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I have a CAD file (DWG) that need to load into a GIS. It does not have any projection information, just its own internal coordinate system: Extent: (287073.083996, -667765.958872) - (857074.052952, -268711.958871). I don't think this corresponds to any known projection with an EPSG code. I believe it comes from the world of BIM. I think the whole extent is vaguely 1km x 1km.

How can I go about creating a projection to locate this data in geographic space?

Ideally I would use graphical tool to position each corner where I think they go. (I know how to do this with a raster image).

I'm able to load the data into QGIS but I don't know how to create a custom projection, other than by entering in a WKT string (which I obviously don't have).

Steve Bennett
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  • Is the CAD data in 2d or 3d? In which case, you will need to start with a reference spheroid/datum. Given the localised nature of the data, you could just use generic one. But start with that problem first. – nr_aus Aug 31 '21 at 01:32
  • Would not go the projection way but use vector georeferencing using something like https://plugins.qgis.org/plugins/VectorBender/ or recipe from https://gis.stackexchange.com/questions/30559/computing-parameters-for-qgis-affine-transformation/127653#127653 There is an ongoing effort to make a Vector georeferencer version natively in QGIS https://github.com/qgis/QGIS/pull/41386 but not merged at the moment... – ThomasG77 Aug 31 '21 at 01:56
  • @nr_aus 2D. I was hoping that because the scale is small, the datum/spheroid won't make any difference? But I don't really know what I'm talking about. – Steve Bennett Aug 31 '21 at 04:07
  • Looks like UTM coordinates to me, possibly the wrong UTM zone! – Alex Leith Aug 31 '21 at 04:07
  • Well, I ran it through https://stevage.github.io/WhatTheProj/ which didn't find anything closer than 938km away. – Steve Bennett Aug 31 '21 at 04:14
  • @ThomasG77 Thanks, that VectorBender plugin looks like it probably does the right thing, but I can't understand how to use it. I think it wants me to draw lines representing the start and final position of each control point, but in my case, the original CRS is so wildly different I can't even wrap my head around how I could draw such a line. – Steve Bennett Aug 31 '21 at 04:54

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