CH Geodesy & Coordinate Systems 3 — Questions and Answers
Question 1: What is the primary reason hydrographers must apply a sea-level correction factor when working in high-elevation inland waterways?
- Grid distances on the projection are shorter than ground distances (Correct answer)
- GPS signals are weaker at altitude
- Refraction corrections increase with elevation
- Magnetic declination changes with altitude
Correct answer: Grid distances on the projection are shorter than ground distances
At elevation, the Earth's surface is farther from the projection plane, so grid distances underestimate ground distances; the sea-level (elevation) factor corrects for this.
Question 2: In a State Plane Coordinate System (SPCS) zone that uses a Lambert Conformal Conic projection, what feature remains geometrically true?
- Distances along all meridians
- Angles and shapes of small areas (Correct answer)
- Areas of large polygons
- Straight rhumb lines
Correct answer: Angles and shapes of small areas
Lambert Conformal Conic is a conformal projection, so small shapes and local angles are preserved, making it suitable for zones with greater east-west extent.
Question 3: Which of the following is a defining characteristic of the World Geodetic System 1984 (WGS 84) ellipsoid?
- Semi-major axis a = 6 356 752.314 m
- Semi-major axis a = 6 378 137.0 m (Correct answer)
- Flattening f = 1/299.152
- Identical to the GRS 67 ellipsoid
Correct answer: Semi-major axis a = 6 378 137.0 m
WGS 84 is defined with a = 6 378 137.0 m and f = 1/298.257223563, making it nearly identical to GRS 80 but with a slightly different flattening.
Question 4: A hydrographer needs to report soundings relative to chart datum. Which vertical reference surface is most commonly used as chart datum in US waters?
- Mean Higher High Water (MHHW)
- Mean Lower Low Water (MLLW) (Correct answer)
- North American Vertical Datum 1988 (NAVD 88)
- International Great Lakes Datum (IGLD 85)
Correct answer: Mean Lower Low Water (MLLW)
MLLW is the standard chart datum for nautical charts in US coastal waters, ensuring soundings represent the shallowest expected conditions.
Question 5: What is the angular difference called between astronomic north (defined by Earth's rotation axis) and geodetic north (defined by the ellipsoid normal)?
- Magnetic declination
- Meridian convergence
- Deflection of the vertical (Correct answer)
- Azimuth correction
Correct answer: Deflection of the vertical
The deflection of the vertical is the angle between the gravity vector (plumb line) and the ellipsoidal normal, separating astronomic and geodetic latitudes/longitudes.
Question 6: On a UTM grid, what is the false easting assigned to the central meridian of each zone to avoid negative coordinates?
- 100 000 m
- 500 000 m (Correct answer)
- 1 000 000 m
- 250 000 m
Correct answer: 500 000 m
A false easting of 500 000 m is added to all easting values so that points west of the central meridian still have positive coordinates.
Question 7: Which geodetic quantity is determined by spirit leveling and represents height above the geoid?
- Ellipsoidal height (h)
- Orthometric height (H) (Correct answer)
- Dynamic height
- Normal height
Correct answer: Orthometric height (H)
Orthometric height H is the height above the geoid measured along the plumb line, obtained by spirit leveling with gravity corrections.
CH Geodesy & Coordinate Systems 3
What is the primary reason hydrographers must apply a sea-level correction factor when working in high-elevation inland waterways?