[mkgmap-dev] HGT - getElevation()

Thu Jan 18 02:08:48 GMT 2018

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