From Gerd Petermann gpetermann_muenchen at hotmail.com on Wed Jan 31 11:03:14 GMT 2018

```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
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 at lists.mkgmap.org.uk> im Auftrag von lig fietser <ligfietser at hotmail.com>
Gesendet: Mittwoch, 31. Januar 2018 09:48
An: mkgmap-dev at lists.mkgmap.org.uk

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

[1] http://www.mkgmap.org.uk/
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