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[mkgmap-dev] Incorrect compilation of grid lines

From Felix Hartmann extremecarver at gmail.com on Fri Jul 25 10:12:40 BST 2014

not an idea - but a comment. I think that for such a special nonusual 
case, there should be no degradation for the general use. Wouldn't it be 
enough to just switch off douglas peucker algo for maps where you want 
to include long "straigt" lines?
On 25.07.2014 10:46, 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
> ------------------------------------------------------------------------
> Date: Wed, 25 Jun 2014 12:12:18 +0100
> From: roger at rogercalvert.me.uk
> To: mkgmap-dev at lists.mkgmap.org.uk
> Subject: [mkgmap-dev] Incorrect compilation of grid lines
> I have a file containing the British National Grid lines, as printed 
> on Ordnance Survey maps. This can be found at 
> http://www.rogercalvert.me.uk/osm/GBGrid/GBGrid.zip (this contains two 
> files, they are the same except for the addition of the 'version' tag 
> in the GBGrid06.osm file). I compile this file with mkgmap and overlay 
> it on my GPS map.
> This works correctly with mkgmap 2654 and It displays correctly in JOSM.
> With mkgmap version 3241, the lines are misaligned and out of 
> position. The behaviour is the same on MapSource, BaseCamp and my 
> device. No error messages are produced.
> (I have not used any versions of mkgmap in between these two).
> My style file allocates codes in the 0x10300 range to these lines, but 
> the effect is the same when I use code 0x21 (contour) for all lines.
> Attached are screen dumps for a small part of the map, from JOSM, 
> mkgmap 2654 and mkgmap 3241, and also the batch file I used to compile it.
> I would greatly appreciate any advice.
> Many thanks,
> Roger
> -- 
> ------------------------------------------------------------------------
> Roger Calvert
> http://www.rogercalvert.me.uk
> ------------------------------------------------------------------------
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