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| Digital
Image Rectification
Rectification
of aerial photographs involves the establishment of ground control
points that link each image to its corresponding aerial coverage
on a digital orthophoto quarter quad (DOQQ) that serves as the
base map. Points are chosen on the image that can be matched to
points on the DOQQ. Road intersections and other cultural features
are preferred as reference points rather than natural features.
However, in many cases cultural features are lacking and features
such as trees, shrubs, and the edges of water bodies are used.
Where possible, points are evenly spaced across the image, with
special emphasis on the edges of the image, and on areas near
to the shoreline.
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Once
all the ground control points have been established, the
image is rectified. Once the rectification is complete,
the image is made semitransparent and overlain on the DOQQ.
In some areas a "double image" occurs, indicating
that either more ground control points are needed, or one
or more existing ground control points have been placed
incorrectly. In these cases, images are reprocessed to correct
any remaining distortion.
Ground
control point selection in ER Mapper's geocoding wizard.
The program calculates the approximate RMS error for each
point.
Click
on images to enlarge.
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| A
poorly rectified image overlain on a DOQQ. A double image
indicates that more ground control is needed. |
A
well-rectified image overlain on a DOQQ as shown by a sharp
image. |
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Mesh of Delaunay triangles. |
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Rectification
by Triangulation
In
a Delaunay triangulation, each triangle is shaped
as equilaterally as possible by connecting each ground
control point (GCP) to its two nearest neighbors.
During rectification each GCP snaps to its corresponding
location on the base map and each triangle is stretched
and squeezed to fit by calculating the polynomial
coefficient of each triangle. Because multiple polynomials
are used, local distortion can be corrected. A large
number of well-placed GCP's could result in an almost
complete correction for any geometrical
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If the GCP's could be placed with 100 percent accuracy, accuracy
would be a function of image resolution and photograph scale. |
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Sources
of Error
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| Errors
in the data are difficult to measure and have several possible
causes: |
| •Lack
of suitable ground control point locations on some images |
•Artifacts
caused by the triangulation process |
| •Poor
choice of ground control points because of interpretation
error |
•Incorrect
position of mapped shoreline due to poor quality of image
or interpretation error |
| •Severe
image distortion |
•Positional
inaccuracies in the base map DOQQ |
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