## Using Differential GPS Positioning For Elevation Determination

### Quiz Questions:

1. FGCC is currently known as ________________.

FGCS
USCG
USGS

2. Global Positioning System (GPS) surveying produces a set of X-Y-Z coordinates which can be transformed into geodetic latitude, longitude, and ellipsoidal height by using an reference ellipsoid to model the earth.

True
False

3. A determination of a NAVD88 elevation using GPS measurements at a given point requires transformation between ellipsoid and geoid based height systems.

True
False

4. ________ is the reference system most frequently used with off-the-shelf GPS equipment.

WGS74
WGS84
WGS94

5. The ortho-metric height of a point is the distance from the reference surface to the point, measured along the line perpendicular to every equi-potential surface in between.

True
False

6. The NAVD88 datum is the product of a horizontal adjustment of leveled height difference measurements made across North America.

True
False

7. Geoidal heights or (geoid height values) represent the geoid-ellipsoid separation distance measured along the ellipsoid normal and are obtained by taking the difference between ellipsoidal and orthometric height values.

True
False

8. The geoid itself is defined as a potential surface and natural variations in gravity induce a smooth, continuous, curvature to the plumb line and therefore physical equi-potential surfaces which are normal to gravity do not remain geometrically parallel over a given vertical distance.

True
False

9. NAVD88 elevations are not ortho metric heights that represent the geometric distance from the geoid to the terrain measured along the plumb line.

True
False

10. The World Geodetic System of 1984 reference ellipsoid is

Geocentric
Bi-axial
An ellipsoid of revolution
All of the above

11. The GRS80 ellipsoid is ______________________.

Exactly geocentric
Not exactly geocentric

12. The geoid model is constructed ____________________.

From a truncated functional series approximation
Using a spherical harmonics expansion
Using an extensive set of globally available gravity data
All of the above

13. Most recent geoid models have an absolute accuracy for _________________.

Geoid heights of one centimeter
Geoid heights of a few centimeters
Geoid heights of a few decimeters
Geoid heights of one meter

14. The lowest geoid heights in the US NGS GEOID96 model are

-7.2 meters
-21.5 meters
-39.8 meters
-51.6 meters

15. The geoid model GEOID96 is based on

NAVD29

16. HPGN is abbreviated as _______________________.

High Precision Geodetic Networ
High Probability Geodetic Network

17. The geoid height is added to the published orthometric height at the GPS reference station to determine its ellipsoid height to the accuracy level of the geoid model.

True
False

18. Once the reference stations ellipsoidal, orthometric and geoidal heights have been fully determined, elevations are transferred from the reference stations to the remaining points in the network according to the relation __________________.

Hi = hi − NI
Hi = Ni − hI

19. GPS baselines are not observed to determine ellipsoidal height differences across a network of stations.

True
False

20. GPS reference stations for accurate vertical adjustment should include at least __________ NAVD first order benchmarks.

Two
Three
Four

21. An error estimate of ______________ is commonly seen as the minimum baseline error produced from static type surveys.

10 mm
20 mm
30 mm

22. Relative geoidal height precision from geoid modeling can have an expected standard deviation of __________.

10-20 mm
10-30 mm
20-30 mm

23. In areas with obstructions, dense vegetation, or high relief between monuments or projects site, GPS cannot exceed leveling accuracy when time is critical to the project.

True
False

24. It was determined that higher accuracy elevations are obtained by the transfer of ellipsoidal height differences and relative geoidal heights from a station with a known NAVD88 elevation, than is possible from the direct application of absolute geoid heights to GPS networks.

True
False

25. GPS relative positioning accuracy depends in part on the length of the measured baseline.

True
False