a two points for the reference line that are
>> directly North/South of each other, or on the equator as these should
>> result in the reference line being the same regardless of the method of
>> determining it.
>
> Good idea! I thought I did that before, but maybe with wrong values.
> I found that the formula (or my implementation) is wrong.
> Now I can try to find the reason...
>
> Gerd
>
>>
>>    Bill
>>
>> p.s. Thank you, and everyone else who is continuing to improve and
>> support mkgmap.
>>
>>
>> On 07/31/2014 09:41 AM, GerdP wrote:
>> > Hi Andrzej,
>> >
>> > thanks for the hints, I'll have a look at them tomorrow. I think
>> > "Spherical versus ellipsoidal model" as a reason is unlikely, the
>> difference
>> > should
>> > be < 1 %, not ~50%
>> > Today I looked at various webpages which calculate the distance between
>> > two points, e.g. airports. It is quite funny how different the results
>> are,
>> > probably because some are using R=6371 km and others 6378.135 m or
>> > values between.
>> >
>> > Gerd
>> >
>> >
>> >
>> > popej wrote
>> >> Hi Gerd,
>> >>
>> >> I have looked at your example file in QGIS and Global Mapper and they
>> >> show distance like JOSM. I don't know why there is so big difference.
>> My
>> >> guess is that it could be caused by spherical versus ellipsoidal model
>> >> of the Earth. Maybe you could test algorithms based on Vincenty's
>> >> formulae? See for example:
>> >>
>> >>
>> http://www.gavaghan.org/blog/free-source-code/geodesy-library-vincentys-formula-java/
>> >>
>> >> http://geographiclib.sourceforge.net/html/java/
>> >>
>> >> --
>> >> Best regards,
>> >> Andrzej
>> >> _______________________________________________
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>> >> mkgmap-dev at .org
>> >> http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev
>> >
>> >
>> >
>> >
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