[Qgis-user] Custom Projection in QGIS

[Kristian Evers]

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:CFranklin at rml.com.au] 
Sendt: 30. november 2017 12:24
Til: Kristian Evers <kreve at sdfe.dk>; qgis-user at lists.osgeo.org
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" <kreve at sdfe.dk<mailto:kreve at sdfe.dk>> 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:qgis-user-bounces at lists.osgeo.org] På vegne af Franklin, Chris - Perth
Sendt: 30. november 2017 03:47
Til: Qgis-user at lists.osgeo.org
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