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 Projections and datum
 200 m E-W error for Gauss-Krueger projection
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ralfhesse

Germany
6 Posts

Posted - 31/10/2013 :  12:55:03  Show Profile  Reply with Quote
I am working with maps and geodata in Gauss-Krueger projection (zone 3, DHDN). When I use OkMap to convert geotiff files to oruxmaps files, there is a 200 metre offset (map seems to to shifted east). Did anybody else run into this problem, and what can be done about it?

okmap

Italy
2352 Posts

Posted - 31/10/2013 :  13:08:50  Show Profile  Visit okmap's Homepage  Reply with Quote
Please can you send your map so I can do some tests?
If it's heavy please use one of web free services to send big attachments.
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okmap

Italy
2352 Posts

Posted - 31/10/2013 :  15:53:11  Show Profile  Visit okmap's Homepage  Reply with Quote
Hi Ralf,
I've tried to georeference your map manually using data retrieved at: http://seer.science.oregonstate.edu/cwis438/Websites/EPSG/CRS_Info.php?CoordRefSystemCode=31467&CallingPage=%2Fcwis438%2FWebsites%2FEPSG%2FCRS_List.php%3FTakeAction%3DReturned%26CurrentRow%3D0%26OrderByField%3Dcoord_ref_sys_name%26DescendingFlag%3D0%26SearchString%3D%26FirstNameLetter%3D%26AdvancedSearch%3D0%26AreaOfUseCode%3D0%26SearchIn%3D0%26MatchTo%3D0%26CRSCode%3D31467%26EPSG_CRS_Kind%3D0%26AreaOfUseCode%3D0%26DatumCode%3D0%26CRSCode%3D31467&CallingLabel=To%20Coordinate%20Reference%20Systems&WebsiteID=1
Parameters seem all correct but map continues to show offset.
Are you sure that geotiff tag inside your tif file are correct?
Have you any other map of the same zone (and projection)?
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ralfhesse

Germany
6 Posts

Posted - 31/10/2013 :  16:10:45  Show Profile  Reply with Quote
Hi,

the geotiff file loads correctly in Globalmapper and gvSIG. I have run into this problem repeatedly when working with Gauss Krueger data but never when working with UTM data. Of course I could first convert my maps to UTM and then run OkMap to get an Oruxmaps file (which is my usual workaround), but that's an extra step in the workflow (almost ALL my data are Gauss Krueger). I suspect that there is a bug in the projection definition/calculation in OkMap.
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okmap

Italy
2352 Posts

Posted - 31/10/2013 :  16:37:26  Show Profile  Visit okmap's Homepage  Reply with Quote
It's improbable there is a bug in the projection algorithm because works properly for other datums.
It's possible there is a mistake in the datum definition but I've checked with data found on the net and seems correct.
Please give me still a little time to analyze the question.
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ralfhesse

Germany
6 Posts

Posted - 31/10/2013 :  16:44:52  Show Profile  Reply with Quote
In the xml file created by OkMap, the projection parameters are OK but the datum is set to WGS 1984 rather than DHDN as it should be. Maybe that's the problem?

Ellipsoid for Gauss Krueger should be Bessel 1841 with semi-major axis 6377397.155 and inverse flattening 299.1528128. Projection details should be Transverse Mercator with semi-major axis as above and semi-minor axis 6356078.962818189.

Edited by - ralfhesse on 31/10/2013 16:48:11
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okmap

Italy
2352 Posts

Posted - 31/10/2013 :  17:07:11  Show Profile  Visit okmap's Homepage  Reply with Quote
I've solved.
The correct datum is "Potsdam Rauenberg DHDN".
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ralfhesse

Germany
6 Posts

Posted - 31/10/2013 :  17:17:36  Show Profile  Reply with Quote
quote:
Originally posted by okmap

I've solved.
The correct datum is "Potsdam Rauenberg DHDN".




Will this be changed in the next version of OkMaps?

Edited by - ralfhesse on 31/10/2013 17:46:01
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okmap

Italy
2352 Posts

Posted - 31/10/2013 :  17:45:00  Show Profile  Visit okmap's Homepage  Reply with Quote
I'll change in the next version.
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okmap

Italy
2352 Posts

Posted - 01/11/2013 :  14:29:04  Show Profile  Visit okmap's Homepage  Reply with Quote
Try the new version of OkMap
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ralfhesse

Germany
6 Posts

Posted - 21/11/2013 :  09:30:06  Show Profile  Reply with Quote
I finally was able to test it, and it works well now. Thanks a lot!

A small issue remains: When I load the original GeoTiff in a GIS and plot the bounding and/or calibration coordinates from the OruxMaps XML file, they are not exactly in the corners of the map - there is still an offset of approx. 5 m. So it's certainly good for orientation with standard GPS now, but may become tricky when trying to locate a spot very accurately.
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okmap

Italy
2352 Posts

Posted - 21/11/2013 :  09:38:30  Show Profile  Visit okmap's Homepage  Reply with Quote
Oruxmaps does not support this datum, so are necessary some conversions to produce final OruxMaps files.
5 m. are under the standard GPS error and might be accetable.

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ralfhesse

Germany
6 Posts

Posted - 21/11/2013 :  13:33:56  Show Profile  Reply with Quote
Yes, as I said it's OK as long as one does not require better accurracy than standard GPS.
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mettsmith

USA
2 Posts

Posted - 03/12/2018 :  08:27:07  Show Profile  Reply with Quote
Not really sure why this is happening. Is your GeoTIFF tag correct inside the tif file? If it is incorrect then the error might occur, so do check that. And if still, you don't get any positive results with Phallosan forte, you may revert back to us.

---

My Blog
https://buffaloreport.com/phallosan-forte-review-results/

Edited by - mettsmith on 07/05/2019 10:02:05
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