surveying iii (topographic and geodetic surveys) - Modern Prepper

exercise extreme caution in applying GPS height determinations to projects that are based onconventional orthometric elevations.The heights obtained from GPS are in a different height system than those historically obtainedwith geodetic leveling. GPS data can be readily processed to obtain the ellipsoidal height. This is theheight above or below a simple ellipsoid model of the earth. Geodetic leveling gives rise to aorthometric height, often known as the height above the MSL. These heights are found on topographicmaps, stamped on markers, or stored in innumerable digital and paper data sets. To transform betweenthese height systems requires the geoid height. These height systems are related by the followingequation:Where--h = H + Nh = ellipsoidal heightH = orthometric heightN = geoid height5-27. Differential Error Sources. Error sources encountered in using differential GPS positioningtechniques are the same as for absolute positioning. In addition to these error sources, the user mustensure that the receiver maintains satellite lock on at least three satellites for 2D positioning and foursatellites for 3D positioning. When loss of lock occurs, a cycle slip (discontinuity of an integer numberof cycles in the measured carrier beat phase as recorded by the receiver) may occur. In GPS absolutesurveying, if satellite lock is not maintained, positional results will not be formulated. In GPS staticsurveying, if satellite lock is not maintained, positional results may be degraded resulting in incorrectformulations. In GPS static surveying, if the observation period is long enough, postprocessing softwaremay be able to average out loss of lock and cycle slips over the duration of the observation period andformulate positional results that are adequate. If this is not the case, reoccupation of the stations may berequired. In all differential-surveying techniques, if loss of lock does occur on some of the satellites,data processing can continue easily if a minimum of four satellites are tracked. Generally, the moresatellites tracked by the receiver, the more insensitive the receiver is to loss of lock. In general, cycleslips can be repaired.5-28. Differential Accuracies. There are two levels of accuracy obtainable from GPS using differentialtechniques. The first level is based on pseudorange formulations, while the other is based on carrier beatphase formulations.a. Pseudorange formulations can be developed from either the C/A-code or the more precise P-code. Pseudorange accuracies are generally accepted to be 1 percent of the period between successivecode epochs. Use of the P-code, where successive epochs are 0.1 millisecond apart, produces resultsthat are about 1 percent of 0.1 millisecond (about 1 nanosecond). Multiplying this value by the speed oflight gives a theoretical-resultant range measurement of around 30 centimeters. If using pseudorangeformulation with the C/A-code, results can be ten times less precise (a range-measurement precision ofaround 3 meters). Point-positioning accuracy for aEN0593 5-14