[mkgmap-dev] DEM Resolution and size savings

Sat Mar 31 11:33:44 BST 2018

Yes I know there are still improvements to be done. It was just a
suggestion because the result is much better than saving space/data by
decreasing the dem-dist value. Even resolution 22 as highest value is still
pretty good - but with 22 on 3312 you start to see some very small changes
already. Still way better than 6624 at resolution 24.
Actually with resolution 22 it just looks a little bit flatter but level of
detail still seems to be the same (similar to decreasing the elevation
exageration by 20% in Basecamp). Only at resolution 21 you really start to
miss detail (in general it seems to me that the DEM detail is not that good
in Basecamp - but that also applies to original garmin maps).

Maybe to save size (because right now DEM at resolution 24 can get quite
huge) - there could be an option to have the DEM always saved like this -
so same as 3312 on resolution 22 but at 24....

On 31 March 2018 at 08:40, Gerd Petermann <gpetermann_muenchen at hotmail.com>
wrote:

> 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.
>
> Gerd
>
> ________________________________________
> 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
> _______________________________________________
> mkgmap-dev mailing list
> mkgmap-dev at lists.mkgmap.org.uk
> http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev
>

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