2

I've created a grid and points in QGIS which I used to survey. Using a test image at the moment and I'm trying to fit this image as an overlay within a boundary. When I insert the image it has a white background which I can't remove in QGIS.

Image 1 is what the area looks like, I want to get the survey results inside the red line. Image 2 is what happens when I insert the image, the inserted image of the geophysical results is a .BMP file, the same happens with .TIFF files.

The image is moved into place by georeferencing corners of the boundary area or points on the grid.

Image 1:

image 1

Image 2:

image 2

1

If the file is a tif, you can add transparency to it, either by

  1. adding a transparency channel, or

  2. with additional NODATA values.

For the first solution, you might clip your data by a polygon layer of the red line.

For the second solution, you can use the information tool on the raster layer, note the raster value in the black and white areas you want to remove, and add those as NODATA in the layers transparency tab. This might affect data inside your area of interest too, in which case solution 1 is necessary.

  • I'm assuming I'd have to add transparency in another program? If I can't do it in QGIS I might have a problem, or have to beg my manager to get some suitable software. Your second solution would likely wipe some of the data out since the geophysics data is all in greyscale, even missing one or two values wouldn't be good. – Peter B Jul 9 '15 at 13:19
  • 1
    Clipping by polygon can be done inside QGIS with Raster -> Clipper. For BMP it might not work since that format won't supprt transparency. gdal_translate can add an alpha channel (and convert to TIF), you can use it also via the Raster menu entries of QGIS. – AndreJ Jul 9 '15 at 13:31
  • I can access clipper via the Raster dropdown menu Raster -> Extractions -> Clipper but as far as I can work out can only clip in a square shape? I don't know if I'm just doing it wrong. – Peter B Jul 9 '15 at 13:39
  • 1
    Check the Mask Layer button, select a polygon layer (must be in the same projection as the raster), and activate the alpha band. – AndreJ Jul 9 '15 at 14:46
  • 1
    Most likely the polygon has a different CRS. Switch On-the-fly-reprojection OFF to check that. The raster must be a TIFF, because BMP has no spatial information stored inside. – AndreJ Jul 9 '15 at 15:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.