[mkgmap-dev] HGT - getElevation()

Wed Jan 10 18:40:35 GMT 2018

Am 10.01.2018 um 18:15 schrieb Gerd Petermann:
> Hi Peter,
> 
> I think the only alternative to interpolation is to use the height of the nearest hgt point for a given DEM point.

That is my theory, but I can't prove ....

lg Peter

> One may minimize this distance by chosing proper dem-dist values and by aligning the tiles to the raster
> given by hgt. Example:
> If the upper left corner of your img file is at lat 51.0 , lon 10.0 and dem-dist is 2^32/(360*3600) ~ 3314,
> the DEM points will be close to the hgt points in file N51E010.hgt if that is in 1'' resolution.
> 
> Gerd
> 
> ________________________________________
> Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Peter Danninger <peter at danninger.eu>
> Gesendet: Mittwoch, 10. Januar 2018 17:53:46
> An: mkgmap-dev at lists.mkgmap.org.uk
> Betreff: Re: [mkgmap-dev] HGT - getElevation()
> 
> I don't think rounding or interpolating
> will deliver any better results.
> If you have 1" .hgt files, you get
> an altitude value every 30m.
> With 3" .hgt files much worse, every 90m.
> In the mountains, between 2 points
> may be a big wall with some 100m
> height-difference.
> So if you have small deviations in lat/lon,
> you will evtl. get very false elevation values.
> 
> If you interpolate altitude-values,
> you calculate wrong values by design :-(
> 
> May be I'm wrong, but this is my theory.
> 
> lg Peter
> 
> 
> Am 10.01.2018 um 17:39 schrieb Gerd Petermann:
>> Hi Andrzej,
>>
>> if you want to try this:
>> you can easily change the code in interpolatedHeight to return the height in feet, just make sure that you don't convert UNDEF.
>>
>> The only other change that is needed is  in this line in DemHeader:
>> writer.putInt(0); // 0: elevation in metres, 1: foot
>>
>> well, the comment should say feet , not foot
>>
>> Gerd
>>
>> ________________________________________
>> Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Frank Stinner <Frank.Stinner at kabelmail.de>
>> Gesendet: Mittwoch, 10. Januar 2018 17:27:22
>> An: mkgmap-dev at lists.mkgmap.org.uk
>> Betreff: Re: [mkgmap-dev] HGT - getElevation()
>>
>> Hi Gerd and Andrzej,
>>
>> i think "overkill" is a good word.
>>
>> For algorithm that'a only numbers, it does not matter. The questions is, how exact are the hgt-values. We don't know that, but i don't believe it is
>> +-1m or so. I'm not wondering, when the hgt's have +-5m or +-10m. The copernicus-data have +-7m and i don't believe the technic was worse.
>>
>> That's why feets are overkill. The numbers are 3 times greater, that's why the dem's are greater. It's not worth it.
>>
>>
>> Frank
>>
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