[mkgmap-dev] Please help with news about new DEM options

Wed Jan 31 13:31:34 GMT 2018

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 at lists.mkgmap.org.uk> im Auftrag von osm at pinns <osm at pinns.co.uk>
> Gesendet: Mittwoch, 31. Januar 2018 13:32
> An: mkgmap-dev at 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 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
>> 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
>>
>> [1] http://www.mkgmap.org.uk/
>> _______________________________________________
>> mkgmap-dev mailing list
>> mkgmap-dev at lists.mkgmap.org.uk
>> http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev
>> _______________________________________________
>> mkgmap-dev mailing list
>> mkgmap-dev at lists.mkgmap.org.uk
>> http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev
> _______________________________________________
> mkgmap-dev mailing list
> mkgmap-dev at lists.mkgmap.org.uk
> http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev
> _______________________________________________
> mkgmap-dev mailing list
> mkgmap-dev at lists.mkgmap.org.uk
> http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev