•  About Photogrammetric Base Maps
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About Photogrammetric Base Maps
Photogrammetry can be defined as the science of obtaining reliable measurements and producing maps by means of photography. Photogrammetric base maps are prepared from aerial photographs, called orthophotos, which have been corrected for distortion due to unintentional camera tilt and relief displacement. Photogrammetric techniques are commonly used in the preparation and revision of USGS topographic maps, and are essential to engineering and other mapping applications.

Photogrammetric base map production makes use of stereoscopic plotting equipment. Stereoplotters consist of a projection system, a viewing system, and a measuring and tracing system. First, orientation procedures are used to eliminate distortion due to camera tilt and relief displacement. The projectors are rotated about their x, y, and z axes to recreate the position of the cameras at the time of exposure of the stereopair, and the images are oriented to ground control points. The stereopair can then be used to create a terrain model allowing the operator to measure elevations and to trace features from the air photos onto a map sheet.

More Information
The UK's Photogrammetric Society maintains a list of WWW servers that provide information about obtaining photogrammetric base maps. The National Geodetic Survey (NGS) also has some information on its website relating to photogrammetry.

Also, visit our Aerial Photography section to learn more about photogrammetry and orthophotos.