[mkgmap-dev] DEM: performance with bicubic interpolation

Tue Jan 23 14:05:07 GMT 2018

Hi Henning,
yes, we have to scale hgt resolution to garmin resolution, this is not about voids.

We may use the same algo to fill voids. I think this link is quite interesting:
https://web.archive.org/web/20051024202307/http://www.geovista.psu.edu/sites/geocomp99/Gc99/082/gc_082.htm

Gerd

________________________________________
Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Henning Scholland <osm at hscholland.de>
Gesendet: Dienstag, 23. Januar 2018 14:09:17
An: mkgmap-dev at lists.mkgmap.org.uk
Betreff: Re: [mkgmap-dev] DEM: performance with bicubic interpolation

For srtm I found elevation accuracy of 6m in wikipedia, so my 10m was not that unrealistic.

Btw. We are talking about interpolating from hgt-data position to Garmin-data position, aren't we?

For filling voids, it would be a good idea to use some spline interpolation as written here:
https://en.wikipedia.org/wiki/Shuttle_Radar_Topography_Mission#Void-filled_SRTM_datasets

Henning

On 23.01.2018 20:59, Henning Scholland wrote:

Hi Andrzej,

I also suggest to make interpolation optional.

So far I don't understand your argument. I agree, compilation time is
not the only criteria. The question is, what is the benefit for the user.
For example: If accuracy of srtm is +- 10m and the difference between
with/without interpolation is +-1m, then it's definitely not worth
spending any effort on interpolation. Only in cases, where interpolation
accuracy is on same level than srtm accuracy, it starting to be worth
spending time for it.

Do you have any values for differences with/without interpolation with
same input data?

Henning

On 23.01.2018 20:22, Andrzej Popowski wrote:

Hi Gerd,

there are different kind of bicubic interpolations. I'm not good at
this math, I think the previous version was actually bicubic spline
interpolation. See other possibilities here:
http://mrl.nyu.edu/~perlin/cubic/Cubic_java.html

I don't know, which type of spline or cubic interpolation is best for
DEM. Maybe differences aren't big, but I think it is better to have
good interpolation than fast one. You compile a map once but then many
people can use it.

It shouldn't be difficult to include an option like:
--dem-interp=..

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