surveying iii (topographic and geodetic surveys) - Modern Prepper

All carrier phase relative-surveying techniques, except OTF/RTK, require postprocessing ofthe observed data to determine the relative baseline vector differences. OTF/RTK can beperformed in real time or in the postprocessed mode. Postprocessing of observed satellitedata involves the differencing of signal phase measurements recorded by the receiver. Thedifferencing process reduces biases in the receiver and satellite oscillators and is performed ina computer. When contemplating the purchase of a receiver, the computer requirementsnecessary to postprocess the GPS data must be considered. Most manufacturers require apersonal computer (PC) with a math coprocessor. All baseline reductions should beperformed in the field (if possible) to allow an onsite assessment of the survey adequacy.a. Static Surveying. Static surveying is the most common method of densifying project networkcontrol. Two GPS receivers are used to measure a GPS baseline distance. The line between a pair ofGPS receivers from which simultaneous GPS data have been collected and processed is a vector referredto as a baseline. The station coordinate differences are calculated in terms of a 3D ECEF coordinatesystem that utilizes X, Y, and Z values based on the WGS-84 ellipsoid. These coordinate differences arethen subsequently shifted to the local-project coordinate system. GPS receiver pairs are set up overstations of either known or unknown locations. Typically one of the receivers is positioned over a pointwhose coordinates are known (or have been carried forward as on a traverse) and the second ispositioned over another point whose coordinates are unknown, but desired. Both GPS receivers mustreceive signals from the same four (or more) satellites for a period of time ranging from a few minutes toseveral hours, depending on the conditions of observation and the precision required.(1) Station occupation time is dependent on the baseline length, the number of satellitesobserved, and the GPS equipment. A good approximation for a baseline occupation of 1 to 30kilometers is 30 minutes to 2 hours. A rough guideline for estimating station occupation time is shownin Figure 5-5.Figure 5-5. Station Occupation TimeEN0593 5-28