[mkgmap-dev] Incorrect compilation of grid lines

Fri Jul 25 18:31:20 BST 2014

Gerd,

Thanks for diagnosing this.

It would be possible for me to re-structure the file so that lines are 
no longer than (say) 10km. This would increase the source (and 
presumably .img)  file size somewhat, but should be handleable. I would 
need to do the same for my latitude/longitude grid generator. (This 
might also help with a rather different problem I have had with Osmand, 
which does not always show labels on the lines within the screen area - 
if the lines were much shorter, the labels would be more likely to 
appear on screen.)

What maximum length of line would produce sensible results?

Of course, this would not solve the problem for other lines such as 
contour lines, mentioned by Felix. Perhaps automatic detection of 
excessively long lines, with some default action, combined with a 
warning, would be appropriate. This would give the user the chance to 
find and perhaps change the offending data.

Thanks again,

Roger

On 25/07/2014 09:46, Gerd Petermann wrote:
> Hi Roger,
>
> the problem is caused by the routine that removes obsolete points from 
> straigt lines.
> This routine doesn't care (enough) about the spherical distortian.
> The routine uses herons formula to "calculate distance of point in the 
> middle to line c1,c2".
> This formula is for 2D, but used to work good enough with the rather 
> short ways
> we have in OSM. (The formula is also used in the Douglas-Peucker filter).
> With long lines the as in your data, the errors are too big and the 
> routine returns garbage.
>
> If I got that right, we have to use the formula for the "Cross-track 
> distance" as described here:
> http://www.movable-type.co.uk/scripts/latlong.html
>
> Unfortunately, this routine uses more trigonometrical calculations, so 
> it will be slower.
>
> @Programmers: Any other ideas?
>
> Gerd
>
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-- 
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Roger Calvert
http://www.rogercalvert.me.uk
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