[mkgmap-dev] HGT - getElevation()

Wed Jan 10 12:58:18 GMT 2018

Hi Henning,

hgt files contain voids, and we don't write 0 height for such a void, instead we write a value that
is considered to be invalid. When you hover over such a point MapSource doesn't display a height.
I did not yet try to find out how this influences the ele values in calculated gpx tracks.
I assume that a gpx point without ele attribute is created.

Gerd

________________________________________
Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Henning Scholland <osm at hscholland.de>
Gesendet: Mittwoch, 10. Januar 2018 13:48:11
An: mkgmap-dev at lists.mkgmap.org.uk
Betreff: Re: [mkgmap-dev] HGT - getElevation()

On 10.01.2018 20:33, Andrzej Popowski wrote:
> > i would be very defensive with interpolation in the case of
> > missing values.
>
> I got the same feeling. The best way to fill voids is to process whole
> HGT. And I believe that DEM providers already have done it, so any
> attempt at simple extrapolation would rather add errors than make
> output better. In my code I preserve voids. If requested coordinates
> are near void HGT node, then returned value is void.
>
Hi Andrzej,

>From user perspective, I think it's more correct to somehow fill voids
based of surrounding data than using elevation = 0. Elevation=0 I would
only use as a fall back, if whole hgt-file is missing. I agree with you,
that we shouldn't spent a lot of code for guessing.

To have a simple example: If there would be a cone-shape mountain in
reality, but in DEM the top of the cone is missing, I would suggest to
fill the hole with same elevation as the highest circle. So result in
DEM would be a truncated cone shape. Of course reality is not that
simple. :'( ;-)

Henning

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