GPS uses a constellation of 28 satellites in orbit to locate positions on the Earth's surface. To locate a position on the surface, a minimum of 4 satellites are needed to give precise coordinates in 3 dimensions. Signals from satellites are received on the ground by users, and receiver units calculate the time delay of the signal to calculate a distance from the satellite; with the distances from all available satellites a fix of position is possible. While GPS can be an extremely precise mode of surveying, there are potential errors that can compromise the accuracy of the data. Types of errors are: atmospheric interference, electromagnetic interference near the receiver, multipathing, and dilution of precision (DOP). Of greatest concern to our survey are multipathing and DOP. Multipathing is the reflection of signals off of other objects altering the signal received, this is minimized by working in relatively open areas, and with receivers that use a differential GPS capability. DOP is caused by poor geometry of the satellites and range errors, these are common when working in confined areas, such as ravines or dense canopy, where signals from available satellites may be lost.
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