[Qgis-user] Non-Earth Coordinates

[.]

Grant will no doubt correct me on this, but I’d imagine thot the use case he has in mind is something like this.

A lot of the work that I do involves local planar grids that are set up with north off set to normal. One of the grids I’m working on at the moment has a north that is 12.5 degrees east of the MGA94 UTM coordinate system anchored to an origin point (which is hopefully something better than 30m SE of the corner fence post).

In Mapinfo (which in our part of the word is the dominant GIS) you can set these up as “Non-Earth” coordinate systems and MapInfo will create an affine transform for moving data to and from the local grid and your projects real CRS. It also allows you to overlay (OTF) data from the local grid and the more standard CRS.

This last bit is quite useful. Say you have some historic drill points, which you have the original local coords for. When you where on site last, you found some old drill holes (with no labels) which you recorded in MGA94. This allows you to quickly visually check which holes you found and also how good the conversion really is.

One of the usual end points is to be able to move data from these local grids into something more standard. That said there are still people out there working in these local grids, so sometimes you need to go the other way.

I understand that QGIS depends on PROJ which is a good choice - however for this sort of exercise I’ve not been able to work out how to get PROJ to build a custom grid at an angle to north. Sometimes you can sort of fudge it, say if you happen upon a grid that is at the right angle. I’d be extremely happy if I was wrong on this and someone could explain how to do this. (The PROJ syntax does my head in - and I’d rather go learn python properly)

On python. There is a plugin that does Affine transforms, but it assumes you know the values for the transform (not likely in this use case).
I did put some work toward building something in python to calculate these, it’s extremely rough and not well tested.

-ramon.


> On 17 Jan 2017, at 10:21, Michael Treglia <mtreglia at gmail.com> wrote:
> 
> Hi Grant - good question. Looks like a similar question came up on Stack Exchange last spring. Does it sound like this answer works for you?  http://gis.stackexchange.com/questions/187699/how-to-create-a-qgis-map-of-unprojected-data <http://gis.stackexchange.com/questions/187699/how-to-create-a-qgis-map-of-unprojected-data> 
> 
> (Ultimately the suggestion is to assign it to a UTM or foot based coordinate system, as that should work fine for display and such - if you find out there is a real Geographic coordinate system later, you can just assign as appropriate).
> 
> If not, curious to hear more about the specific case you're dealing with.
> mike
> 
> 
> On Mon, Jan 16, 2017 at 9:10 PM, Grant Boxer <boxerg at iinet.net.au <mailto:boxerg at iinet.net.au>> wrote:
> Is there an option in QGIS to select a non-earth coordinate system? This is useful when you get data in a local grid without a readily available conversion to real earth coordinates. There is this option in MapInfo and it is really handy when dealing with old data. The “local grid” can then be transformed later when a  conversion can be determined.
> 
>  
> 
> Grant Boxer
> 
> Perth, Western Australia
> 
>  
> 
> 
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