GIS Files 2: Geographical data

2.2: Surveying and remote sensing (2)

Photogrammetry – remote sensing

Photogrammetry is the science of measuring objects from photographs. Historically, this meant using aerial photographs to capture topographic information. The first photogrammetric surveys were conducted more than 100 years ago. Now satellite pictures are also used to record the location and geometry of features on the ground. Remote sensing is another term describing the use of aerial and space imagery to record geographical information. It includes the interpretation of other phenomena such as vegetation type or land use shown in the Earth's reflectiveness to different wavelengths of electromagnetic radiation.

Initially, maps were created from aerial photographs by various kinds of tracing mechanism. Sophisticated devices were engineered to allow an operator to view and trace a pointer around the visible features on the photograph. Using a system of wheels and pulleys, this motion was mechanically reproduced by a drawing arm. Such machines used stereoscopic viewing to survey in 3-D (three dimensions). As technology has advanced, the techniques of digitising and scanning have become important aspects of photogrammetry. Ordnance Survey has captured a significant amount of its detailed mapping by digitising from aerial photographs. Remote sensing by satellite is now widely used for data capture and, as the accuracy increases, this method could replace ground survey and aerial photos.

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