[mkgmap-dev] some findings about NT format

Fri Mar 11 11:46:36 GMT 2022

Hi Gerd

Just to say you're doing a great job ! It must be like one step fw 2 
steps back ...

btw hgt2osm is written by Frank, the one who discovered the DEM structure.

Nick

On 11/03/2022 11:00, Gerd Petermann wrote:
> Hi all,
>
> I am still struggling with the NET 4 structures.  Whenever I find a pattern I also find exceptions to it ;)
>
> Reg. NET 5:
> It is a replacement for NET 3 (sorted roads)
> - first two bits give the label number
> - next n bits give the offset, n is the minimal number of bits needed to address all bytes in NET 4. Something like this
>    n = Integer.numberOfTrailingZeros(Integer.highestOneBit(<bytes in NET 4>)) + 1;
> - the remaining bits are not yet clear to me.
>
> Gerd
>
> ________________________________________
> Von: mkgmap-dev<mkgmap-dev-bounces at lists.mkgmap.org.uk>  im Auftrag von Gerd Petermann<gpetermann_muenchen at hotmail.com>
> Gesendet: Samstag, 5. März 2022 11:40
> An: Development list for mkgmap
> Betreff: Re: [mkgmap-dev] some findings about NT format
>
> Hi all,
>
> I think today I've now identified most of the structures of the NET file. Quite a lot of fields and bitstreams and I still don't know much about the meaning of them.
> Most of the findings described before were correct as long as some more flags are considered, but there is much more. Bit 0x20 in the first flag byte signals another
> bunch of possible structures following.
>
> I think this allows to read NET4 without looking at NOD, and there are some roads which are not referenced in RGN (and ignored by GPXSee). They possibly have no extra lines
> or the same lines as they preceding road.
> I've not yet made up my mind if I can add code to the existing NetDisplay in display tool or if we need a new GmpDisplay program.
>
>
> Reg. NET5:
> Sample from NET header:
> 000006d9 | 2d 10 1f 00             | NET5 ???, offset 1f102d
> 000006dd | c4 9d 01 00             | NET5 ???, length 105924
> 000006e1 | 27 00                   | ???
>
> The header has a record length (0x27 =39)  but that seems to give the number of bits, not bytes. At least I see some patterns when I print the bitstreams with that length.
> In general the record size seems to depend on the number of roads, the more roads the larger the record size.
> I guess there is a header which tells us what the bits mean. Todo...
>
> ciao,
> Gerd
>
> ________________________________________
> Von: mkgmap-dev<mkgmap-dev-bounces at lists.mkgmap.org.uk>  im Auftrag von Gerd Petermann<gpetermann_muenchen at hotmail.com>
> Gesendet: Mittwoch, 23. Februar 2022 09:45
> An: Development list for mkgmap
> Betreff: Re: [mkgmap-dev] some findings about NT format
>
> Hi all,
>
> I've identified a few more bytes following the road labels.
> If there are labels there is always an additional byte array.
> This starts with a length field (varying number of bytes).
> First byte in the array is a flag byte, the lower 4 bits each indicate the existance of another byte array with length prefix.
> Example:
> buf=[1d, 07, 07, 04, 01, 39, 07, 04, 01, 5b, 09, 2a, f1, 46, 66]
> 1d == 29 -> length 14
> 07 flag byte
> 07 1st array length -> 3
> array1=[04, 01, 39]
> 07 2nd array length -> 3
> array2=[04, 01, 5b]
> 09 3rd array length -> 4
> array3=[2a, f1, 46, 66]
>
> The top 4 bits are probably also flags. I've not yet understood what they mean, each seems to flag the existence of further bytes, but the number of those bytes vary.
> The flag byte might be 0:
> buf=[03, 00]
>
> array1 and array2 are more often found for major roads, but also on some minor ones. No idea yet what they mean.
> I am pretty sure that array3 contains the house numbers, because this array never shows up on motorways.
> Our code to read them doesn't (always) work. It seems to extract the base number correctly but then often fails.
> So I guess Garmin has extended the format. Maybe Numbers like 1a,1b are supported now?
> Another example:
> buf=[11, 84, 09, 2a, 30, 00, 27, 02, 57]
> 11 (=17) -> length 8
> 84 flag byte : (only) array4 exists, and 0x80 is set
> 09 array4 length -> 4
> array4=[2a, 30, 00, 27]
> The remaining bytes x02,x57 are probably a flag and a counter (or id). The flag probably indicates if the counter fits into one or two bytes.
> The counter itself simply increases for all roads with labels and the 0x80 bit in the flag byte set. No idea yet where this is used.
>
> I'll soon have access to a Garmin Nüvi. I hope that allows me to load the map in Mapsource or Basecamp. Maybe this will tell me what
> additional data we can find.
>
> Gerd
>
> ________________________________________
> Von: mkgmap-dev<mkgmap-dev-bounces at lists.mkgmap.org.uk>  im Auftrag von Gerd Petermann<GPetermann_muenchen at hotmail.com>
> Gesendet: Mittwoch, 16. Februar 2022 17:45
> An:mkgmap-dev at lists.mkgmap.org.uk
> Betreff: [mkgmap-dev] some findings about NT format
>
> Hi all,
>
> my findings so far which are not used / needed in GPXSee. Maybe it helps someone else:
> - method linkLabel() could extract further road labels, bit 0x800000 flags the last of up to 4 labels.
> - method linkLabel()  extracts a flag, the bit 0x08 in this flag seems to signal a tunnel. No idea yet what the other bits mean.
> - the flags field in struct LinkInfo is used like this (netfile.cpp)
>          bool firstIsShape = (linkInfo.flags >> 10) & 1;
>          bool singleTopology = (linkInfo.flags >> 9) & 1;
>          bool hasLevels = (linkInfo.flags >> 11) & 1;
> My guesses about the meaning of the lower bits:
>          //      int     road_speed = linkInfo.flags  & 7;
>          //      bool oneway = (linkInfo.flags >> 3) & 1;
>          //      int     road_class = (linkInfo.flags >> 4) & 7;
>
> - The NET header can have two more sections, e.g. I see
> 00000684 | f6 cb 13 00             | NET5 ???, offset 13cbf6
> 00000688 | 96 0a 01 00             | NET5 ???, length 68246
> 0000068c | 23 00                   | ???
> 0000068e | 8c d6 14 00             | NET6 ???, offset 14d68c
> 00000692 | 28 05 00 00             | NET6 ???, length 1320
> 00000696 | 03 00                   | NET6 ??? record size 3
> 00000698 | 02 00 00 00             | ???
>
> NET6 contains a list of offsets into NET4, they are in sorted order
> and each offset points to the byte following the flag byte that is read in  linkLabel()
> I see 440 recrods here, and 441 NOD6 records in the corresponding NOD file,
> so they are obviously related (field blockIndexId).
>
> I've not yet found where the address info like city name, zip name, and house numbers are
> stored. Maybe that's in NET5.
>
> I think in the old non-NT format there is no need to know NOD data to be able to read NET, so
> I woulld expect that this is also possible with NT maps.
>
> Gerd
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