CH Geodesy & Coordinate Systems 2 — Questions and Answers
Question 1: Which ellipsoidal parameter defines the degree of flattening of a reference ellipsoid?
- Semi-major axis
- Semi-minor axis
- Inverse flattening (1/f) (Correct answer)
- Eccentricity squared
Correct answer: Inverse flattening (1/f)
Inverse flattening (1/f) directly expresses how much the ellipsoid departs from a perfect sphere, and is a defining parameter for datums like WGS 84 (1/f ≈ 298.257).
Question 2: In the context of hydrographic surveying, what does the term 'geoid undulation' (N) represent?
- The difference between ellipsoidal height and orthometric height (Correct answer)
- The curvature of the Earth's surface
- The tidal correction applied to depth soundings
- The angular separation between geodetic and magnetic north
Correct answer: The difference between ellipsoidal height and orthometric height
Geoid undulation N = h − H, where h is ellipsoidal height (GPS) and H is orthometric height (leveling), and it varies globally from about −105 m to +85 m.
Question 3: A hydrographer converts UTM coordinates to geographic coordinates. Which projection property is preserved in UTM?
- Equal area
- Equidistant
- Conformal (angle-preserving) (Correct answer)
- Azimuthal
Correct answer: Conformal (angle-preserving)
UTM is based on the Transverse Mercator projection, which is conformal, meaning small shapes and angles are preserved locally.
Question 4: What is the central meridian scale factor (k₀) used in the standard UTM projection?
- 1.00000
- 0.99960
- 0.99996 (Correct answer)
- 0.99980
Correct answer: 0.99996
The standard UTM k₀ = 0.99996 reduces distortion so that scale error stays within ±0.04% across the 6° zone width.
Question 5: Which geodetic coordinate is measured as the angle between the equatorial plane and the normal to the ellipsoid at a point?
- Geocentric latitude
- Geodetic (geographic) latitude (Correct answer)
- Astronomic latitude
- Reduced latitude
Correct answer: Geodetic (geographic) latitude
Geodetic latitude is the angle between the equatorial plane and the ellipsoidal normal at the point, which differs from geocentric latitude except at the poles and equator.
Question 6: When performing a datum transformation from NAD 27 to NAD 83, which method provides the highest accuracy for hydrographic applications in the US?
- Molodensky 3-parameter
- Bursa-Wolf 7-parameter
- NADCON grid-based interpolation (Correct answer)
- Helmert 6-parameter
Correct answer: NADCON grid-based interpolation
NADCON (National Adjustment of Datum for Continental US) uses grid-based interpolation of known control point shifts, achieving accuracy of ~0.15 m or better.
Question 7: A survey vessel records an ellipsoidal height of 52.4 m at a benchmark where the known orthometric height is 45.8 m. What is the geoid undulation at that point?
- −6.6 m
- +6.6 m (Correct answer)
- +98.2 m
- −98.2 m
Correct answer: +6.6 m
N = h − H = 52.4 − 45.8 = +6.6 m; a positive N means the ellipsoid lies above the geoid at this location.
CH Geodesy & Coordinate Systems 2
Which ellipsoidal parameter defines the degree of flattening of a reference ellipsoid?