Chris, The towgs84-parameter is specifically used to do a 3D Helmert transformation from one system TO WGS84. The 3D Helmert is different from the 2D, so you can "cheat" by e.g. only using some of the 7 parameters in the 3D Helmert. It also sounds like your destination system is not WGS84.
The Swedish geodetic authority, Lantmäteriet, has worked around the problem of not having a 2D Helmert available in common GIS system by exploiting that a change of prime meridian in a given projection in principle rotates the coordinate system. This is a practical solution made many years ago because of the circumstances at the time and should not be repeated :-) Kristian -----Oprindelig meddelelse----- Fra: Franklin, Chris - Perth [mailto:[email protected]] Sendt: 30. november 2017 12:24 Til: Kristian Evers <[email protected]>; [email protected] Emne: Re: [Qgis-user] Custom Projection in QGIS Hi Kristian, I was thinking maybe I needed to use the towgs84 in the proj.4 but maybe I was wrong, so what can I do in QGIS, will ask tomorrow to see if there is a 4 point projection parameters I can use. Regards Chris On Thu, Nov 30, 2017 at 5:05 PM +0800, "Kristian Evers" <[email protected]<mailto:[email protected]>> wrote: Hi Chris, From your description I have no idea of what you are trying to do. It does however look like the parameters you have is for a 2D Helmert transformation. That is not something you can do in QGIS today. All transformations in QGIS are carried out by PROJ.4. The good news is that in the coming version of PROJ.4 the 2D Helmert is available (as well as a bunch of other geodetic transformations). Eventually this will be available to you in QGIS as well. /Kristian Fra: Qgis-user [mailto:[email protected]] På vegne af Franklin, Chris - Perth Sendt: 30. november 2017 03:47 Til: [email protected] Emne: [Qgis-user] Custom Projection in QGIS I have been given the parameters for a local grid - it's a version of UTM Zone 29N but I'm not sure where I need to change the syntax The parameters I have been given are: rl_correct = -24.273 yd1= 22737.030 xd1= 17332.250 yd2= 25987.560 xd2= 17893.870 ym1= 1193806.320 xm1= 820224.327 ym2= 1196784.417 ym2= 821648.929 Note the Z adjustment This results in the following transformation Scale= 1.00801358 Rotation= 15.4548 Shift x= 1171069.29 Shift y= 802891.987 Can someone point me in the right direction as to where these are entered? +proj=utm +zone=29 +datum=WGS84 +units=m +no_defs Thanks Chris _______________________________________________ Qgis-user mailing list [email protected] List info: https://lists.osgeo.org/mailman/listinfo/qgis-user Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-user
