surveying iii (topographic and geodetic surveys) - Modern Prepper

e. Field Processing and Verification. It is strongly recommended that GPS data processing andverification be performed in the field (when applicable) so that any problems may be identified andcorrected before returning from the field.5-34. Absolute Positioning. The accuracy obtained by GPS point positioning is dependent on the user'sauthorization. The SPS user can provide an accuracy of 80 to 100 meters. SPS data are most oftenexpressed in real time; however, the data can be postprocessed if the station occupation took place over aperiod of time. Postprocessing produces a best-fit point position. Although this will provide a betterinternal approximation, the effects of S/A (when activated) still degrade positional accuracy up to 80 to100 meters. The PPS user requires a decryption device within the receiver to decode the effects of S/A.The PPS provides an accuracy reading between 10 to 16 meters when a single-frequency receiver is usedfor observation. Dual-frequency receivers using the precise ephemeris may produce an absolutepositional accuracy on the order of 1 meter or better. These positions are based on the absolute WGS-84ellipsoid. The PPS that uses the precise ephemeris requires the data to be postprocessed. The militaryuses a GPS-S receiver that is capable of meter-level GPS point positioning without postprocessing.5-35. Differential Positioning. Differential GPS surveying is the determination of one location withrespect to another location. When using this technique with the C/A- or P-code, it is called differentialcode phase positioning. Differential code phase positioning has limited application to detailedengineering surveying and topographic site mapping applications. Exceptions include generalreconnaissance surveys and operational military or geodetic-survey support functions. Additionalapplications for differential code phase positioning have been on the increase as positional accuracy hasincreased. The code phase tracking differential system is a functional GPS-S for positioninghydrographic-survey vessels and dredges. It also has application for topographic, small-scale mappingsurveys. The collected data is used as input for a geographic information system database. A real-timedynamic DGPS positioning system includes a reference station, a communication link, and remote-userequipment. If results are not required in real time, the communication link can be eliminated and thepositional information is postprocessed.a. Accuracy of Differential Global Positioning System Surveys. Differential code phase surveyscan obtain accuracies of 0.5 to 0.05 meter. This type of survey can be used for small-scale mapping oras input to a geographic information system (GIS) database.b. Reference Station. A reference station is placed on a known survey monument. It is an area withan unobstructed view of at least four satellites, 10° above the horizon. It consists of a GPS receiver, aGPS antenna, a processor, and a communication link (if real-time results are desired). The referencestation measures the timing and ranging information broadcast by the satellites and computes andformats range corrections for broadcast to the user equipment. Using differential pseudoranging, theposition of a survey vessel is found relative to the reference station. The pseudoranges are collected bythe GPS receiver and transferred to the processor where PRCs are computed and formatted for dataEN0593 5-26