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I'm trying to form a mosaic from two overlapping images of an old tithe map. I've used the georeferencer plugin using one of the images as canvas to generate a modified version of the other. I then used Raster>Miscellaneous>Merge. This generates the following commandline:

gdal_merge.py "/home/ian/Maps/Stubbin/1801 map Stubbin prt 1.JPG" "/home/ian/Maps/Stubbin/1801 map Stubbin prt 2_modified.jpg"

This gives an error:

ERROR 1: Attempt to create 2768x-1659 dataset is illegal,sizes must be larger than zero.

This seems to be related to the fact that image metadata for the original images shows the layer extents as 0.0, -1651.0 : 2339.0, 0.0 and other digital camera images also have -ve extents.

How do I work round this?

Platform is QGIS 1.8.0 on Debian Squeeze 64-bit.

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To give your images positive coordinates, try georeferencing one image first with approximate real-world coordinates (make a points layer with known coordinates and then overlay the image); then, georeference the second with the edges of the first.

Assuming that the tithe map comes from the same map (approximately the same projection / scale?), you would want to make sure that the first image is not distorted - use a linear transformation with only two points on the first image.

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  • Thanks, @Simbamangu, I'm getting there. I built a points file to set three of the corners to what they should be, eg. top left, which was 0,0 becomes 0, 1651 & bottom left which was 0,-1651 becomes 0,0 & applied that as a linear transform. I then georeferenced the other image as before. This looked OK when displayed as layers with a spline transform but has a wedge-shaped gap when I generate the merge. So I'll go back & try your suggestion of georeferencing for the edges only.
    – Ian Goddard
    Commented Oct 2, 2012 at 13:54
  • I tried another merge on the pair of images which gave be the black wedge above & this time ticked the No data box & the generated image is now OK. How do I update the title to say solved?
    – Ian Goddard
    Commented Oct 2, 2012 at 14:06
  • Hi @IanGoddard welcome to GIS.se, and glad that worked! Interesting to know that the nodata box helps with the black margins. There's a little 'check box' near the upper left of my solution here which you can tick - see this link for an explanation of how this (and other) sites work.
    – Simbamangu
    Commented Oct 2, 2012 at 14:35
  • Thanks @Simbamangu. It didn't, in the end, get me to where I wanted. When I then try to overlay onto a scanned OS map however many or few GCPs I provide it puts it down about half a mile S of where it should be. I think I'm going to have several problems with this project & ISTM that this particular one was somewhere in the middle. I'll start a new question at the beginning & see how much else falls into place
    – Ian Goddard
    Commented Oct 3, 2012 at 16:26

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