Geodesy & Coordinate Systems Flashcards

Which of the following best describes the difference between NAD 83 and ITRF for high-precision hydrographic control?

Answer: NAD 83 is fixed to the North American plate while ITRF tracks tectonic motion globally

NAD 83 is a plate-fixed frame that moves with North America, so ITRF coordinates for US points drift ~2 cm/yr relative to NAD 83 due to tectonic motion.

A hydrographer working in Alaska notices that UTM zones overlap with Universal Polar Stereographic (UPS). At what latitude does UPS begin to replace UTM?

Answer: 84°N

UTM covers 80°S to 84°N; north of 84°N (and south of 80°S), the Universal Polar Stereographic system takes over to avoid extreme distortion near the poles.

What is the purpose of the 'epoch' specification when citing a dynamic reference frame such as ITRF2014?

Answer: It records the date at which coordinates were valid due to tectonic motion

Because tectonic plates move continuously, coordinates in a dynamic frame like ITRF are only valid at a specific epoch; specifying e.g. 2014.0 freezes the coordinate epoch.

Which NOAA tool is used to transform coordinates between NAD 83 and NAVD 88 using a hybrid geoid model?

Answer: VDatum

VDatum integrates tidal, orthometric, and ellipsoidal datums, allowing transformations among NAVD 88, MLLW, and ellipsoidal heights in a single tool.

On a Lambert Conformal Conic projection, along which lines is the scale factor exactly equal to 1.0000?

Answer: The two standard parallels

On a two-standard-parallel Lambert Conformal Conic, the cone intersects the ellipsoid at the two standard parallels, where scale is exact (k = 1.0000).

A benchmark has a published NAVD 88 orthometric height of 3.251 m. The GEOID18 model gives N = −28.421 m at that location. What is the WGS 84 ellipsoidal height?

Answer: −25.170 m

h = H + N = 3.251 + (−28.421) = −25.170 m; a negative ellipsoidal height means the point is below the WGS 84 ellipsoid.

Which parameter in the Helmert (7-parameter) datum transformation accounts for a slight rotation of the coordinate axes between two reference frames?

Answer: Rotation angles (ωx, ωy, ωz)

The three rotation angles ωx, ωy, ωz (often in arc-seconds) rotate one Cartesian frame into alignment with another, and are distinct from the translational shifts.