exercise extreme caution in applying GPS height determinations to projects that are based on

conventional orthometric elevations.

The heights obtained from GPS are in a different height system than those historically obtained

with geodetic leveling. GPS data can be readily processed to obtain the ellipsoidal height. This is the

height above or below a simple ellipsoid model of the earth. Geodetic leveling gives rise to a

orthometric height, often known as the height above the MSL. These heights are found on topographic

maps, stamped on markers, or stored in innumerable digital and paper data sets. To transform between

these height systems requires the geoid height. These height systems are related by the following

equation:

Where--

techniques are the same as for absolute positioning. In addition to these error sources, the user must

ensure that the receiver maintains satellite lock on at least three satellites for 2D positioning and four

satellites for 3D positioning. When loss of lock occurs, a cycle slip (discontinuity of an integer number

of cycles in the measured carrier beat phase as recorded by the receiver) may occur. In GPS absolute

surveying, if satellite lock is not maintained, positional results will not be formulated. In GPS static

surveying, if satellite lock is not maintained, positional results may be degraded resulting in incorrect

formulations. In GPS static surveying, if the observation period is long enough, postprocessing software

may be able to average out loss of lock and cycle slips over the duration of the observation period and

formulate positional results that are adequate. If this is not the case, reoccupation of the stations may be

required. 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 more

satellites tracked by the receiver, the more insensitive the receiver is to loss of lock. In general, cycle

slips can be repaired.

differential techniques. The first level is based on pseudorange formulations, while the other is based on

carrier beat phase 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 successive

code epochs. Use of the P-code, where successive epochs are 0.1 millisecond apart, produces results

that are about 1 percent of 0.1 millisecond (about 1 nanosecond). Multiplying this value by the speed of

light gives a theoretical-resultant range measurement of around 30 centimeters. If using pseudorange

formulation with the C/A-code, results can be ten times less precise (a range-measurement precision of

around 3 meters). Point-positioning accuracy for a