|
Digital Photogrammetry
|
Implementation of Digital Photogrammetry for monitoring The Tsaolin Area in Taiwan
After completion of orientation process, stereo images were generated by resampling the original
images. While viewing the stereo images in stereo, topographic features including contour lines, spot heights, peaks, and drains were digitized. These topographic features were then used to build a TIN (triangulated irregular network) model. In addition, a GRID model, similar to DTM, was produced from the TIN model. To attain accurate illustration of the relief of the study area, the vertices of contour lines were sampled at 5-meter interval. The pixel size of the GRID model was set to 9 m x 9 m. This pixel size was selected because high quality orthophotomaps can be achieved using the existing map setting system. The GRID model, or DTM, was then used to orthorectify the photographs. Figure 3 shows the shaded relief and an image created by draping orthophoto image on top of the relief.
Figure 3. Shaded relief and orthophoto image draped over the relief.
The topographic features and DTM can be used for further analysis by using GIS software. Using the software developed by Jan et al. (2000), several physiographic parameters of the study area such as watersheds, stream networks, stream orders, stream lengths, drainage density, mean elevation, median elevation, bifurcation ratio, streams lengths ratio, stream areas ratio, and stream frequency can be readily derived from the DTM.
4. Discussion
In this study, the aerial photographs were obtained about three months after the earthquake. By examining the photographs, it is apparent that the landscape of the study area has been changed drastically. To understand the impacts of earthquake on the environment, and the risks resulted from the natural dam on the surrounding villages, a large-scale surveying about the area is very needed. This study shows that photogrammetry is very suitable for acquiring information about a large area, particularly when cost, effectiveness, efficiency, and accuracy are all considered.
The digital photogrammetric approach utilizes digital images to do all the measuring and digitizing works, therefore it is essential to have high-quality digital images. Extra caution should be exercised to retain appropriate contrast of the images both during the processes of duplicating film and scanning.
The accuracy of the photogrammetric mapping process highly depend on the accuracy of control points, equipment used for measurement, and most importantly, well-trained people that execute the tasks. To insure satisfactory results, ground survey is necessary to obtain data for the control points. In addition, the results of aerotriangulation provides the basis for model adjustment and further measuring tasks, therefore the aerotriangulation process has to be done carefully.
To estimate the volume of the natural dam and the amount of water, it requires data about the geomorphic status of the study area prior to the earthquake. Therefore, the photographs obtained before the earthquake can be used for photogrammetric processing. Further study is suggested to monitor the geomorphic changes in the study area in order to prevent disaster from occurring in the future.
5. Conclusion
The following conclusions are drawn from this study:
- Aerial photogrammetry
is an effective approach for acquiring
information about a large area, particularly
when cost, effectiveness, efficiency, and
accuracy are all considered. In addition, the
results produced using digital photogrammetric
approach can be readily used to establish
geographic database, which can be further
analyzed with GIS software. Integrated with GIS,
digital photogrammetric approach can provide
timely and accurate information about an area,
which may be valuable for various disciplines.
- High-quality digital
images are required for use in digital
photogrammetric processing. Therefore, the
processes of duplicating film and scanning
positives should be done with extra caution.
- The results of
aerotriangulation provide the basis for model
adjustment and further measuring tasks,
therefore the aerotriangulation process has to
be done carefully. When necessary, ground survey
should be done to acquire data about the control
points.
- Further study is suggested to monitor the geomorphic changes of the study area. It is recommended to acquire aerial photographs both prior and after the earthquake in order to monitor temporal changes in this area. Considering the cost and time constraint, as well as the desirable accuracy of the data generated from the mapping process, digital photogrammetric approach and GIS may be most suitable to meet the objective.
Reference
- American Society of
Photogrammetry, 1980. Manual of Photogrammetry.
Eds. Slama, C.C., C. Theurer, and S.W.
Henriksen, American Society of Photogrammetry,
VA, 1056 p.
- Chen, Y.K., J.F. Jan,
and H.Y. Lai, 1998. Mapping and establishing
topographic layers for GIS in mountainous forest
area by digital photogrammetric technique.
Quart. Journ. Exp. For. National Taiwan
University, 12 (3), pp. 139-156. [in Chinese
with English summary]
- Dowman, I.J., H. Ebner,
and C. Heipke, 1992. Overview of European
developments in digital photogrammetric
workstations. Photogrammetric Engineering and
Remote Sensing, 58 (1), pp. 51-56.
- Heipke, C., 1995.
State-of-the-Art of digital photogrammetric
workstations for topographic applications.
Photogrammetric Engineering and Remote Sensing,
61 (1), pp. 49-56.
- Jan, J.F., Y.K. Chen,
C.C. Cheng, H.Y. Lai, and S.C. Liaw, 2000.
Automatic extraction of watershed physiographic
parameters using digital terrain model and
geographic information system. Proceedings of
The 19th Symposium on Surveying Research and
Applications, August 24-25, Changhua, Taiwan,
pp. 455-464.
- Skalet, C.D., Y.G. Lee, and L.J. Ladner, 1992. Implementation of softcopy photogrammetric workstations at the U.S. Geological Survey. Photogrammetric Engineering and Remote Sensing, 58 (1), pp. 57-64.
|
|
|
|
|
