[mkgmap-dev] How to handle missing hgt files or files with voids?

Mon Jan 15 13:25:30 GMT 2018

Hi Henning,

I'll see if I can use srtm.zip.
I think voids are interesting. When I started coding DEM support I searched my PC for *.hgt files and found them in several places.
Later I found out that some of those files have voids and that better data exists (assuming that files without voids have fewer errors)
So, I think it would be good to have a list of those files that have voids.

I'll see what is needed for this.

Gerd

________________________________________
Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Henning Scholland <osm at hscholland.de>
Gesendet: Montag, 15. Januar 2018 12:21:09
An: mkgmap-dev at lists.mkgmap.org.uk
Betreff: Re: [mkgmap-dev] How to handle missing hgt files or files with voids?

This would be the list of all hgt-files available on viewfinder.

In each zip-file of viewfinder there are 6° in E-W direction and 3° in
N-S. The scheme of naming is also simple. That list I also attached.

In general I think it's better to just output a list, which
hgt-files/viefinder-tiles are missing than writing a downloader. I mean
in total it's 13GB of data. If you need to download it manually, you are
more aware of what you are doing and not just download them all.

Regarding voids: I think if there is a possibility the user can fix the
hgt file, A list can be helpful. Otherwise I don't know if a list would
be helpful. Only use case I see is to ask the user to use better data.
But then we need a list, which source is best in which area. And finally
all the different sources need to match each other. Otherwise it will
lead to a hard border between 2 sources. I think this will be much to
complicated...

Maybe for advanced users there can be a Warning message, hgt-file
N00E100.hgt contains 3 voids. So user can compare different sources
manually.

Henning

On 15.01.2018 16:55, Gerd Petermann wrote:
> Hi Henning,
>
> checking data is not that easy, but I'll think about that option.
>
> I'd like to add a short howto that describes ways to get good hgt data, but I don't know any method for Windows users.
> phyghtmap is good, but installation on Win is complex and doc for that looks out-aged.
> Srtm2Osm seems out-dated, as well as Groundtruth.
>
> So, up to now I try to guess what areas I have to download from viewfinder.
> Any hints ?
>
> Gerd
>
> ________________________________________
> Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Henning Scholland <osm at hscholland.de>
> Gesendet: Montag, 15. Januar 2018 09:05:24
> An: Development list for mkgmap
> Betreff: Re: [mkgmap-dev] How to handle missing hgt files or files with voids?
>
> Hi Gerd,
>
> regarding 1) I can create a list of existing tiles of Viewfinder (which covers all land area so far I see). But it will be a long list. I remember it's about 25000 files in total. So maybe it's easier to create a polygon or at least take the list and let software create the polygon. An easier way can be to check if data is available. No data might be sea. Of course it's not 100% sure, as there are deserts and ferries.
>
> Another idea I had during the weekend was to output a osm-file with missing areas. So then user can check. But I came to conclusion, that it's easily visible in the map later. So I'm not sure it's useful at all.
>
> Henning
>
> On 2018-01-15 15:45, Gerd Petermann wrote:
>
> Hi all,
>
> 1) The current code (only) writes a log message with severity WARNING for a missing hgt file:
> "file xxx.hgt not found. Is expected to cover sea."
> I wonder if we can create a list of files that can exist or maybe a polygon that can be tested so that
> tiles which are knwon to cover land are reported as errors.
> When you look at http://viewfinderpanoramas.org/Coverage%20map%20viewfinderpanoramas_org3.htm
> you can see that such a polygon would be quite complex.
> Did anybody already try to define such a polygon or list of files that would cover only ocean?
>
> 2) Should mkgmap print a warning when voids in the hgt file(s) have an influence on the DEM data?
> I think yes, but it is a bit more complex to measure the real effect.
> Reason: We read the 4 hgt values next to the wanted DEM point and use interpolation to calculate the height
> that is stored in the DEM file.
> If one or two of the 4 points are voids they may still have no influence if the wanted point is very close to one that
> is not a void. Another problem is that a single void might be used several times.
> So, another option would be to add an option like --dem-check-hgt which could read all existing hgt files and
> report the number of voids for each. I'd prefer this because it would have no impact on performance for normal
> processing. Or maybe there is already a tool for this that works on all platforms (win,linux,mac) ?
>
> Comments?
>
> Gerd
>
>
>
>
>
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