CH Geodesy & Coordinate Systems 3

Question 1

What is the primary reason hydrographers must apply a sea-level correction factor when working in high-elevation inland waterways?

Accepted Answer

Grid distances on the projection are shorter than ground distances

Answer

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?

Accepted Answer

Angles and shapes of small areas

Answer

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?

Accepted Answer

Semi-major axis a = 6 378 137.0 m

Answer

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?

Accepted Answer

Mean Lower Low Water (MLLW)

Answer

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)?

Accepted Answer

Deflection of the vertical

Answer

The deflection of the vertical is the angle between the gravity vector (plumb line) and the ellipsoidal normal, separating astronomic and geodetic latitudes/longitudes.

(CH) Certified Hydrographer Practice Test

(CH) Certified Hydrographer Practice Test

CH Geodesy & Coordinate Systems 3