[mkgmap-dev] Garmin uses overlapping tiles

Wed Feb 7 09:07:35 GMT 2018

Hi experts,

this is quite complex. My current understanding is this:
1) Garmin maps can have overlapping areas in TRE, boundaries are stored with 24 bit resolution rounded to
either multiples of 2 or 4. Overlaps are also either 2 or 4. In current mkgmap maps there is no overlap.
2) The *.tdb file in Garmin maps describes areas that do not overlap. The boundaries are stored in 32 bit resolution,
probably not rounded.
3) The DEM areas in Garmin maps overlap even the TRE areas and only the position of the upper left
corner is explicitly stored in 32 bit resolution, and is always a multiple of the dem-dist. The position of the
lower right corner could be calculated but I assume that it is not important.

Changing the boundaries in TRE has an effect reg. the crash.
Changing the position of the upper left corner boundaries in DEM has an effect reg. the crash.
Changing the position of the lower right corner and therefore the overlap has an effect reg. the crash, probably
the current code in mkgmap is off by one in both directions (too small).

The current code in mkgmap calculates the *.tdb file boundaries from the TRE boundaries.
This might cause problems when I change the code to write TRE boundaries that overlap.

Maybe we have to align tiles to certain multiples, maybe it is sufficient to align the TRE boundaries to certain
values.
I still did not find a working rule. The TRE bounds may depend on the DEM dist value or on certain prefered values,
maybe there is a flag in the DEM or TRE header that has to match.
Maybe the DEM bounds depend on the TRE bounds, maybe the *.tdb file also is important here.

Please let me know if you find different results in routable Garmin maps with DEM.

Gerd

________________________________________
Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Gerd Petermann <gpetermann_muenchen at hotmail.com>
Gesendet: Dienstag, 6. Februar 2018 16:51
An: Development list for mkgmap
Betreff: Re: [mkgmap-dev] Garmin uses overlapping tiles

Hi Andrzej,

changing the TRE bounds helps sometimes, but not always :-(

@Frank: All Garmin TRE files that I looked at where aligned to multiples of 4 map units.
I tried this as well as aligning to multiples of 2, nothing worked without crashes so far.

Anyhow, it seems to be the right thing to look at.

Gerd

________________________________________
Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Andrzej Popowski <popej at poczta.onet.pl>
Gesendet: Dienstag, 6. Februar 2018 15:49
An: mkgmap-dev at lists.mkgmap.org.uk
Betreff: Re: [mkgmap-dev] Garmin uses overlapping tiles

Hi Gerd,

I guess basemap is overview map. 0x4A objects are derived form
backgrounds of detailed tiles, but with resolution of overview map, it
would be difficult to guess, if background overlap or not.

I hope extending values written to TRE will work.

--
Best regards,
Andrzej
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