[mkgmap-dev] DEM Resolution and size savings

Sat Mar 31 12:00:05 BST 2018

Hi Felix,

Sorry, I don't understand how you connect resolution and DEM. Can you explain this more detailed?

Gerd

________________________________________
Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Felix Hartmann <extremecarver at gmail.com>
Gesendet: Samstag, 31. März 2018 12:33:44
An: Development list for mkgmap
Betreff: Re: [mkgmap-dev] DEM Resolution and size savings

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<mailto: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<mailto:mkgmap-dev-bounces at lists.mkgmap.org.uk>> im Auftrag von Felix Hartmann <extremecarver at gmail.com<mailto: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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--
Felix Hartman - Openmtbmap.org & VeloMap.org
Schusterbergweg 32/8
6020 Innsbruck
Austria - Österreich