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I have four .tif images of the same (MxN) size, which I want to merge together. The images are not georeferenced (i.e., pixels are not associated to latitude/longitude coordinates). The 4 images are part of a same global map of the Mars surface and correspond to the top-left, top-right, bottom-left and bottom-right "portions" of such global map.

When I load the 4 images as raster layers in QGIS, they all overlap, since they are not georeferences. Therefore, when I use the function Merge, I obtain a (MxN) image instead of the (2Mx2N) desired image.

Is there a way to easily merge them in the correct way (i.e., telling that image 1 is the top-left one, image 2 is the top-right one, etc.)?

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  • 1
    Well, there's always georeferencing them relative to each other.
    – Vince
    Apr 6, 2021 at 0:16
  • 2
    For non-referenced images is preferable to use The Gimp; not QGIS.
    – xunilk
    Apr 6, 2021 at 2:01

2 Answers 2

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As I said in my comment, for non-referenced images is preferable to use The Gimp; not QGIS. However, if I use only The Gimp in my answer, I think there will be no implicit GIS component. Therefore, I will try to refer to QGIS from here on. First of all, I loaded in QGIS a non-referenced image (png format) that represents a Cylindrical Equidistant Projection of world (2048×1024) centered on the Pacific Ocean. It can be observed as follows:

enter image description here

By using The Gimp, I crop above image in four .png images of identical size (1024x512) following clock-wise sense (named img1, img2, img3, img4; respectively). As in your situation, when I load the 4 images as raster layers in QGIS, they all overlap, since they are not georeferences.

However, I know that original image has this reference in WGS84:

xmin  ymax  xmax  ymin
0      90    360   -90

So, by using gdal_translate command in OSGeo console of QGIS as follows:

gdal_translate -a_srs WGS84 -a_ullr   0    90   180   0    img1.png img1.tif
gdal_translate -a_srs WGS84 -a_ullr 180    90   360   0    img2.png img2.tif
gdal_translate -a_srs WGS84 -a_ullr 180     0   360 -90    img3.png img3.tif
gdal_translate -a_srs WGS84 -a_ullr   0     0   180 -90    img4.png img4.tif

I got four referenced .tif images. They can be merged without any problem. This merged image, completely indistinguishable of four individual images, it can be observed in following image.

enter image description here

You can adapt this approach for referencing and merging the 4 images of Mars surface.

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  • Perfect, I used gdal_translate to georeference the images, then I merged them in QGIS. Instead of WGS84, I defined another Coordinate Reference System, which was based on a spherical datum with radius 3396000 m, in accordance with the map-projection information contained in the labels attached to the images.
    – Simone
    Apr 6, 2021 at 15:14
  • It is valid too. With The Gimp this process is easier. In my case, I'd create an empty image with 2048x1024 size. Afterward, would load simultaneously the four individual images as layer, would move each one to corresponding position and, finally, would export resulting image as tif. I don't have to know about any georeference.
    – xunilk
    Apr 6, 2021 at 15:27
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convert PNG to GeoTIFF with GDAL has an answer that may be helpful to you.

You just specify corner coordinates and get a geoTif. Load them all in QGIS and you're done. You could also merge them with gdal_merge, or create a virtual raster etc.

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