A place in history: a guide to using GIS in historical research

CHAPTER 3: ACQUIRING SPATIAL DATA

3.8 Primary data sources

Two main primary data sources are data from satellite imagery, and Global Positioning Systems (GPS). Satellite imagery is a form of raster data in which each pixel represents a part of the earth's surface. The exact dimensions of the pixel depends on both the type of scanner used and the post-processing applied, but is usually a square with sides of from 1 metre to around 100 metres. For each pixel some information is provided about the light that was reflected back from that part of the earth's surface when the image was captured. Again, the exact details depend on the type of scanner and the post-processing applied. From this basic information, more sophisticated knowledge can be developed on, for example, types of land use, health of vegetation, and so on. It is not the intention to describe remotely sensed satellite imagery here, as many good guides are available. Obviously, satellite imagery has only become available in relatively recent times but it may provide useful information for historians, for example, in determining the location of certain features in remote areas, or to attempt to determine past land-uses. Areal photographs may be an alternative (secondary) source of data on land-use in the past.

GPS receivers are an easy way of primary spatial data capture. The simplest form of GPS receivers give the coordinate for the current location of the user. This is calculated from a network of satellites launched by the United States military. Initially the accuracy of these locations was deliberately degraded for non-military users to around 100 metres. This has now stopped and accuracies of only a few metres are available to all users. More sophisticated systems allow the user to capture multiple points as the receiver is moved and download them directly into a computer. If more accuracy is required differential GPS (DGPS) is used. This requires the use of two receivers, one of which is kept stationary at a known location to assist in measuring the location of the roving receiver. This can potentially produce sub-metre accuracy.