CHANGE SERVICE REQUESTED The California Surveyor ... - CLSA

Continued from page 30
• By shifting the AbGPS-only control values as a group back
to the true position of HV-11, residuals were reduced to an
impressive 1’ level.
• The camera center shifts were in the same direction as the
AbGPS AT runs yet magnitudes did vary.
Conclusions
Ground control-less photogrammetry has many possibilities
yet the user must be aware of the shortcomings and have
ways of determining their existence, location, and magnitude.
In all of the sets of data shown above, a distinct bias can be
seen. This implies that “sky control” has good relative accuracy
yet weaker absolute accuracy. By removing this bias as is
seen in set 3, accuracies are quite impressive.
• AT set 1 may be usable for low accuracy orthophotography.
The largest error is seen in the Z, which usually has less effect
on ortho accuracy, however, without ground truthing errors
can only be estimated.
• The accuracy improvements seen in Set 2 show the importance
of some ground control. Using only one point lacks the
necessary redundancy.
• Set 3 revealed potential accuracies +/-6’ horizontally and
better than a foot vertically, suitable for smaller scale/larger
contour interval mapping.
• Vertical control accuracy should be better than 1/25000 of
the flying height for 6” photography. This translates to 0.14’ for
the 600-scale photography used for this project. Photography
at this scale is suitable for mapping at a scale of 1”=100’ with
a two-foot contour interval. The sky control measured in this
study will not support a two-foot contour interval mapping.
Additional ground control points would need to be added to
provide the required strength.
• Inaccuracies in the AbGPS-determined camera/photo center
positions (particularly vertical) can translate to even larger
inaccuracies of ground positions for this single model scenario
due to the shorter distance between photo centers (2200’)
transferred 3600’ to the ground (short backsite/long foresight
syndrome).
• Having some kind of ground truthing is very important.
The photogrammetric journey from an aerial photograph to a
map contains many components (all of which contain errors)
that can contribute to the final accuracy. Photo scale, AbGPS
sampling interval, satellite geometry, base station distance
from project, number of photos in block, aerotriangulation,
stereoplotter operator quality, etc., all contribute at various
levels. Once this product has been created, an assessment
and statement (metadata) of accuracy needs to be made. It all
comes back to the ground truthing. ❖
Caltrans Laser Scanning Research Update
By: Kevin Akin, PLS of Caltrans and Ty Lasky of University of California, Davis
Following is the research update that Kevin promised us in his article
“Caltrans Laser Scanning Research” in Issue #153. - Editor
The Advanced Highway Maintenance and Construction Technology Center (AHMCT) of the University of California at
Davis has completed a research project on stationary time of flight Laser Scanners for the California Department of
Highways (Caltrans). The research consisted of field trials of laser scanners on pavement, a bridge, and test objects.
The objective of this testing was to clarify the common limitations of 3D laser scanners, recommend mitigation methods,
help engineers and surveyors to select the right scanner, and determine optimum scanning settings for survey
applications in diverse situations. CAD data formats that should be used for archival and exchanged purposes are also
covered. This information will be used to create laser scanning survey specifications in the Caltrans Survey Manual. The
completed report can be found at:
http://www.ahmct.ucdavis.edu/images/AHMCT_LidarFinalReport.pdf. ❖
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