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_082h04_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:
AREA_OR_POINT=Area
Image Structure Metadata:
INTERLEAVE=BAND
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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