Geodesy & Coordinate Systems Flashcards

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

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.

In a State Plane Coordinate System (SPCS) zone that uses a Lambert Conformal Conic projection, what feature remains geometrically true?

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.

Which of the following is a defining characteristic of the World Geodetic System 1984 (WGS 84) ellipsoid?

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.

A hydrographer needs to report soundings relative to chart datum. Which vertical reference surface is most commonly used as chart datum in US waters?

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.

What is the angular difference called between astronomic north (defined by Earth's rotation axis) and geodetic north (defined by the ellipsoid normal)?

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.

On a UTM grid, what is the false easting assigned to the central meridian of each zone to avoid negative coordinates?

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.

Which geodetic quantity is determined by spirit leveling and represents height above the geoid?

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.