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When we convert data from bessel1841 to WGS84, parameter setting must be used because it is impossible automatic convert on QGIS.

Reference: Converting shp file from bessel1841 to WGS84

Is it possible to convert data from WGS84 to Bessel1841? Your advice will be big support for me.

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QGIS can convert in both directions. Just give it a try: If you have Bessel data save it to WGS84 and then back to Bessel. –  underdark Feb 10 '13 at 9:47
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1 Answer

Basically transformation can be done vice versa by just swapping the signs. So if your MGI to WGS84 Parameters look like this:

+towgs84=550.499,164.116,475.142,5.80967,2.07902,-11.62386,0.99999445824

...the corresponding WGS84 to MGI Parameters would be:

+tomgi=-550.499,-164.116,-475.142,-5.80967,-2.07902,11.62386,-0.99999445824

BTW: Keep in mind that the schema of these 7 parameters is:

shiftX,shiftY,shiftZ,turnX,turnY,turnZ,scale

There are some specific details in each software which could make this not fully correct. In ArcGIS for example you also have to be aware of which transformation method is used, because there are different variants of this Helmert (7 Parameter) Transformation. Depending on which you choose, you have to or dont have to swap the signs of the 3 turn parameters.

Unfortunately I am not experienced enough to tell you the exact way to perform this transformation in QGIS or with proj.4, but I think when you know, that the parameters are the same, only signs have to be swapped, you should be able to do it.

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When I register CRS, I got below message. "This proj4 projection definition is not valid. Please correct before pressing save" Following is my setting. Let me know why it does not work. +proj=tmerc +lat_0=0 +lon_0=21 +k=0.9999 +x_0=7500000 +y_0=0 +ellps=wgs84 +tobessel=-574.027,-170.175,-401.545,-4.88786,0.66524,13.24673,-0.99999445824 +units=m –  Jermain Marthin Feb 10 '13 at 9:10
    
As reference, following is used Bessel(MGI)toWGS [+proj=tmerc +lat_0=0 +lon_0=21 +k=0.9999 +x_0=7500000 +y_0=0 +ellps=bessel +towgs84=574.027,170.175,401.545,4.88786,-0.66524,-13.24673,0.99999445824 +units=m] –  Jermain Marthin Feb 10 '13 at 9:13
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Ok, sorry my fault, according to proj.maptools.org/gen_parms.html the tobessel parameter does not exist. In my understanding you just have to register your CRS correctly with the right towgs84 Parameter values. Proj.4 is able to do the vice verse transformation then, because it just swaps the signs. So in my understanding there is nothing left to do, layers with this CRS should transform on the fly. –  Jürgen Zornig Feb 10 '13 at 9:21
    
Thank you for your reply. I understood your advice. Additionally, On the fly does not work for bessel and WGS. –  Jermain Marthin Feb 10 '13 at 10:28
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