[mkgmap-dev] Garmin uses overlapping tiles

Tue Feb 6 10:45:55 GMT 2018

Hi all,

so far I've never seen the crash in MapSource using the Garmin maps, although they also have
tiles with "invalid heights" and therefore the tile headers have the "Extra flag".

my idea: 
- use mkgmap to create DEM data for exactly the same two areas as in a Garmin map [1]
- Try to reproduce the crash with those two areas 
- replace Garmin DEM by mkgmap DEM and see what happens in MapSource

The interesting point: Garmin tiles overlap. I've created this areas.list from the information 
in two Garmin TRE files:

00000029: 2174824,745652 to 2190360,768956
# 46,6667,16,0000 47,0000,16,5000 
00000030: 2174824,768952 to 2190360,792260 
# 46,6667,16,4999 47,0000,17,0001

Note that  the first tile has eastern border at 768956 while the 2nd uses 768952 for the western border.

When I use exactly those values I cannot reproduce the crash.
When I move the boundary of  one of the two tiles so that they are equal I can reproduce the crash,
but not always with the same routes. I tried 768952, 768954, 768956.
So, I tried to change mkgmap so that it calculates DEM as if the tile boundaries are overlapping,
but with TRE where tiles have same 768956 --> crash possible.

The next interesting point is that the 0x4a polygons of these two tiles do NOT overlap.
They seem to share  768896 (!).
So, I used a new   areas.list with this value and still see the crash.

My conclusion:
We have to modify mkgmap to write a different TRE file, probably it has to cover the DEM area as well.
Does that make sense?

Gerd

[1] TopoGuide Hungary 2.12c 