I want to use QGIS to do some georeferencing and image registration. On ArcMap10 I can define an "Unknown Projection", basically a default metric grid, and use it as a grid for georeferncing.

In QGIS however, when I tried to georeference an image with GDAL, it gave me the following error:

Failed to compute GCP transform: Transform is not solvable

I've looked around and figured that the problem lies with the projections.

I've imported a 4 line CSV with 4 points demarcating the image extent in meters (BL 0,0, UR 450,450), but the layer properties tell me it's in WGS 84. because I do not know the actual coordinates of the image, I have to use just a basic internally coherent 5*5 metric grid, that I can georeference with corresponding marks in the image. This can only work with a datumless metric projection, which I do not know how to define (and save it as a custom defined projection)

I'm new to QGIS, so the problem might be remarkably easy to solve, but after a few hours of searching, I still haven't found it.

appreciate any help, H.


I don't know a definite solution, but First, I'd try loading a value of "-1" for the CRS code for QGIS. Or, if that doesn't work, and you have Qspatialite functional within QGIS, then I would try loading your CSV data into a spatialite database, using SRID:-1 [Undefined - Cartesian] as the spatial reference ID, then loading that into QGIS as a spatialite layer. Spatialite atleast recognizes -1 as [Undefined - Cartesian] and also SRID:0 [Undefined - Geographic Long/Lat]

[edit below] 2nd idea on what to do: I think you may have QGIS settings setup to automatically use the default CRS of WGS 84.

(Settings) menu [Options] dialog box tab

uncheck [ ] enable 'on the fly' reprojection by default

select [ ] Prompt for CRS

load your spatial file and set it's undefined CRS to WGS 84 Pseudo Mercator (units are meters)

or create your own generic projection

I don't think that QGIS supports a Generic cartesian 2D (see SR-ORG:7203 at www.spatialreference.org) coordinate system, after all, anything that's geographic is better represented on a globe, so with QGIS you will have to pretend your map comes from a Globe rather than a flat world--actually your map is coming from a Globe, its just that if it's a small area it doesn't really matter.

I'm guessing this would be good generic projection to create

------Create a generic CRS with meters---- (Settings) menu [Custom CRS] dialog box new CRS

NAME: Test1 +proj=tmerc +lat_0=0 +lon_0=0 +k=1 +x_0=0 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs [click the FILE] save CRS. Remember your name and apply it to your layer.


All CRSs are simple XY grids. If you are not interested in fitting your project onto the "real world" you could use any projection you like. World Mercator (EPSG:3395) would do, for example.


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