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I took some points with a GPS, and I'm trying to convert from GCS to OSGB 1936 British National Grid in SAGAGIS. I'm working with a shapefile. To reproject the vector layer to the OSGB projection I have used Proj. 4 (Command Line Arguments, Shapes).

Once the vector layer is reprojected, I loaded a .tif image to check that the projected points lie in their correct places. I set its projection using Proj.4 --> Set Coordinate Reference System.

However, when I try to overlay layer with the image, and the shapefile with the re-projected data, I can't overlay properly, and only appear the layer with the gps points but not the image.

How can I overlay those two properly in order to check that the points are in the adequate place?

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You did not tell the CRS of the image, but the lack of information itself tells that you missed the first step to deal with a non-georeferenced image.

The tool you used on the image Set Coordinate Reference System does set the CRS information by over-riding existing metadata. However, it does nothing if there is no metadata to be modified.

Follow these steps:

  1. Activate Define Georeference for Grids (under Projection > Georeferencing).
  2. Check the Grid system of your input image. Most likely you will see only column / row numbers, and it does not have cell size or extent information. Define cell size and origin (lower-left or upper-left corner) and run the tool. You will obtain a raster with grid definition, with a blank CRS.
  3. Activate Set Coordinate Reference System and run the tool to assign appropriate CRS to the raster.

Should the shapefile reprojected correctly, the (georeferenced) image will overlay properly.

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