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

Tue Apr 21 10:19:27 BST 2020

Hi Carlos,

yes, I see that the source code handles this format. I finally found
https://dds.cr.usgs.gov/srtm/version2_1/Documentation/MIL-PDF-89020B.pdf
I think "TABLE I.  Matrix intervals for DTED Level 0."
shows how to interpret the data. It seems there are a few more variants...
For now I'll try to support also the 1801 * 3601 * 2 format

Gerd

________________________________________
Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Carlos Dávila <carlos at alternativaslibres.org>
Gesendet: Donnerstag, 16. April 2020 17:41
An: mkgmap-dev at lists.mkgmap.org.uk
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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