Dear All --
First of all: This is my first posting to this
list, and I would like to take the opportunity
and say a big "Thank You" to the developers of
GeoTools! Your library has allowed me to do some
immensely productive Java GIS coding.
But now I've run into a dead end (or so it seems).
I am trying to use GeoTools' "ProjectiveTransform(Builder)"
classes to transform the pixel coordinates of images
to real-world coordinates. I made some tests with
"AffineTransform" first and this worked fine, but
the images have a perspective distortion and a
simple affine transformation won't suffice.
Then I tested "ProjectiveTransform" with some synthetic
data, and the results looked promising. However, with
real data, the transformation seems to go out of control.
Below is a list of six points. On line "Transform",
you will find the image coordinates and the world
coordinates to which they are _supposed_ to map.
Under "World", you find the _actual_ mappings and the
difference (in map units, meters). As you can see,
the two are way off, with a maximum distance of
278 meters. Also note the huge RMS:
PROJECTIVE FORWARD RESULTS (IMAGE TO WORLD):
RMS = 134.46109310005252
Pt.: 1
Transform: 138.2/385.3 -> 513356.295/280624.993.
World X = 513393.74146133795 (37.446461337967776).
World Y = 280638.165346099 (13.172346098988783).
Dist = 39.695694583763874
Pt.: 2
Transform: 1267.3/415.3 -> 513357.71/280624.795.
World X = 513482.11524025456 (124.40524025453487).
World Y = 280597.9072626728 (-26.8877373271971).
Dist = 127.2777051229511
Pt.: 3
Transform: 2516.7/509.4 -> 513359.273/280624.553.
World X = 513583.4260212551 (224.1530212551006).
World Y = 280556.36530408286 (-68.18769591714954).
Dist = 234.294982473546
Pt.: 4
Transform: 2517.4/1589.3 -> 513359.071/280623.21.
World X = 513637.1007852404 (278.0297852404183).
World Y = 280607.33684666886 (-15.873153331165668).
Dist = 278.48252813687935
Pt.: 5
Transform: 1510.2/1744.6 -> 513357.854/280623.122.
World X = 513573.2481027579 (215.39410275791306).
World Y = 280647.9796511657 (24.857651165744755).
Dist = 216.82371255092065
Pt.: 6
Transform: 237.0/1685.9 -> 513356.421/280623.314.
World X = 513480.8793013462 (124.4583013462252).
World Y = 280688.37563165755 (65.06163165753242).
Dist = 140.43818813958055
Now, I have tested the same data with QGIS and
its projective referencing mode and it manages
to hit the target spot-on. So the problem is not
that the image is too distorted for a projective
transform. The RMS should be at most a few cm.
Also, if I scale the pixel coordinates to bring
them roughly into the same range as the real-world
coordinates (say multiply by one million), then
I get substantially better results (however, results
are still off the mark by several meters). This
leads me to suspect that there is a precision problem
involved here.
I use a "ProjectiveTransformBuilder" with a list of
"MapPosition" to build the "ProjectiveTransform". I
have tried toying with
"MappedPosition.setAccuracy(double)", but no luck
(I also don't understand how to set the double
parameter).
I have tested GeoTools 11 through 13 and am getting
the same results with each.
If anyone could point me into the right direction
for solving this problem, I would be very grateful.
I will gladly elaborate and also send sample data
via PM if anyone wants to take a closer look.
Thanks and best regards,
Ben
--
Dr. Benjamin Ducke
{*} Geospatial Consultant
{*} GIS Developer
Spatial technology for the masses, not the classes:
experience free and open source GIS at http://gvsigce.org
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