[mkgmap-dev] misshapen roundabouts

Thu Jul 18 08:25:58 BST 2013

I think I finally found what the problem is: the garmin devices can't 
handle the precise data I want them to display, look at these two quotes:

1. from http://www.cgpsmapper.com/download/GM8DocV2.pdf

When declaring the location of objects in the world, the highest 
precision available uses 24 bits of resolution, which resolves to an 
accuracy of about 2.5 meters.

2. in 'Garmin IMG File Format' by John Mechalas:

The reference coordinates in the TRE section are given as 24-bit 
integers (three bytes). This implies that the maximum granularity for 
map elements in the image file is approximately .0000214576721 degrees 
of latitude or longitude. At the equator, a degree of latitude is 
roughly 365228.16 feet, making the highest resolution for map points 
about 7.8369461 feet.

So even if we have precise coordinates in OSM, in the garmin image they 
will be rounded to the nearest(?) grid point. In fact, everything on the 
garmin map looks like it's been snapped-to-grid, if you look at it closely.

Sorry for the noise; maybe my posting is still helpful for others facing 
the same problem.

Kay