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[mkgmap-dev] "Small" SRTM1 files and DEM maps

From Carlos Dávila carlos at alternativaslibres.org on Fri Apr 17 09:32:24 BST 2020 

I also searched for info about this format before posting to the list, 
but could not find anything either:-(

El 17/4/20 a las 6:19, Gerd Petermann escribió:
> Hi all,
>
> I could not find any information about this special format. Would be great if someone could point me to a detailed documentation.
>
> Gerd
>
> ________________________________________
> Von: Gerd Petermann <gpetermann_muenchen at hotmail.com>
> Gesendet: Donnerstag, 16. April 2020 18:06
> An: Carlos Dávila
> Betreff: AW: [mkgmap-dev] "Small" SRTM1 files and DEM maps
>
> Hi Carlos,
>
> the hgt files are very simple. They contain an array of n * n 16-bit values, nothing else. The 16-bit value gives the elevation, and the file name gives the position of the southwest corner, see https://stackoverflow.com/questions/357415/how-to-read-nasa-hgt-binary-files
> Maybe the 12.4 MB files use 8-bit values for the elevation, but 12.970.802 / 3601 gives 3602, so there is probably an extra byte giving a delta value.
> Coud not find a desription of this format so far. I'll try to find out more.
>
> Gerd
>
> ________________________________________
> Von: Carlos Dávila <carlos at alternativaslibres.org>
> Gesendet: Donnerstag, 16. April 2020 17:39
> An: Gerd Petermann
> Betreff: Re: [mkgmap-dev] "Small" SRTM1 files and DEM maps
>
> HI Gerd
> I don't think conversion is wrong. If input geotiff file is 24MB, output
> hgt file is 24 MB, if input tiff is 12MB, output hgt is 12MB
> I use this command on Linux:
> for i in *.tif ; do gdal_translate -strict -q -eco -of SRTMHGT $i
> `basename $i tif`hgt ; done ; rm -f *.hgt.aux.xml
> BTW, gdal is also able to deal with small SRTM files
>
> El 16/4/20 a las 11:31, Gerd Petermann escribió:
>> Hi Carlos,
>>
>> so your conversion routine is wrong. The file size is 12.970.802 bytes. There is no integer n for which 2 * n^2 results in this value.
>>
>> Gerd
>>
>> ________________________________________
>> Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Carlos Dávila <cdavilam at orangecorreo.es>
>> Gesendet: Donnerstag, 16. April 2020 11:19
>> An: mkgmap-dev at lists.mkgmap.org.uk
>> Betreff: Re: [mkgmap-dev] "Small" SRTM1 files and DEM maps
>>
>> Hi Gerd
>> Forgot to mention files are downloaded as geotiff, but I then convert
>> them to hgt, sorry.
>> You can download a sample one from
>> https://alternativaslibres.org/tmp/N50E118.hgt
>>
>> El 16/4/20 a las 9:18, Gerd Petermann escribió:
>>> Hi Carlos,
>>>
>>> are you talking about geotiff format? I think someone documented how that format can be converted to *.hgt.
>>> Can you post a link to such a file?
>>>
>>> Gerd
>>>
>>>
>>>
>>> ________________________________________
>>> Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Carlos Dávila <cdavilam at orangecorreo.es>
>>> Gesendet: Montag, 13. April 2020 14:08
>>> An: mkgmap-dev at lists.mkgmap.org.uk
>>> Betreff: Re: [mkgmap-dev] "Small" SRTM1 files and DEM maps
>>>
>>> Hi Gerd
>>> They are downloaded by phyghtmap. This is what I found digging into
>>> phyghtmap files:
>>> |def getSRTMFileServer(self, resolution, srtmVersion):||
>>> ||        if srtmVersion == 2.1:||
>>> ||            return
>>> "https://dds.cr.usgs.gov/srtm/version2_1/SRTM{0:d}".format(resolution)||
>>> ||        elif srtmVersion == 3.0:||
>>> ||            if resolution == 1:||
>>> ||                urlRe =
>>> "https://earthexplorer.usgs.gov/download/8360/SRTM1{:s}V3/GEOTIFF/EE"||
>>> ||            elif resolution == 3:||
>>> ||                urlRe =
>>> "https://earthexplorer.usgs.gov/download/4960/SRTM3{:s}V2/GEOTIFF3/EE"||
>>> ||            return urlRe|
>>> They should come from the second URL above.
>>>
>>> El 13/4/20 a las 8:24, Gerd Petermann escribió:
>>>> Hi Carlos,
>>>>
>>>> sorry for the late response. I found no hint on the phyghtmap page http://katze.tfiu.de/projects/phyghtmap/
>>>> regarding a new format for srtm data. The hgt format is not compressed. Do you know from where these 12.4 MB files came from?
>>>>
>>>> Gerd
>>>>
>>>> ________________________________________
>>>> Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Carlos Dávila <cdavilam at orangecorreo.es>
>>>> Gesendet: Samstag, 11. April 2020 17:09
>>>> An: Development list for mkgmap
>>>> Betreff: [mkgmap-dev] "Small" SRTM1 files and DEM maps
>>>>
>>>> I use phyghtmap to build contour lines and also to download hgt data
>>>> that I then use to build DEM maps with mkgmap. For a long time, SRTM1
>>>> files (and also VFP1) have all been ~24.7 MB in size, but lately (I
>>>> don't remember since when) SRTM1 are also served as 12.4 MB files.
>>>> phyghtmap is able to generate contour lines from those small files, as
>>>> they seem to contain the same information but with a different
>>>> compression ratio, but mkgmap fails to read them, and report them as
>>>> missing. Would it be possible to get support for such files in mkgmap?
>>>> _______________________________________________
>>>> mkgmap-dev mailing list
>>>> mkgmap-dev at lists.mkgmap.org.uk
>>>> http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev
>>>> _______________________________________________
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