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I have a map here with Coordinates from the Schwarzeck Map Datum Format 1903. They have the following format

e.g.: y: 41 944,31 x:363 810,71

I want these point to be pictured in QGis... how is it done?

Thank you in advance for your help and time!

Lukas

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Is it possible to share the map image? Where are we, in South Africa? N. – nhopton Sep 25 '12 at 15:27
Yes I can share the map:plus.google.com/photos/101147296834514371043/albums/… – Lukas Sep 25 '12 at 19:23

2 Answers

up vote 2 down vote accepted

The official CRS paramters can be found here: http://www.mme.gov.na/gsn/nam-map-system.htm.

So it should be Albers Equal Area or LO Gauss conformal. Qgis only offers EPSG 4293 (lat/lon) and EPSG 29333 (UTM 33S). Both do not match with the given example.

You may try one of these: http://spatialreference.org/ref/?search=Schwarzeck

SOLUTION A

For the local LO 22/17 system EPSG 29377:

The page does not have a proj string, but OGC WKT. I saved it to 29377.prf, and converted the points with gdaltransform from OSGeo4W:

gdaltransform -s_srs 29377.prf -t_srs EPSG:4326 < input.txt > out.txt

Input Y X without minus, and blanks as delimiters. After adding to the Output file a line

E S Z

you can add the file as delimited text layer with blanks as delimiters, marking the E and S column as X and Y. Layer CRS should be set to EPSG:4326.

SOLUTION B

For LO 22/17, you can test this proj string under settings/custom CRS:

 +proj=tmerc +lat_0=-22 +lon_0=17 +k=1 +x_0=0 +y_0=0 +a=6377397.155 +rf=299.1528128 +towgs84=616,97,-251,0,0,0,0 +units=m +no_defs

(Not sure where to bring the "German legal meter" in)

You can add a line with

X Y

before your data, and import in Qgis as delimited text with blank as delimiter. Then apply the user-created CRS to that layer.

Comparing both with Bing Aerial map in Qgis Openstreetmap plugin looks as follows. The difference between Solution A and B is about 18 cm. Openstreetmap even knows the place name Ombili ;-)

http://i.stack.imgur.com/WlbIp.jpg

ANNOTATION

I think the coordinates should both be negative. "Transverse Marcator - south orientated" should have positive values to the south and west of the reference point (-22/+17). But until someone comes up with a correct projection string, it is easier to swap plus/minus on the input side. This would also apply to the WKT format given at spatialreference.org.

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This one should be the one: spatialreference.org/ref/epsg/29377 Lo22/17. But you are right, it's not in QGis. – Lukas Sep 25 '12 at 19:33
I'm sorry I was without Internet for a while... I didn't get the solution A, I tried to discover all the abbreviations but I failed. The second solution is at least understandable for me :). I addded a custom pojection the following way: settings->user projection->parameter: [your parameters]. Then I clicked on the save button and then ok. Now I can see the points but they are on a point where they can't be. So either way... how can I understand Solution A, or whyt did I do wrong on solution B? – Lukas Sep 29 '12 at 15:09
I did try and make a custom south-oriented TM CRS using these parameters. +proj=tmerc +lat_0=-22 +lon_0=17 +k=1 +x_0=0 +y_0=0 +axis=wsu +ellps=bess_nam +towgs84=616.8,103.3,-256.9,0,0,0,0 +to_meter=1.0000135965 +no_defs But I use QGIS Master, which only has GDAL/OGR 1.7, so I can't test it. N. – nhopton Oct 2 '12 at 15:06
It works good in Qgis 1.8.0, with an offset of about 8 metres to the previous calculations. Maybe a matter of the +towgs84 parameters. – Andre Joost Oct 2 '12 at 17:23
I'm still confused why the OGC-WKT projection parameters EPSG 29377 expect positive values to give the same results, although it claims to be south-orientated. – Andre Joost Oct 3 '12 at 7:35
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up vote 1 down vote

You need to know the exact parameters of this coordinat system, for instance longitud of origin, false easting, false northing etc. Check this

http://proj.maptools.org/gen_parms.html

Once you know them, you can try defining a custom crs at settings/custom CRS. You can test some points in lat long wgs84 in this place also. CRS definition must be in proj4 format. You can test some points in lat long wgs84. The only problem could be your datum, though...

hope it helps

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In this Map there are these parameters: Latitude of origin: -22 ; Central Meridian: 17. So how can I add these parameters to Qgis, so that it works? I have these points entered into a csv-textfile to import it. Should I beforehand define a custom projection? How Do I do this? Thank you very much! – Lukas Sep 25 '12 at 19:12
The CRS is Schwarzeck Lo22/17, which is Transverse Mercator south oriented. I honestly don't know if QGIS can handle this. The coordinate you gave above (y=41944.31 x=363810.71) appears to translate to something like lat -18.713829 lon 17.396855, could you fly to this coordinate in Google Earth and let me know the size of the error? N. – nhopton Sep 26 '12 at 10:05
Looks pretty much the same as my calculation, except that the y coordinate in the map is 41954.31. Openstreetmap knows the place name around there. Bing aerial maps are a bit better down there than Google satellite. – Andre Joost Sep 26 '12 at 10:42
@andrejoost how do I calculate this? I'm feeling now very stupid I learned everything from QGIS on my own we only had ARCGIS courses where I study. – Lukas Sep 29 '12 at 15:22

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