[GIS] Georeferencing an image with GDAL, based on QGIS Georeferencer Plugin

gdalgdal-translategeoreferencingqgisraster

I would like to georeference an image with GDAL. The image already has a spatial reference set (EPSG:31276). For the georeferencing, I have a list of 4 ground control points (also in EPSG:31276).

Using the QGIS Georeferencer Plugin, I can correctly georeference the image. I then select "Generate GDAL Script", which results in the following GDAL commands:

gdal_translate -of GTiff -gcp 6.55253e+06 -5.07402e+06 6.55273e+06 5.07395e+06 -gcp 6.55246e+06 -5.0711e+06 6.55263e+06 5.07106e+06 -gcp 6.55477e+06 -5.07382e+06 6.55511e+06 5.07368e+06 -gcp 6.55483e+06 -5.07197e+06 6.55525e+06 5.07197e+06 "C:/gis_sample_data/sample_image/image1.tif" "C:/Users/XYZ/AppData/Local/Temp/image1.tif"

gdalwarp -r near -order 1 -co COMPRESS=NONE  "C:/Users/XYZ/AppData/Local/Temp/image1.tif" "C:/gis_sample_data/sample_image/image1_georef.tif"

Those commands run without an error, but the resulting image is not correctly projected. Here is a small part from gdalinfo "C:/gis_sample_data/sample_image/image1.tif" (the original image):

Corner Coordinates:
Upper Left  ( 6551897.000, 5074277.000) ( 18d40' 4.21"E, 45d48'39.73"N)

whereas gdalinfo "C:/gis_sample_data/sample_image/image1_georef.tif" (result from the GDAL call) shows

Corner Coordinates:
Upper Left  ( -534350.794,  156938.268) ( 34d51'24.74"W,  0d52'47.63"N)

How would I have to modify the GDAL commands to correctly georeference the image?

The sample image is part of the Baranja Hill data set, which can be downloaded from here.
The ground control points are those:

mapX,mapY,pixelX,pixelY,enable
6552733.64613180514425039,5073946.04477077350020409,6552533.84482758585363626,5074021.48275862075388432,1
6552633.32449856773018837,5071059.01110315229743719,6552461.44827586226165295,5071104.32758620660752058,1
6555111.64040114637464285,5073678.52041547279804945,6554765.36206896603107452,5073821.32758620660752058,1
6555249.11819484271109104,5071965.62141833826899529,6554833.5,5071973.08620689623057842,1

When copied into a text editor and saved with the .points extension, they can be imported in the QGIS Georeference Plugin.

EDIT: Changed negative to positive values in gdal_translate. Results of gdalinfo:

Corner Coordinates:
Upper Left  ( -591887.923,  223739.838) ( 35d 8'28.93"W,  1d14'58.74"N)

By the way, I also tried adding -t_srs EPSG:31276, -s_srs EPSG:31276 and -a_srs EPSG:31276 and various combinations thereof to the gdal commands, but it didn't fix the problem.

Best Answer

The coordinates in your gdal_translate command should be all positive.

The GCP list is, so I think this could be a bug of the GDAL script creator.

EDIT

I could replicate your error, and think that your use case is not the intended use of GCP.

In "normal" georeferencing, the GCP points map pixel coordinates from an unreferenced picture to projected or geographical coordinates. The pixel coordinates are relative to the upper left corner.

In your use case, you take projected coordinates from a georeferenced tif, and compare them to "correct" coordinates. The coordinates on the left side are not relative to the upper left corner, but to the origin of the projection. If you start the reprojection process, this information will be distroyed and replaced by the GCP information. This does not seem to work.

In the GDAL batch file, you can add -of VRT to get vrt instead of tif files. You can open those vrt in a text editor and see that the matching goes terribly wrong.

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[GIS] Georeferencing png using gdal

Here's a world file for you (as per Gene's suggestion). Cut and paste the following into a text editor and name the file the same as your full-sized map but give it the extension '.pgw' instead on '.png':

1.0
0.0
0.0
-1.0
0.0
3231.0

I've made the following assumptions:

Your map has a resolution (pixel size) of 1m square (i.e. each pixel is a reasonable stride length for a fantasy game humanoid character). If I am wrong, edit lines 1 and 4 accordingly.
I have assumed that your map is not rotated.
I have assumed that your LL corner is at 0,0 in your world. If your pixel size is bigger than 1m square then you will need to multiply your Y coordinate (3231.0) accordingly.
Technically the UL coordinate (last two lines) is the center of your pixel. I have been lazy and set it to the corner. This in theory means that your coordinates would be consistently half a meter out but it's a fantasy world so nobody will know; it is consistent so won't screw up any distant calculations; and it saves a negative value as one of your origin coordinates. Adjust if you are sufficiently worried :)

I would then re-tile your map from the georeferenced full-sized one (this saves you making a world file for each tile)

Now, if you load your fantasy map into a GIS or similar system, you will probably be asked for the Spatial Reference System (SRS). The World file doesn't encapsulate that. It just locates and orients the image file in space relative to a nominal 0,0 origin. Since this is a fantasy map, all you need to do is pretend that it is in the real world somewhere and pick any coordinate system that gives you a flat projection with units in meters (or feet if you prefer but you will may have to adjust the values for resolution in the world file). A couple of suggestions for SRS could be:

EPSG: 3857 (aka 900913)
EPSG: 27700 (UK National Grid - the advantage of a national grid system UK or otherwise, is that you mostly stay clear of the poles and the international dateline and can hopefully avoid any weirdness which may result from such a location)

Your choice might not matter at all... on the other hand, you choice of SRS might be influenced by what you intend to do with your KML/CSV file (like you might be making a game with a post-apocalyptic theme but set in Quebec ...with zombies...). If you are using Google Earth or similar as a 'platform' for your game/fantasy world, so long as all your other data are in the same coordinate system, all should be well.

However, I do wonder if you may be better sticking with your PNG file as a text format like KML/CSV will be big. A PNG may be better for an internet-based game (reducing bandwidth). If you have a stand-alone scenario, most programming languages understand image formats well (especially if you call the GDAL libraries) and so I'm not convinced that a text-based format is even necessary here - but that's your choice of course.

[GIS] Georeferencer QGIS not working

If you look at the ground control points in the generated gdalwarp command you should see that they do not make sense.

     pixel   row     x        y 
-gcp 324.814 728.712 11.3582 43.5428 
-gcp 354.101 637.672 11.3582 45.5428 
-gcp 476.351 216.605 11.3586 43.5427 
-gcp 518.207 168.548 11.3585 43.5426 
-gcp 342.315 362.074 11.3585 43.5428

Two of the points have exactly same coordinates but still different pixel/row. Three other points have almost identical coordinates.

The reason may be that QGIS is automatically truncating coordinates at fourth place which is too coarse for you. As a workaround measure your Ground Control points in EPSG:32632 units or measure as EPSG:4326 but convert to EPSG:32632 for QGIS. You can do that with gdaltransform http://www.gdal.org/gdaltransform.html.

gdaltransform -s_srs epsg:4326 -t_srs epsg:32632
11.3582 43.5428
690515.193804498 4823796.38860415 0

Alternatively, if you have accurate EPSG:4326 coordinates you can edit the gdal_translate command by hand and run gdal_translate directly from the command shell.

Best Answer

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