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[mkgmap-dev] DEM File Format and mkgmap

From Gerd Petermann gpetermann_muenchen at hotmail.com on Sat Dec 16 09:57:35 GMT 2017

Hi all,

FYI: I have started to add code in the dem-tdb branch which was created by Steve.
Up to now the code in the branch is completely experimental and it doesn't write anything to the img files.

At the moment I am looking at Franks code to understand how to fill the DEM header structures based on the (knwon) information in TRE,
next I have to write the code to read and transform the hgt files, this looks rather simple compaired to the DEM structures.
One open question is if splitter has to be changed so that it doesn't calculate tiles which are too large for DEM.

Maybe we have something useful in mkgmap till the end of the year...


Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von osm at pinns <osm at pinns.co.uk>
Gesendet: Freitag, 15. Dezember 2017 20:34:39
An: mkgmap-dev at lists.mkgmap.org.uk
Betreff: Re: [mkgmap-dev] DEM File Format and mkgmap

Hi Andreas

I have been able to make it work (tapping on the arrows) on my Oregon 600 but its very sluggish and at one stage it just freezes.

I think you are right ; there is an issue.

On 15/12/2017 19:30, Andreas Schmidt wrote:
Hi Frank,

with ypur great tool I am able to

- have3d - View in basecamp
- hillshading on Oregon600

Thats really amazing !

In Oregon there is also a menu item „3D“. Even if the map Shows Up hillshading,, this function doesnt work. Oregon complains that the map does Not have enough hight information.

Could you make it work on your device?


Gesendet mit der Telekom Mail App<http://www.t-online.de/service/redir/emailmobilapp_ios_smartphone_footerlink.htm>

Von: Frank Stinner <frank.stinner at leipzig.de><mailto:frank.stinner at leipzig.de>
Betreff: Re: [mkgmap-dev] DEM File Format and mkgmap
Datum: 08.12.2017, 09:27 Uhr
An: mkgmap-dev at lists.mkgmap.org.uk<mailto:mkgmap-dev at lists.mkgmap.org.uk>

i must confess, i have never tried to decompilea garmin dem map. I have only playing with bit-patterns and then lookingfor the result in mapsource. But i'm sure, that we have different algorithmsor one algorithm with different special cases.

My description in the pdf is (only) validfor the „TOPO Deutschland v3“. Please see the pages 3 and 4. There area few "unknown values".

For example see "Codierungstyp"(coding type) for a 64x64 tile. I found in different garmin maps valuesfrom 0 to 6. 0 is the standard. It seems to be, that the coding type haveno influence of the algorithm, but i'm not sure. Presumable is this typeonly important for different interpretations of the height values. BuildDEMFileuse type 1 for areas with "unknown height". This is in this casethe interpretation for height=maxvalue.

Unknown is also the byte on position 0x25in the dem header. In the „TOPO Deutschland v3“ there we have 0x1 buti have also seen in other maps 0x0.

Interesting is also the word in 0x12 in evereyzoomlevel definition (see page 26 in the pdf). In the „TOPO Deutschlandv3“ thats all 0. But i have also found 0x100, 0x200, 0x400. Perhaps akind of multiplier for heights?

Very important is the bytes 0x0 (and 0x1)in everey zoomlevel definition. In the „TOPO Deutschland v3“ 0x0 is ever0. 0x1 seems to be a number as "link" to a maplevel. But i don'treally understand the sense ot that. We can have 6 maplevels and we cansay the maplevel 2 and 4 are without dem's?

In a little demo map from garmin i have see"twins" of zoomlevelnumbers, one with value 0 on 0x0 and onewith 1 on 0x0. Perhaps 0x0 is for different and optimized output on pcand gps?

I think, a decompiler with the algo frommy pdf is working only for this special case:
0x15 in dem header should be 0 (meter)
0x25 in dem header should be 1
0x0 and 0x12 in every zoomlevel should 0

By the way, have anybody see a dem with valuesin foot? The values in such a dem should be greater then for a dem in meter.Have anybody height values with a precision of foot? The conversion fromfoot to meter would be a first compression step. If garmin really use "foot",then a different/better compression algo make sense.


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