Digital Photogrammetric Mapping

In the early stages, this Branch introduced aerial survey technology to support forest resource and land use surveys as well as the production of basic aerial photographic maps for Taiwan. Aircraft and aerial cameras were acquired to carry out aerial image collection, and analog orthophoto mapping technology from Germany was introduced, marking the transition into the era of mechanical stereoscopic mapping. To enhance mapping production techniques, a digital mapping system was introduced in 1995. Aerial photographs were scanned at high resolution and processed using newly acquired digital mapping equipment (P33) integrated with personal computers. This allowed for the simultaneous production of digital topographic maps, advancing the mapping operations from traditional mechanical stereoscopic methods to a new domain of digital cartography.
With continued technological advancements, a new digital aerial survey camera was procured in 2007, along with digital photogrammetric workstations and stereo mapping software packages. When combined with Position and Orientation System (POS), this significantly improved the efficiency of orthophoto production. Currently, the Branch’s primary mapping operations include the following:

(1)Digital Orthophoto Production

The digital orthophotos produced by this Branch are primarily generated using positional data acquired through the POS system, which provides the camera’s position, orientation, and exterior orientation parameters. By incorporating external digital terrain models, the production time is significantly reduced, allowing for rapid batch generation of orthophotos that maintain consistent scale and are georeferenced with geographic coordinates.
To ensure accuracy, these orthophotos undergo quality assessment using ground control points derived from field surveys or other reliable sources. Once confirmed within acceptable accuracy limits, the imagery undergoes tonal balancing and image mosaicking. The outputs are standardized according to the specifications of 1:5,000 scale topographic map sheets to facilitate efficient management and application. Finally, sensitive areas are masked, and map sheet metadata is compiled, resulting in official orthophoto map products for distribution.

Digital Orthophoto Production
　

(2)Orthorectification of Analog Aerial Photographs

To extend the temporal coverage of aerial imagery applications and enhance the usability of early analog (film-based) aerial photographs, historical negatives are digitized using high-precision photogrammetric scanners. For historical aerial photographs that lack orientation information, photogrammetric methods are applied to reestablish positional data and produce orthophotos.
During the orthorectification process for analog aerial photographs, internal orientation is first established using the fiducial marks on the scanned images. Then, through a manual point-matching approach, numerous unchanged ground features are identified and matched between existing orthophotos and the analog imagery. By incorporating digital terrain model data, the three-dimensional ground coordinates of these feature points are computed. Using these measurements, the exterior orientation parameters of the analog photographs are determined through either single-photo space resection or aerial triangulation block adjustment. With these parameters established, the orthorectification process can then be completed.

Scanned Images of Analog Aerial Photographs
　

Analog Aerial Photograph Frame Mark Measurement Operation