[gdal-dev] Warping an already georeferenced image with control points

Jan Hartmann j.l.h.hartmann at uva.nl
Wed Feb 18 09:17:22 EST 2009

Thanks to Brent Fraser, I think I know now were the problem lies. He 
even sent me some suggestions for a workaround, without even exactly 
completely comprehending the problem (isn't that brilliant?). I want to 
rubbersheet maps (gdalwarp -tps), using already georeferenced maps and 
using control points in georeferenced coordinates. I do this to align 
exactly historical maps, but it is also something that is useful in 
edge-matching adjacent sheets of recent topographical maps. This 
*cannot* be done by the gdal_translate / gdalwarp suite. The problem is 
that control points have to be added to a tiff-file with gdal_translate, 
and when that is done, the file has lost its georeferencing information: 
SRS and dimensions. This is a restriction of the geotiff format, so the 
problem is unsolvable.

However, computationally everything is available in GDAL to implement 
this functionality. With gdaltransform, I can do exactly what I want: 
here, I translate a coordinate in the Dutch SRS (epsg:28992) one meter 
east, based on four control points, also in epsg:28992:

gdaltransform -s_srs epsg:28992 -t_srs epsg:28992                      
                       -gcp 179624 318062 179625 318062
                      -gcp 181371 318062 181372 318062
                      -gcp 181371 320751 181372 320751
                      -gcp 179624 320751 179625 320751 <<eof
 180000 320000
 180001 320000 0

So the problem is not that GDAL can't do it (GDAL can do practically 
anything:-)), it is just that the Geotiff file format isn't up to the 

Now, this geotiff file with gcps is just an intermediate file. When 
warping with control points, you need first to gdal_translate the 
original image into a second file with gcps added, and than you can 
gdalwarp that second file into the final map, a third file. I have 
always wondered why this intermediate gcp-file was needed: you could 
just as well transform directly from image to map, *if* gcps could be 
indicated with gdalwarp, or am I overlooking something? Adding a -gcp 
option to gdalwarp would AFAICS not be very difficult, it wouldn't break 
existing applications, and it would make warping by control points more 
efficient, because no intermediate files have to be generated any more. 
I think I understand how the gcp option came to be added to 
gdal_translate: some image providers, especially satellite images, 
provide their images with control points within the file, so the -gcp 
option has to be retained for gdal_translate.

So, even if I have been unable to make clear my exotic problems with 
historical maps, could this feature, adding a -gcp parameter to 
gdalwarp, be implemented with a reasonable amount of programming effort?



