[mkgmap-dev] HGT - getElevation()

Thu Jan 18 06:03:31 GMT 2018

Hi Andrzej,

these are good hints. I already wondered why upper left corner is not the same at different zoom levels.

I'll try to change the code to match these conditions, maybe this solves the problem with "dem movement".

Gerd

________________________________________
Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Andrzej Popowski <popej at poczta.onet.pl>
Gesendet: Donnerstag, 18. Januar 2018 03:08:48
An: mkgmap-dev at lists.mkgmap.org.uk
Betreff: Re: [mkgmap-dev] HGT - getElevation()

Hi Gerd,

I have looked a bit at Garmin's DEM and I have some guesses.

* Coordinate resolution for DEM is 28 bits. Since it is shifted to the
left, 4 lower bits are always 0.

* The same goes for pixel distance, lower 4 bits should be 0. This is
the reason, why Garmin uses 3312 (CF0) instead of 3314 (CF2).

* Coordinates of left top corner for each DEM layer are always exact
multiple of pixel distance. Since layers get different distances, each
layer get a bit different coordinates too.

I would expect, that aligning of HGT in mkgmap should at least support
3-rd requirement, but it is not the case. I don't know why. Differences
looks too big to be explained by rounding errors.

--
Best regards,
Andrzej
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