Decoding the city database
Note: This post is a copy of the original web page I set up in 2005.
D-Sat 1
For current development status see below.
Introduction
D-Sat 1 is a satellite atlas of Germany, consisting of pictures taken by russian COSMOS satellites in 1991. Besides viewing the pictures, it is also possible to search for over 13000 German cities (see screenshot) and to read the coordinates of the locations under the mouse pointer. D-Sat 1 was published in 1996, several versions followed (up to D-Sat 6 AFAIK), the companies which programmed and distributed D-Sat (SCOUT Systems and TopWare) are bankrupt now.
Besides the proprietary file format and compression algorithm of the pictures, the coordinate system (projection/datum) are AFAIK also unknown. My intention is to decode at least the data of the city database and to use that data to find out the used coordinate system.
The file dsatnord.mp contains the whole city database (dsatsued.mp also, but not tested). The first location is "Aach", offset unknown (extracted it once and then forgot it ...). I decoded the main parts of the city database (wasn’t so difficult, see below), but I recognized that the coordinates in the database differ from the coordinates which are shown if I put the mouse pointer on the basis of the "town sign".
So the basic problem is to find the connection between screen coordinates (under the mouse pointer), city coordinates (from the database) and true coordinates. Unfortunately, it is very boring work to write down the screen coordinates, because it is impossible for me to get them automatically. So the database of screen coordinates is very small. On the other hand, I have over 13000 coordinates of German cities with the problem that some of them are completely wrong (there existed a corrected update to D-Sat 1, I'm trying to decode that) and there is rule which part of the city has been chosen as reference point. Therefore, automatic comparison, for example, with OpenGeoDB data is very difficult.
Unfortunately, some rumours in the Internet also say that the pictures of D-Sat 1 are very badly geo-referenced. So maybe it is impossible to discover projection and datum, because it is just very badly geo-referenced GKK/Potsdam?
format of city data base
The format of the data is 64 bytes per city:
- 40 Bytes cityname (ASCII)
- 24 Bytes coordinates + other data (to identify), consisting of
-
Bytes 1-4: have exclusively the following values:
0 0 0 0 0 0 0 128 20 174 71 225 20 174 71 97 41 92 143 194 41 92 143 66 61 10 215 163 82 184 30 133 195 245 40 92 215 163 112 61 236 81 184 158 236 81 184 30 -
Bytes 5-7: easting, stored as little endian
-
Byte 8: always 65
-
Bytes 9-11: always 0
-
Byte 12: 1995 x 0; 584 x 32; 1878 x 64; 622 x 96; 1825 x 128; 593 x 160; 1889 x 192; 618 x 224
-
Bytes 13-15: northing, stored as little endian
-
Bye 16: always 65
-
Byte 17: 8486 x 9; 37 x 11; 3 x 17; 447 x 26
-
Bytes 18-24: always 0
-
Note: Bytes 1-4, 12 and 17 are geographically independent, that is, the values have nothing to do with the location of the cities.
