How to create a “Datum-less” projection in QGIS?

Question

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.

Answer 1

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.

Answer 2

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.

Nick.