ellipsoidal height differences, and the quantity *)***N **is the

relative geoidal heights from a station with a known

NAVD88 elevation, than is possible from the direct

geoidal height difference computed from the geoid model

application of absolute geoid heights to GPS networks.

(see Figure 1).

for determining NAVD88 elevations from GPS

from using GPS relative positioning and modeled geoid

ellipsoidal height data. These methods were tested on a

heights and the above relation can be calculated by the

network of points having known first-order leveled

summation of variance components corresponding to the

orthometric heights that were tied to first-order vertical

accuracy of the published orthometric height, the GPS

control. Results of the testing indicated that GPS-based

relative height determination, and the computed geoid

surveys could determine NAVD88 elevations to an

height differences.

accuracy of 30 mm when relative heights and differences

in geoid heights are applied.

benchmarks must be obtained from published sources

based on the results of a vertical network adjustment.

determined from DGPS-derived heights and geoid

Without this information it is presumed that a fixed

modeling is dependent on the accuracy of the GPS

vertical control point contributes no additional error to the

coordinate solution and the geoid model.

height of the unknown stations. The uncertainties in GPS

relative heights are estimated from the vertical component

error estimate produced from the GPS data processing

and adjustment software. An error estimate of 10 mm is

commonly seen as the minimum baseline error produced

from static type surveys. Relative geoidal height (F)**N**)

In addition to the guidelines discussed in section A-5, the

following procedures and methods are recommended and

precision from geoid modeling can have an expected

should be implemented when using GPS for elevation

standard deviation of 10-20 mm.

determination:

expected uncertainty in final orthometric height at the

unknown station of approximately 30 mm (at the 95%

depends in part on the length of the measured baseline.

confidence level) relative to the published elevation at the

Positioning errors grow in direct proportion to baseline

benchmark reference station. A repeatable accuracy of 30

length at a rate of approximately 1 part per million. For

mm meets or exceeds most feature elevation tolerances

networks with an area less than 20 km, the distance

specified for many USACE surveying and mapping

dependent error in the GPS vertical component (relative

projects, excepting certain high precision surveys such as

ellipsoid height) will be limited. Occupation times of less

for structural deformation monitoring. In areas with

than 1 hour (i.e., 20-30 min) should produce good results

obstructions, dense vegetation, or high relief between

for these shorter baselines. For project areas greater than

monuments or projects site, GPS can exceed leveling

20 km, the occupation times should be increased to a

accuracy when time is critical to the project.

minimum of 2 hours for primary and secondary control

points. Control points should be spaced throughout

(surrounding and within) the project area.

modeling software capabilities by CETEC, it was

Dilution of Precision (VDOP) is a measure of vertical

determined that higher accuracy elevations are obtained

positioning accuracy (due mainly to satellite geometry)

by the transfer of ellipsoidal height differences and