some sample data
Byte 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Alsenz __ 41 92 143 194 184 76 25 65 0 0 0 192 140 11 85 65 9 0 0 0 0 0 0 0
Alsfeld __ 20 174 71 225 165 173 31 65 0 0 0 224 81 122 85 65 26 0 0 0 0 0 0 0
Alsheim __ 41 92 143 194 10 159 27 65 0 0 0 192 170 15 85 65 9 0 0 0 0 0 0 0
Alsleben/ Saale __ 0 0 0 128 40 234 36 65 0 0 0 0 183 228 85 65 26 0 0 0 0 0 0 0
Alt Bennebek __ 0 0 0 128 2 30 32 65 0 0 0 128 57 2 87 65 9 0 0 0 0 0 0 0
Alt Bork __ 236 81 184 158 134 88 39 65 0 0 0 64 148 28 86 65 9 0 0 0 0 0 0 0
Alt Duvenstedt __ 0 0 0 128 72 138 32 65 0 0 0 96 47 0 87 65 9 0 0 0 0 0 0 0
Alt Golm __ 41 92 143 66 103 241 41 65 0 0 0 32 105 46 86 65 9 0 0 0 0 0 0 0
Alt Krenzlin __ 236 81 184 30 67 245 35 65 0 0 0 64 118 146 86 65 9 0 0 0 0 0 0 0
Alt Madlitz __ 41 92 143 194 244 64 42 65 0 0 0 64 137 55 86 65 9 0 0 0 0 0 0 0
Alt Mahlisch __ 41 92 143 194 33 130 42 65 0 0 0 192 12 65 86 65 9 0 0 0 0 0 0 0
Alt Meteln __ 41 92 143 194 123 252 35 65 0 0 0 96 18 193 86 65 9 0 0 0 0 0 0 0
Alt Mölln __ 236 81 184 30 106 154 34 65 0 0 0 64 141 177 86 65 9 0 0 0 0 0 0 0
Alt Rehse __ 41 92 143 194 34 181 39 65 0 0 0 64 40 171 86 65 9 0 0 0 0 0 0 0
Alt Schwerin __ 41 92 143 66 217 15 38 65 0 0 0 128 252 169 86 65 9 0 0 0 0 0 0 0
Alt Schönau __ 41 92 143 194 133 207 38 65 0 0 0 64 22 179 86 65 9 0 0 0 0 0 0 0
Alt Sührkow __ 41 92 143 194 180 156 38 65 0 0 0 160 111 202 86 65 9 0 0 0 0 0 0 0
Alt Tucheband __ 0 0 0 128 189 174 42 65 0 0 0 192 165 73 86 65 9 0 0 0 0 0 0 0
Alt Zauche __ 0 0 0 0 203 206 41 65 0 0 0 128 246 3 86 65 9 0 0 0 0 0 0 0
Alt Zeschdorf __ 0 0 0 0 228 146 42 65 0 0 0 128 30 61 86 65 9 0 0 0 0 0 0 0
Alt-Schadow __ 20 174 71 97 112 159 41 65 0 0 0 192 164 25 86 65 9 0 0 0 0 0 0 0
Altbach __ 20 174 71 97 167 30 32 65 0 0 0 32 249 158 84 65 9 0 0 0 0 0 0 0
Altbrandsleben __ 0 0 0 128 188 249 35 65 0 0 0 0 170 11 86 65 9 0 0 0 0 0 0 0
put into an image ...
OK, so let us take all 13378 cities, their easting/northing and put them into a cartesian coordinate system: orte.ps.gz. THe function I used to transform the coordinates into PostScript-compatible values is: PS_E = (500.0/1835000.0) * (E-1114000), PS_N = (1000.0/262144.0) * (N-5505000)\
Nice, isn't it? :-)\ (besides an east-west-compression – which may result from conversion to Postscript coordinates?!)
For me this picture was the proof that I really found the coordinates of the cities and not some other data. I also tried to colorize the points, depending on the value of the unidentified bytes, but that yielded no pattern – the colors were spread all over Germany.
comparison to known data
explanation of the columns in the following table:
- file_E/file_N: easting/northing which I got from the decoded city database
- screen_E/screen_N: easting/northing which is shown on screen, when I put the mouse pointer on the basis of the town sign (see screenshot)
- GKK/Potsdam, UTM/WGS84: I tried to find the locations of the town signs in the topographical map 1:25000 and wrote down the coordinates (see red cross in screenshot TK25 and compare with screenshot D-Sat 1). Of course, this is very error prone, because it's impossible to get the exact location.
