Page 67 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

46 Maritime Archaeology: A Technical Handbook, Second Edition
be the same for both units). Using DGPS with conventional GPS units it was possible to determine position with simple hand-held instruments to ±5 m. This is an accuracy acceptable for most maritime archaeological work, although it is expensive to do in real time and time-consuming to do post survey. When SA was turned off (thus canceling the dither) the accuracy of the system was then limited by instrumental errors introduced by atmos- pheric effects and the accuracy of the operational unit. This meant that to further increase the accuracy of the system, it was necessary to have more precise instrumentation and more control. For normal survey work, a hand- held instrument usually has a theoretical resolution of three decimal points in minutes (1.852 minutes). An experiment over an extended period of several hours showed that position converted to meters for GPS gave a standard deviation of 1.0 m and for the DGPS system a standard deviation of 1.5 m, indicating for some unknown reason that the DGPS quality was not quite as good as the GPS. In addition, the two systems gave slightly dif- ferent positions in latitude (a difference of about 4 m). The most significant factor, however, is that the standard deviation of position in the GPS is about equal to the resolution of the system (Figure 3.15). Using a standard GPS system, over a period of time the position will tend to wander and standard deviations of 5 to 10 m over a period of a year are expected.
GPS accuracy can be reduced by the geometry of the constellation of the satellites. This is known as dilution of precision (DOP). All receivers have some form of DOP calculation to determine which of the satellites in view are the best ones to track. The most frequently used is position dilution of precision (PDOP). PDOP factors in elements of both horizontal DOP and vertical DOP, and degraded PDOP will occasionally occur somewhere on the Earth’s surface. This area of degraded geometry is often referred to as a PDOP “spike.” Thus it is important to monitor PDOP to ensure that the position has not been degraded. In many cases the GPS alerts the operator to this situation, but when tracking where the operator is not usually mon- itoring the system this can fail to be noticed.
The GPS usually has an option to output data and these data can contain a number of different types of information. For the purposes of survey work, the most relevant data will be latitude, longitude, and time. Other information may relate to what satellites are being used and what the PDOP is. Output options usually include an internal system used by the manufacturer and a system known as National Maritime Electronics Association (NMEA) sentences. These sentences have a variety of prede- termined fields that the GPS outputs. You simply select the sentence that suits your needs and program that into the unit. From then on the unit will download data to a computer or data logger at the rate you specify for the unit.