Implementation of Digital Photogrammetry for monitoring The Tsaolin Area in Taiwan
2.3 Analysis methods
In order to obtain high-quality mapping products, standard photogrammetric procedure was followed to acquire aerial photographs and process the related data. Figure 2 depicts the workflow of digital photogrammetric processing. In somewhat more detail these steps are described as follows.
Figure 2. Workflow of digital photogrammetric processing.
For each photograph comprising the stereo models, the control points including ground points of known horizontal position and/or elevation, and natural targets discernible on the photographs were identified. These points were then carefully measured using C-130 analytical stereoplotter. Triangulation was done using PAT-M local adjustment software.
In order to retain as much detail of the original film as possible, high-quality positive films were used to duplicate them. The duplicated positive films were then scanned using Intergraph Photo Scan system. The resolution used in the scanning process was 15 µm, or approximately 1,700 dots per inch.
The orientation process includes three steps, that is, interior orientation, relative orientation, and absolute orientation. Interior orientation reconstructs the bundle of light rays so that they are geometrically identical to those entered the camera lens at the time of exposure. Relative orientation reproduces the same perspective conditions between a pair of photographs so that the corresponding light rays in these two photographs intercept in space and a stereo model is formed. Follows relative orientation, the process of absolute orientation involves using control points with known horizontal and/or vertical positions to make the stereo model conform in scale and position with respect to the reference plane of the map sheet. At the completion of absolute orientation, the position of any point in the stereo model can be measured at the intersection of two corresponding light rays (American Society of Photogrammetry, 1980). The orientation process was completed using the mensuration software (ISDM) module provided by Intergraph ImageStation.
After completing the orientation process, the original imageries were resampled to generate epipolar images, which were used to form the stereo model. The images were viewed in stereo on the monitor using the CrystalEyes stereo viewing system. While viewing the images in stereo, topographic features of interest such as streams, contour lines, spot heights, and DTM (digital terrain model) grids were collected. The DTM were then used to orthorectify the photographs to produce orthophotos.
Together with other reference data, the images and collected topographic features were analyzed and special features were digitized as needed. The final products of the digital photogrammetric process include orthophotomaps and numerical data (e.g. contour lines, DTM), which may be readily used to build a geographic database for use in GIS (geographic information system).
3. Results
Control points are needed to scale and level the stereo model resulting from relative orientation. For each photograph used in the photogrammetric process, several natural targets distributed evenly over the photograph were identified. Together with existing known ground control points and control points obtained from control survey, all the points were carefully measured using Zeiss C-130 analytical stereoplotter. These points were then used as input to the PAT-M software, which was used to perform aerotriangulation. The scale of the maps to be produced was set to 1:5,000. Consequently, aerotriangulation was performed following the mapping standard.
The photographs were scanned at a resolution of 15 µm. The digital images were compressed to about 150 MB in order to reduce the required storage space. In addition, to facilitate rapid display of the images, overviews of the original images were created. These images were then used for orientation. The control points required for absolute orientation were obtained from aerotriangulation. To insure satisfactory orientation results, by repeatedly examining the results for possible errors the orientation process completed only if the error was less than 15µm, i.e. no more than a pixel.
After completion of orientation process, stereo images were generated by resampling the original
