[mkgmap-dev] help needed

Sat Aug 2 14:41:21 BST 2014

Hi Andrzej,

sorry, I forgot the attachment. The difference to yours is that
I created a gpx track, not a route.
Now I can reproduce your results :-)

Reg. Precision:
"Correct" results are an illusion. Even the most complex algos
will use some simplification (earth is a sphere or ellipsoid, but has no
mountains), 
and when it comes to writing data to the img, we are rounding 
so heavily that almost all information gets lost. 

Some numbers: 
With maps produced by mkgmap, the length of an arc is 
saved in units of 4.8m, means, an arcs is 4.8 or 9.6 or 14.4 or ... meter
long.
The bearing is often encoded with only 4 bits, sometimes with 8 bits, means,
we have to accept an error of at least 0.7 degrees, and the device doesn't 
care very much about these values.

The problem is that we do also complex calculations based
on the calculated values, and here the rounding errors
as well as the simplifications play a role.
We use the results of these complex calcs to decide 
if the line a-b-c can be reduced to a-c because 
the distance between b and a-c is so small that it doesn't
matter for the Garmin algos. 
This means the Garmin program will show the same map
and calculate the same routes and travel times with and 
without point b.

I am now going to find out if Garmin draws the line a-c
(saved in the img) as a great circle or not, depending
on that we have to use different algos.

Do you agree?

Gerd

popej wrote
> Hi Gerd,
> 
>  > I use MapSource 6.16.3 and it doesn't show the great circle line
>  > when I open the attached gpx file.
> 
> I can't see your attachment.
> 
> Mapsoruce draws great circle for direct routes (select "Use Direct 
> Routes" in Preferences). I have marked waypoints for LAX an JFK and then 
> created a route form these points. See gpx attached to my post.
> 
> In my opinion you should go for a correct solution regardless of speed 
> penalty. I would prefer reliable program over a fast one. Later you 
> probably can optimize some computations, where you know for sure, that 
> error is small.
> 
> -- 
> Best regards,
> Andrzej
> 
> _______________________________________________
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> mkgmap-dev at .org

> http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev
> 
> lax-jfk.gpx (3K)
> &lt;http://gis.19327.n5.nabble.com/attachment/5813453/0/lax-jfk.gpx&gt;

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