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[mkgmap-dev] DEM Resolution and size savings

From Gerd Petermann gpetermann_muenchen at hotmail.com on Sat Mar 31 07:40:09 BST 2018

Hi Felix,

yes, the DEM format is not yet fully understood. I assume what you have is a map that uses a shrink factor <> 1.
The shrink factor is used like this:
The height deltas are devided by this value before encoding and multiplied when extracting. The effect is that the deltas
are smaller and therefore the size is also smaller, but of course you also lose a bit of information, because only the integer
part is stored.
The problem is that Garmin also uses slightly different rules for the encoder, and we did not yet find out all details.
Frank Stinners program BuildDEMFile allows to use this but sometimes produces invalid data.
The tool DemDisplay shows my current knowledge.


Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Felix Hartmann <extremecarver at gmail.com>
Gesendet: Freitag, 30. März 2018 23:07:10
An: Development list for mkgmap
Betreff: [mkgmap-dev] DEM Resolution and size savings

Hi everyone,

I noticed that the DEM layer if created for resolution 23 only (with a map that has not 24 resolution) will only be half the size of the DEM in resolution 24 (dem-dist=3312) - however in Basecamp/Mapsource the detail is virtually identical - I cannot see any difference in quality.

So I think there must be some way to still save a lot of data/space - but it's not by going for dem-dits=6624 - that will result in much worse DEM detail.

(I still really haven't found a good solution for DEM on GPS devices though. Need more time trying out different values and possibilities. Right now I think best is probably a separate transparent but except for DEM empty DEM only gmapsupp.img).

Felix Hartman - Openmtbmap.org & VeloMap.org
Schusterbergweg 32/8
6020 Innsbruck
Austria - Österreich

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