Photogrammetry and Survey

Photogrammetry and Survey
Photogrammetry is the process of producing maps from aerial photography. Aerial photography has inherent distortions due to the rotation of the aircraft and the relief of the terrain. It is the science and mathematics of photogrammetry and survey that seeks to eliminate these distortions and produce an accurate positional measurement of the terrain and features within the photography.

Survey Operations
Ground survey enables the photogrammetrist to assign an established horizontal and vertical coordinate system and datum to identifiable locations within the stereo photography. Surveyors target locations on the ground and provide a survey control network of these locations.

Stereo Aerial Photography
Aerial photography is obtained as a series of overlapping pairs. This overlap allows the images to be viewed in stereo. The survey targets are visible in the photography at key locations throughout the block of photography.

Aerotriangulation
The aerotriangulation process measures the survey locations and other identical points on successive pairs of stereo images. Combining the stereo measurements, ground survey information, and camera data, the photogrammetrist is able to create a block or bundle adjustment across the entire block of photography. The results of aerotriangulation are real world coordinates of photo centers and the precise rotational parameters of each exposure. With this mathematical solution, measurements can be made from the stereo images that are correctly positioned in three-dimensional real world coordinates.

Mapping Operations
Controlled stereo aerial photography provides the foundation for photogrammetric mapping operations. Operators interpret the stereo imagery on specialized workstations that allow precise 3D measurements. Mapping features such as terrain, structures, roads, hydrology, and vegetation are collected as layers of digital data.

Successful mapping from aerial photography requires photography that is not obstructed by snow, tall grass, leaves on trees or other conditions that impair the accuracy of the mapping. The accuracy of the mapping is directly related to the scale of the photography and to the control.

From the 3D compilation of features a base map is created. Base maps typically include features such as contours, spot elevations, buildings, roads, utilities, hydrology, vegetation, and coordinate grids.

Digital Terrain Models
Contours are created from the triangulation of digital terrain models. Compilation of the terrain typically involves the collection of 3D breaklines along features of break in the topography; e.g., ridges, drains, crests, curbs. Elevation points are collected to supplement breaklines. These 3D breaklines and random elevation points are processed through a triangulation to produce a surface model.

Fugro Horizons, Inc. performs cartographic edit operations on the contours generated from the final surface models. Index contours are depicted with a heavier line style and annotated with elevation values. Contours are often clipped at buildings. In some cases contours may by thinned in areas of heavy relief. In areas that are obscured due to vegetation or shadow in the photography; contours are dashed to indicate that their accuracy may not meet the targeted standard.

Mapping Accuracy
The accuracy of mapping can be described in terms of relative accuracy and absolute accuracy. Relative accuracy refers to the positional accuracy between the various features within the mapping. Absolute accuracy refers to the fit of the map features to their "real world" position. Relative accuracy is determined by the scale of the photography and the method applied to the relative adjustment of the photography. Absolute accuracy is also determined by those factors and by the distribution and quality of the control data.

Fugro Horizons' personnel are experienced in designing the photogrammetric approach to achieve the accuracy requirements of various mapping applications. Project design will establish the control methods and flight planning to ensure the relative and absolute accuracy of each project.

The field of photogrammetry relies upon several standards for the statement of accuracy, these include:

NMAS - National Map Accuracy Standards
NSSDA - National Standards Spatial Data Accuracy
ASPRS - Standards published by the American Society of Photogrammetry and Remote Sensing