[mkgmap-dev] DEM File Format and mkgmap
From osm@pinns osm at pinns.co.uk on Mon Dec 4 15:02:27 GMT 2017
Hi Gerd Thanks for letting me know That is probably why eventually it stopped with repeated plateau len 64 messages on one of the dems builddemfile created I have some 'old' non nt Garmin Topo maps with dem and will have a go using your display.jar r Nick On 04/12/2017 13:49, Gerd Petermann wrote: > Hi Nick, > > I just found out that display tool can not yet read DEM tiles with multiple zoom levels. Working on it now... > > Gerd > > ________________________________________ > Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von osm at pinns <osm at pinns.co.uk> > Gesendet: Montag, 4. Dezember 2017 14:35:15 > An: mkgmap-dev at lists.mkgmap.org.uk > Betreff: Re: [mkgmap-dev] DEM File Format and mkgmap > > Many thanks Gerd > > Will have a go at it today and let you know my findings, if any ! > > Nick > > > On 04/12/2017 13:27, Gerd Petermann wrote: >> Hi Nick, >> >> My understanding is that the prediction starts with the values from the header (min and max height in the tile) >> The first row is indeed rather easy to decode. Frank has described many more details in his pdf, and his program >> seems to create valid DEM data since Basecamp displays it, I just try to reduce the number of special cases now. >> >> A compiled display.jar r505 is here: >> http://files.mkgmap.org.uk/download/361/display.jar >> If you also download and unzip this mkgmap package http://www.mkgmap.org.uk/download/mkgmap-r3997.zip >> you can run e.g. >> java -cp display.jar;mkgmap-r3997\mkgmap.jar test.display.DemDisplay 12345678.DEM > log.txt >> >> I am interested in those files created by BuildDemFile which create a run time exception. If you find one or more, I need the original *.TRE file(s) >> which were the input for BuildDemFile . >> >> Gerd >> ________________________________________ >> Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von osm at pinns <osm at pinns.co.uk> >> Gesendet: Montag, 4. Dezember 2017 13:00:27 >> An: mkgmap-dev at lists.mkgmap.org.uk >> Betreff: Re: [mkgmap-dev] DEM File Format and mkgmap >> >> Hi Gerd >> >> Presumably some hgt data gets stored in a tile from which other data is >> predicted? Perhaps the first 'row'? >> >> Would be great if you could compile the DemDisplay.java so we can do >> ome testing as well. >> >> I'm glad you are almost 'getting there'! >> >> r >> >> Nick >> >> >> On 03/12/2017 11:16, Gerd Petermann wrote: >>> Hi all, >>> >>> I think I understand now much more about this format. I used Frank Stinners code for BuildDEMFile and his pdf to write a decoder in the display tool. >>> I still fails to read some Garmin DEM files but seems to work with the files produced by BuildDEMFile. >>> My current work flow is to generate DEM files with BuildDEMFile and read them with my decoder. If the decoder fails I can find out why because I can trace step by step what BuildDEMFile does. >>> I guess that I did not yet hit all special cases identified by Frank, but I am making good progress now, so I hope I'll be able to start implementing a DEM encoder in mkgmap soon. >>> >>> Reg. the format: >>> It is quite different to those used in other GARMIN sub files like NOD, NET, RGN etc. In short: The data is organized in tiles, typically 64*64 values. Instead of storing each value >>> Garmin uses some statistics to predict the next value based on the already processed values. This predicted value is subtracted from the real value and only the delta is stored. >>> The better the prediction, the closer to 0 is the delta. The prediction also calculates the error-probability of the prediction and uses this to select the number of bits which are >>> needed to store the value. If the delta is larger than expected the so called BigBin encoding is used. >>> The format to store the delta is optimized for values close to 0, so that many deltas are stored with only 1 or 2 bits. >>> A 2nd trick to save space is to use run length encoding for repeated values. >>> So, the tricky part is to find out exactly how the prediction algo by Garmin works. >>> Frank has done really great work here. >>> >>> For those who want to try the display tool: >>> You have to compile on your own and you also need an installed mgkmap.jar. >>> If that is available a command like this (replace d:\... with your paths) should produce a large txt file: >>> java -ea -Xmx1000m -cp d:\display\dist\display.jar;d:\mkgmap\dist\mkgmap.jar test.display.DemDisplay test.DEM > dem_decode.txt >>> >>> I am now searching for DEM files produced by BuildDEMFile which are not properly processed by display tool. When it fails you'll see a traceback like this >>> Exception in thread "main" java.lang.RuntimeException: did not read all avalaible bytes >>> at test.display.DemDisplay$DemTile.decodeBitStreamData(DemDisplay.java:532) >>> at test.display.DemDisplay$DemSection.decodeBitstream(DemDisplay.java:831) >>> at test.display.DemDisplay.readFileHeader(DemDisplay.java:887) >>> at test.display.DemDisplay.print(DemDisplay.java:845) >>> at test.display.CommonDisplay.display(CommonDisplay.java:171) >>> at test.display.CommonDisplay.display(CommonDisplay.java:196) >>> at test.display.DemDisplay.main(DemDisplay.java:903) >>> >>> Gerd >>> _______________________________________________ >>> mkgmap-dev mailing list >>> mkgmap-dev at lists.mkgmap.org.uk >>> http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev >> _______________________________________________ >> mkgmap-dev mailing list >> mkgmap-dev at lists.mkgmap.org.uk >> http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev >> _______________________________________________ >> mkgmap-dev mailing list >> mkgmap-dev at lists.mkgmap.org.uk >> http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev > _______________________________________________ > mkgmap-dev mailing list > mkgmap-dev at lists.mkgmap.org.uk > http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev > _______________________________________________ > mkgmap-dev mailing list > mkgmap-dev at lists.mkgmap.org.uk > http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev
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