Even Rouault wrote:
> Yes, once you have specified GCPs with the -gcp option, you remove the 
> original georeferencing made of the origin coordinates and pixel size.
> Several notes :
> - There is an exclusive test in gdal_translate.cpp that prevents you from 
> setting both both GCPs and (origin coordinates, pixel size) at the same time. 
> That can be easily disabled, by removing the '&& nGCPCount == 0 )' test at 
> line 692 of gdal_translate.cpp. (*)
> - ... but the GeoTIFF driver doesn't support setting both GCPs and (origin 
> coordinates, pixel size) at the same time. It wouldn't read both either. 
> Hacking the GTiff driver might be possible to make it support both. But my 
> understanding of the GeoTIFF specification 
> (http://www.remotesensing.org/geotiff/spec/geotiff2.6.html#2.6.1) is that it 
> is not a foreseen case. Although I don't see it is explicitely forbidden, I 
> would anticipate that it wouldn't be handled well by other software.
> - The VRT driver happens to support writing and reading both both GCPs and 
> (origin coordinates, pixel size) at the same time. But I doubt this is the 
> result of a design decision. It more looks like this works because that's 
> what required the minimum amount of coding !
> - Although I'm not 100% positive, I am not aware of any other GDAL driver that 
> would support both at the same time.
> - I don't think that you actually loose information by transforming the 
> (origin coordinates, pixel size) into 4 GCPs. With the raster dimension and 
> the coordinates, it's easy to retrieve the pixel size.
> Even
> (*) To tell the truth, by looking at the code, I've seen a workaround that 
> doesn't imply changing the code. Just use the -a_ullr option...
> Le Friday 13 February 2009 18:52:29 Brent Fraser, vous avez écrit :
>> Jan,
>> I stepped through your procedure with a Canadian topo map to slightly
>> adjust its positioning, using GDAL 1.6.0:
>> 1. Warp raster image of topo map to a projected coordinate system
>> No Need.  Canadian topos already are georeferenced so I simply downloaded
>> http://ftp2.cits.rncan.gc.ca/pub/canmatrix/50k_300dpi/082/h/canmatrix_082h0
>> 4_tif.zip
>> 2. Attach my new GCPs to the original topo map:
>> gdal_translate --optfile gcps.opt -a_srs "EPSG:26912" 082h04_1_1.tif
>> 082h04_gcps.tif (the GCPs are in UTM zone 12, as indicated by the -a_srs
>> parameter)
>> 3. Warp/resample image using GCPs
>> gdalwarp -tps 082h04_gcps.tif 082h04_warped.tif
>> 4. Examine SRS of output image:
>> gdalinfo  082h04_warped.tif
>> Driver: GTiff/GeoTIFF
>> Files: 082h04_warped.tif
>> Size is 11786, 8881
>> Coordinate System is:
>> PROJCS["NAD83 / UTM zone 12N",
>>     GEOGCS["NAD83",
>>         DATUM["North_American_Datum_1983",
>>             SPHEROID["GRS 1980",6378137,298.2572221010002,
>>                 AUTHORITY["EPSG","7019"]],
>>             AUTHORITY["EPSG","6269"]],
>>         PRIMEM["Greenwich",0],
>>         UNIT["degree",0.0174532925199433],
>>         AUTHORITY["EPSG","4269"]],
>>     PROJECTION["Transverse_Mercator"],
>>     PARAMETER["latitude_of_origin",0],
>>     PARAMETER["central_meridian",-111],
>>     PARAMETER["scale_factor",0.9996],
>>     PARAMETER["false_easting",500000],
>>     PARAMETER["false_northing",0],
>>     UNIT["metre",1,
>>         AUTHORITY["EPSG","9001"]],
>>     AUTHORITY["EPSG","26912"]]
>> Origin = (275804.277072605620000,5462603.410837069200000)
>> Pixel Size = (4.233604626074191,-4.233604626074191)
>> Metadata:
>> Image Structure Metadata:
>> Corner Coordinates:
>> Upper Left  (  275804.277, 5462603.411) (114d 4'56.70"W, 49d16'30.13"N)
>> Lower Left  (  275804.277, 5425004.768) (114d 3'41.57"W, 48d56'14.30"N)
>> Upper Right (  325701.541, 5462603.411) (113d23'49.60"W, 49d17'28.67"N)
>> Lower Right (  325701.541, 5425004.768) (113d22'51.13"W, 48d57'12.15"N)
>> Center      (  300752.909, 5443804.089) (113d43'49.87"W, 49d 6'53.14"N)
>> Band 1 Block=11786x1 Type=Byte, ColorInterp=Palette
>>   Color Table (RGB with 256 entries)
>> The result of step 2 (gdal_translate) DOES remove the image's original
>> georeferencing.  Actually, it replaces it with a set of GCPs (and their
>> SRS).  To leave the original would resulting in two sets of conflicting
>> georeferencing information.
>> And to me it doesn't matter anyway, as that "GCP-ed" image is just
>> temporary.  After the gdalwarp step I get an image with more
>> traditional/well-supported georeferencing, I can delete it.
>> Should gdalinfo (and possibly other programs) use the GCPs to calculate the
>> image extents?  Maybe.  I'd find it useful as a quality-control check to
>> make sure I didn't mis-code a GCP resulting in a huge raster after warping.
>>  Do you have another reason?
>> Brent
>> Jan Hartmann wrote:
>>> Yes exactly! Brent highlights a point I never mentioned, and that may be
>>> the source of the confusion: when I add control points to an already
>>> georeferenced image, I gdalwarp it with the -tps option: a thin spline
>>> transformer, that transforms the control points to the exact
>>> georeferenced position, but stretches the areas in between. AFAIK, this
>>> is done by triangulation. This is different from a traditional warp, in
>>> which the eventual position of the control points on the georeferenced
>>> map has errors, although minimized ones. So in matching roads on two
>>> adjacent images, the road edges on both images are assigned the same
>>> georeferenced coordinates and then, after gdalwarp -tps, they exactly
>>> align.
>>> So my workflow is:
>>> gdalwarp raw image --> georeferenced image with correct projection,
>>> based on the corner points of the original map
>>> gdal_translate : add control points to projected image, essentially
>>> small corrections of road edges and triangulation points for
>>>                         which we know the exact coordinates from later
>>> sources
>>> gdalwarp -tps georeferenced image --> georeferenced image with small
>>> corrections, where roads, churcht towers are on their exact locations