- screen_E-GKK_E: difference between screen-easting and GKK/Potsdam-easting
- screen_N-GKK_N: difference between screen-northing and GKK/Potsdam-northing
The fact, that I have only two geo-referenced Top25-maps is the reason for the few data here. :-(
| city | file_E | file_N | screen_E | screen_N | GKK/Potsdam_(4) | UTM/WGS84_(33U) | screen_E - GKK_E | screen_N - GKK_N |
|---|---|---|---|---|---|---|---|---|
| Salzfurtkapelle | 2488688 | 5629157 | 4507038.0m | 5729278.0m | 4512121E - 5728869N | 304711E - 5730783N | -5083 | 409 |
| Wolfen | 2501091 | 5628611 | 4513170.0m | 5726920.0m | 4518355E - 5726409N | 310938E - 5727642N | -5185 | 511 |
| Sandersdorf (Screenshots: TK25, D-Sat1) | 2501138 | 5627385 | 4513056.0m | 5722012.0m | 4518214E - 5721479N | 310460E - 5723154N | -5158 | 533 |
| Thalheim | 2496565 | 5628002 | 4510842.0m | 5724544.0m | 4515992E - 5724054N | 308346E - 5725823N | -5150 | 490 |
| Jeßnitz | 2507842 | 5629039 | 4516584.0m | 5728546.0m | 4521814E - 5727937N | 314323E - 5729459N | -5230 | 609 |
| Greppin | 2506254 | 5627847 | 4515666.0m | 5723788.0m | 4520853E - 5723274N | 313171E - 5724839N | -5187 | 514 |
| Paplitz | 2662879 | 5640168 | 4595124.0m | 5771098.0m | 4601091E - 5769517N | 395230E - 5767732N | -5967 | 1581 |
| Horstwalde | 2653560 | 5641043 | 4590582.0m | 5774734.0m | 4596532E - 5773231N | 390830E - 5771630N | -5950 | 1503 |
In dsat_difference_vector.ps I have drawn the difference vectors to the real locations (screen_E-GKK_E and screen_N-GKK_N). Note: the signs are drawn wrong (always positive). To me that looks good, that is, there seems to be a systematic error to the real locations. generation of file (plus header/footer):
awk '{f=200} {print "newpath " ($4-4507000)/f " "($5-5720000)/f \
" 1 0 360 arc fill \
newpath " ($4-4507000)/f " " ($5-5720000)/f " moveto " \
($6-4507000)/f " " ((2*$5)-$7-5720000)/f " lineto stroke "}' \
< vergleich_2 > dsat_difference_vector.ps
Difference vectors: 
it gets worse
In D-Sat you can put markers on the map to remember locations. The
location of the markers is saved in dsat.ini in a very simple
format:
M005Key1=5
M005Key2=32838
M005Key3=248766
M005File=
Key1 selects the symbol which is shown on the screen, so the other two values should be the coordinates of the marker. The problem is very simple:
city : file_E : file_N : screen_E : screen_N :dsat.ini :dsat.ini
List : 1853914 : 5719936 : 4261239.0 : 6101200.0 : 32838 : 248766
Oberstdorf : 2242906 : 5509437 : 4371513.0 : 5252560.0 : 881472 : 359034
Selfkant : 1128882 : 5610221 : 4068213.0 : 5665063.0 : 468978 : 55734
Deschka : 2890943 : 5620266 : 4704306.0 : 5688157.0 : 445878 : 691830
(BTW: these are the most northern, southern, western and eastern
cities inside D-Sat.) The first two values file_E/file_N are extracted
by me from dsatnord.mp, the second two values screen_E/screen_N are
shown on the screen and the third two values are from the markers, I
put on the city location.
Help!
If you have ideas that could help to decode any of that coordinates, I would be glad to hear from you. I'm also interested in information regarding the rumour that the pictures in D-Sat 1 are very badly geo-referenced and so it is impossible to get correct coordinates.
Current development status
Thanks to Mikael Rekola, because he pointed out that the coordinates in the city database could also be stored as (long) floating point. And that could be indeed true! I'm still doing some comparisons, but actually the floating point values again give a map of Germany: new.ps.gz. This map is much more distorted, if I don't scale it appropriately.
The second and third column show the new values for locations given above:
ort file_e file_n screene screenn gkk_e gkk_n
sfk 720056.4 5739412.0 4507038 5729278 4512121 5728869
wol 726257.7 5737228.0 4513170 5726920 4518355 5726409
sdf 726281.1 5732326.0 4513056 5722012 4518214 5721479
tha 723994.5 5734795.5 4510842 5724544 4515992 5724054
jes 729633.4 5738941.0 4516584 5728546 4521814 5727937
gre 728839.4 5734175.0 4515666 5723788 4520853 5723274
pap 807151.8 5783457.0 4595124 5771098 4601091 5769517
hor 802492.2 5786956.0 4590582 5774734 4596532 5773231
Last change: 2005-03-26