[mkgmap-dev] splitter r585 implements new option align-for-dem

Thu Jan 18 14:49:05 GMT 2018

Hi Henning,

this is not surprising for me. Tiles should be a bit different, if you look at them in JOSM you have to zoom to maybe 30 m.

Why does the size grow?
The effect of the alignment is higher precision, means, less interpolation. With interpolation, the values are averaged, so
deltas are smaller and thus the number of bits to store the deltas are smaller.
So, if you already used 9942 as dem-dist for level 0 before the only improvement is precision.
Other users use e.g. dem-dists=3312,3312,... and for those I would also expect a decrease in size.

OK?
Gerd

________________________________________
Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Henning Scholland <osm at hscholland.de>
Gesendet: Donnerstag, 18. Januar 2018 15:33:56
An: mkgmap-dev at lists.mkgmap.org.uk
Betreff: Re: [mkgmap-dev] splitter r585 implements new option align-for-dem

Hi Gerd,
I have quite bad news.

My map of China increased size about 300 mb (before 1.6 Gb, now 1.9Gb)
just by following this suggestion adding --align-for-dem=1200.

hgt-files are same, osm-data is same. bounding polygon for splitter is
same and also tiles look similar. I was using latest mkgmap (4070) for
both tests and
x-dem-dist=9942,9942,9942,19884,39768
x-overview-dem-dist=79536

South east asia increased from 1.15 to 1.4 Gb with same work flow.

I'm sorry :-(. If you need anything or if I should test something,
please let me know.

Henning

On 18.01.2018 01:42, Gerd Petermann wrote:
> Hi all,
>
> see http://www.mkgmap.org.uk/websvn/revision.php?repname=splitter&rev=585
>
> If I got that right you can use dem-dist 9942 for level 0 if hgt files are in 3'' resolution and splitter
> was executed with --align-for-dem=1200.
> For hgt 1'' files use dem-dist 3342 for level 0 and --align-for-dem=3600.
>
> This should reduce DEM size a lot AND increase precision.
>
> Gerd
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