FUNDAMENTAL SURVEYING Geodesy and Map Projections 1 — Questions and Answers
Question 1: What shape best approximates the Earth for geodetic purposes?
- A perfect sphere
- An oblate spheroid (ellipsoid) (Correct answer)
- A prolate spheroid
- A geoid exactly
Correct answer: An oblate spheroid (ellipsoid)
The Earth is best approximated as an oblate spheroid (ellipsoid of revolution), flattened at the poles and bulging at the equator.
Question 2: What is the geoid?
- The mathematical ellipsoid used as a datum
- The equipotential surface of Earth's gravity field that approximates mean sea level (Correct answer)
- The topographic surface of the Earth
- A type of GPS satellite orbit
Correct answer: The equipotential surface of Earth's gravity field that approximates mean sea level
The geoid is the equipotential surface of Earth's gravity field that most closely corresponds to mean sea level.
Question 3: What is the difference between an orthometric height and an ellipsoid height?
- They are identical
- Orthometric height is measured above the geoid; ellipsoid height is measured above the reference ellipsoid (Correct answer)
- Ellipsoid height is always larger
- Orthometric height uses GPS; ellipsoid height uses leveling
Correct answer: Orthometric height is measured above the geoid; ellipsoid height is measured above the reference ellipsoid
Orthometric height (H) is measured above the geoid (mean sea level), while ellipsoid height (h) is measured above the reference ellipsoid; h = H + N (geoid undulation).
Question 4: What is the current official horizontal datum for the conterminous United States?
- NAD27
- NAD83 (Correct answer)
- WGS84
- ITRF2014
Correct answer: NAD83
NAD83 (North American Datum of 1983) is the official horizontal geodetic datum for the conterminous US.
Question 5: What is the current official vertical datum in the United States?
- NGVD29
- NAVD88 (Correct answer)
- ITRF96
- GRS80
Correct answer: NAVD88
NAVD88 (North American Vertical Datum of 1988) is the official vertical datum for the US, replacing NGVD29.
Question 6: A 'conformal' map projection preserves which property?
- Area
- Shape (angles) locally (Correct answer)
- Distance everywhere
- Direction from the center only
Correct answer: Shape (angles) locally
A conformal projection preserves local angles and shapes, meaning small features appear in their correct shape even though area may be distorted.
FUNDAMENTAL SURVEYING Geodesy and Map Projections 1
What shape best approximates the Earth for geodetic purposes?