Thanks Gerd One certainly gets more of the lumps and bumps with a distance of 1664 Nick On 31/01/2018 13:20, Gerd Petermann wrote:
Hi Nick,
well, Andrzej reported that this oversampling might improve precision. My understanding is that it reduces the error that is added by interpolation.
For sure it adds a lot of bytes to the img file, so one has to find out what is more imprtant.
Gerd
________________________________________ Von: mkgmap-dev <mkgmap-dev-bounces@lists.mkgmap.org.uk> im Auftrag von osm@pinns <osm@pinns.co.uk> Gesendet: Mittwoch, 31. Januar 2018 13:32 An: mkgmap-dev@lists.mkgmap.org.uk Betreff: Re: [mkgmap-dev] Please help with news about new DEM options
Hi Gerd
Thanks for the detailed explanations.
' if you want more precision and map size doesn't matter half of it'
I'm not quite sure how halving these values produce greater precision for resolution 24.
Interestingly , you mention 1664 .
One of my topos has 1648 as in
1648,3312,13248,53024
Nick
On 31/01/2018 11:03, Gerd Petermann wrote:
Hi Minko,
I'd also like to know a good rule how to calculate the dem-dists values.
Again the basic formulars: Typical hgt formats use a raster with either 1200 (3'') or 3600 (1'') points/degree as resolution. The corresponding resolutions for DEM are 2^32/(1200*360) ~ 9942 or 2^32(3600 * 360) ~ 3314
It is quite obvious that the first value in dem-dists depends on the resolution of the input hgt file(s). My understanding is that you might use a value near the hgt resolution or maybe - if you want more precision and map size doesn't matter - half of it, so 3'' hgt means dem-dist ~ 9942 (or 9942/2 = 4971) 1'' hgt means dem-dist ~ 3314 (or 3314/2 = 1657) Since mkgmap now rounds these values to multiples of 16 you get 2^32/(1200*360) ~ 9942 -> 9936 rounded 2^32/(1200*360) / 2 ~ 4971 -> 4976 rounded 2^32/(3600*360) ~ 3314 -> 3312 rounded 2^32/(3600*360) / 2 ~ 1657 -> 1664 rounded
These are the values for resolution 24. What still seems unclear is how you calculate the values for lower resolutions. Up to now I see no reason to duplicate data with e.g. dem-dists=3312,3312,... but I see this in Arndts "Speichenkarte" which uses resolution 23 for level 1.
I tried some different values on my Oregon 600 and I think the hill shading is not very useful on the device. Maybe that's the reason why the setting Map->Advanced Setup->Shaded relief->Auto means that hill shading is not used unless you zoom out to 5km?
I also tried with only one or two DEM levels, e.g. --dem-dists=1664,3312 or just --dem-dists=3312 It works, the device just uses the DEM from the basemap when you zoom out to e.g. 500 m. The problem: The basemap has a very poor DEM resolution, so it looks wrong when hills suddenly disappear while zooming out.
My current compromise for high precision with 1'' hgt input and default style: 1664,9936,26512,53024
Gerd
________________________________________ Von: mkgmap-dev <mkgmap-dev-bounces@lists.mkgmap.org.uk> im Auftrag von lig fietser <ligfietser@hotmail.com> Gesendet: Mittwoch, 31. Januar 2018 09:48 An: mkgmap-dev@lists.mkgmap.org.uk Betreff: Re: [mkgmap-dev] Please help with news about new DEM options
Yes, it would be helpful if we mention where we can get the hgt files, a few examples of the dem-dist values and an explanation how to calculate it. This information is already there but mentioned in the many updates of the branch versions.
I use http://www.javawa.nl/srtm/index.php?lang=nl because it has a good overview map, the disadvantage is that you need to download every single tile manually and not in one big download. Maybe there are better alternatives?
---------------------
Gerd wrote:
Hi all,
with r4093 I've merged the dem-tdb branch into trunk. I think it would be good to mention this in the "Latest news" on [1] , maybe with one or two screen shots, but I have no idea how to write this "from a users view". Do we need a how-to that describes the sources for hgt files and maybe more about the new options? It would be great if somebody could help with that.
Gerd
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