[mkgmap-dev] Inaccuracy of point coordinates

Thu Feb 19 15:09:30 GMT 2009

On Feb 19, 2009, at 12:38, Paul Ortyl wrote:
> I have noticed, that some ways, which are dead-straight, are not
> presented as such in MapSource and Garmin (etrex Vista HCx).
> It looks like the coordinates have lost its precision.
>
> Take a look at
> http://openstreetmap.org/?lat=48.899634&lon=8.718964&zoom=18&layers=B000FTF
>
> The "double" crossing of Blumenheckstraße and Körnerstraße makes the
> straight Blumenheckstraße being distorted on Garmin.  These two points
> should be in line with the rest of the points on Blumenheckstraße,
> they are not in MapSource.  Some other lines, which are supposed to be
> parallel (they are in josm and mapnik) look really weird in MapSource.

Without further evidence, I assume this is just the low resolution of  
Garmin maps: coordinates are stored in map units, which are 360/2**24  
degrees -- a little over 2m at the equator, less here. Those two nodes  
may well round to the same latitude, hence appear to be on a  
horizontal line.

To test this, you could pass the .osm-file through a conversion to map  
units and back, and see what it looks like in your editor:
x -> y = int(x*2e24/360.0) -> y*360.0/2e24

You might be able to devise some algorithm which displaces nodes  
slightly to keep straight lines closer to straight...

Cheers
Robert