>>> The whole procedure is something I do regularly with ArcGIS, not only
>>> with historical maps but also with things like aerial photographs, just
>>> like Brent describes. It can be done with gdal too, as described above,
>>> if only the second step would work. I still don't see why adding control
>>> points to an image that already has a projection defined, should destroy
>>> that projection information, including its georeferenced boundaries and
>>> dimensions. Even when this is not the case: when  a -a_srs flag is
>>> allowed for gdal_translate, it should be possible to add the complete
>>> geolocation information for that image too: bounds, pixel dimensions and
>>> geotranformation matrix, else the projection information doesn't make
>>> sense.
>>> Have I made my intention a bit clearer now?
>>> Jan
>>> Brent Fraser wrote:
>>>> Jan,
>>>>  I think what you want gdalwarp to do is [Delauney] image
>>>> triangulation.  Not an uncommon (or unreasonable) request.  I
>>>> occasionally have a need for triangulation when mosaicking several
>>>> satellite images.  Its visually pleasing to have the roads match up
>>>> precisely where the images overlap...
>>>> Brent Fraser
>>>> Even Rouault wrote:
>>>>> Jan,
>>>>> (I'm CC'ing the list)
>>>>> I'm not sure what you mean with adapting the pixelsize, now that the
>>>>> output has only GCPs and no more geotransform matrix.
>>>>> As far as including this option in baseline gdal_translate.cpp, I'm
>>>>> currently not really enthousiastic, as there are lots of way to get
>>>>> the result achieved (what is done with that patch could be done
>>>>> similarly with GDAL python API for instance) and the choice of
>>>>> corners is something rather arbitrary. Why should you trust the
>>>>> georeferenced coordinates of the corner points as reliable GCPs ? Why
>>>>> not some regularly discretized points along the edges ? Or a regular
>>>>> grid over the whole raster ? etc. etc. So, I'd prefer hearing from
>>>>> other GDAL developers or users that this patch is a sensical addition
>>>>> before commiting it.
>>>>> Even
>>>>> Le Thursday 12 February 2009 11:47:00, vous avez écrit :
>>>>>> Hi Even,
>>>>>> Thanks a lo. Just one small addition: the pixelsize has also to be
>>>>>> adapted, else you get an image with false dimension. If it's not too
>>>>>> much to add that, I'm going to try it out and will let you know the
>>>>>> results. If it works, I have indeed a practical solution for my
>>>>>> problem.
>>>>>> Even so, I am not convinced that this is an uncommon problem.
>>>>>> Consider a
>>>>>> very basic GIS functionality: edge matching.  You have digitized a map
>>>>>> from many sheets, and at the end the georeferenced images don't match
>>>>>> precisely at the edges. Edge-matching functions (ArcGIS has lots of
>>>>>> them) ensure that the edge of a feature on one map perfectly matches
>>>>>> the
>>>>>> feature edge on the adjacent map. This is exactly the kind of thing
>>>>>> you can do with gdalwarp using control points on a georeferenced
>>>>>> image. It is also more or less the thing I want to do with my
>>>>>> historical maps. Would this be an argument to preserve the boundaries
>>>>>> of a georeferenced image, when adding control points?
>>>>>> Jan
>>>>>> Even Rouault wrote:
>>>>>>> Jan,
>>>>>>> I concur with Jukka's analysis. Your need is rather uncommon, and I
>>>>>>> also
>>>>>>> thinks it shouldn't be a default behaviour. However, adding that
>>>>>>> capability to gdal_translate is quite easy. I've attached a patch for
>>>>>>> gdal_translate.cpp that adds the capability of adding the 4 corners
>>>>>>> as 4
>>>>>>> additional GCPs with the "-add_corners_as_gcps" option. You might
>>>>>>> give it
>>>>>>> at try (it applies cleanly on GDAL 1.6.0 and trunk). I'm not sure
>>>>>>> yet if
>>>>>>> it is valuable enough to include it in baseline gdal_translate.cpp.
>>>>>>> Best regards
>>>>>>> Le Wednesday 11 February 2009 15:59:48 Jukka Rahkonen, vous avez
>>>>>>> écrit :
>>>>>>>> Jan Hartmann <j.l.h.hartmann <at> uva.nl> writes:
>>>>>>>>> Sorry to keep moaning about this, but I need an indication what's
>>>>>>>>> going
>>>>>>>>> on here. Mind, I don't need an immediate solution: for the time
>>>>>>>>> being I
>>>>>>>>> have a workaround. Just an idea whether this a real problem, a dumb
>>>>>>>>> question, something that can be handled in the foreseeable future
>>>>>>>>> (or
>>>>>>>>> not), perhaps with adequate funding. Everything is better than
>>>>>>>>> talking
>>>>>>>>> to a blind wall.
>>>>>>>>> Sorry again Frank,
>>>>>>>>> Jan
>>>>>>>> Hi,
>>>>>>>> I think that your need to keep the original extents but add more
>>>>>>>> ground
>>>>>>>> control points inside the image frame is rather uncommon. Doesn't it
>>>>>>>> mean that you trust that the image corners are correctly warped to
>>>>>>>> a new
>>>>>>>> projection, but there are local distortions in the middle of image
>>>>>>>> which
>>>>>>>> should be corrected with a few extra ground control points? For my
>>>>>>>> mind
>>>>>>>> it shoudn't be the default behaviour of gdal but it might be
>>>>>>>> usable as
>>>>>>>> an user selectable option sometimes.
>>>>>>>> I know that missing gcp's at the image corners often leads to very
>>>>>>>> odd
>>>>>>>> result with polynomial warping because the formula shoots over. 
>>>>>>>> Even unaccurately placed gcp's could help a lot in preserving the
>>>>>>>> original shape of the map. I guess that you are perhaps playing with
>>>>>>>> scanned historical maps?  I have a few old scanned parcel maps which
>>>>>>>> are covering just the area of the farm, and two of the map corners
>>>>>>>> are just
>>>>>>>> white background. It is impossible to measure any real ground
>>>>>>>> control points from the corners because there is nothing on the map
>>>>>>>> to compare
>>>>>>>> with, and warping with all gcp's on the middle area makes really
>>>>>>>> funny
>>>>>>>> looking curves into the hand drawn rectangle framing the original
>>>>>>>> map.
>>>>>>>> -Jukka-
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