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<body class='hmmessage'><div dir='ltr'>Hi Roger,<br><br>the problem is caused by the routine that removes obsolete points from straigt lines. <br>This routine doesn't care (enough) about the spherical distortian.<br>The routine uses herons formula to "calculate distance of point in the middle to line c1,c2".<br>This formula is for 2D, but used to work good enough with the rather short ways <br>we have in OSM. (The formula is also used in the Douglas-Peucker filter).<br>With long lines the as in your data, the errors are too big and the routine returns garbage.<br><br>If I got that right, we have to use the formula for the "Cross-track distance" as described here:<br>http://www.movable-type.co.uk/scripts/latlong.html<br> <br>Unfortunately, this routine uses more trigonometrical calculations, so it will be slower.<br><br>@Programmers: Any other ideas?<br><br>Gerd<br><br><div><hr id="stopSpelling">Date: Wed, 25 Jun 2014 12:12:18 +0100<br>From: roger@rogercalvert.me.uk<br>To: mkgmap-dev@lists.mkgmap.org.uk<br>Subject: [mkgmap-dev] Incorrect compilation of grid lines<br><br>
I have a file containing the British National Grid lines, as printed
on Ordnance Survey maps. This can be found at <a href="http://www.rogercalvert.me.uk/osm/GBGrid/GBGrid.zip" target="_blank">http://www.rogercalvert.me.uk/osm/GBGrid/GBGrid.zip</a>
(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.<br>
<br>
This works correctly with mkgmap 2654 and It displays correctly in
JOSM.<br>
<br>
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.<br>
<br>
(I have not used any versions of mkgmap in between these two). <br>
<br>
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.<br>
<br>
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.<br>
<br>
I would greatly appreciate any advice.<br>
<br>
Many thanks,<br>
<br>
Roger<br>
<div class="ecxmoz-signature">-- <br>
<hr style="width:50%;"><br>
Roger Calvert<br>
<a href="http://www.rogercalvert.me.uk" target="_blank">http://www.rogercalvert.me.uk</a><br>
<hr style="width:50%;"></div>
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