Geodetic Datums & Coordinate Systems Flashcards

What is the purpose of the NGS NADCON5 tool?

Answer: Transforming horizontal coordinates between NAD27, NAD83 realizations, and NATRF2022

NADCON5 provides transformation grids for converting horizontal positions between NAD27, various NAD83 realizations, and the upcoming NATRF2022.

In the upcoming 2022 Terrestrial Reference Frame (NATRF2022), positions will be given at a specific epoch because:

Answer: Tectonic plate motion causes coordinates to change over time

Tectonic plate motion causes ECEF coordinates of fixed monuments to change over time, requiring an epoch to specify when the coordinates are valid.

What is the standard false northing assigned to the equator in the UTM system for positions in the northern hemisphere?

Answer: 0 m

In the northern hemisphere, the equator has a false northing of 0 m, so all northing values increase positively going north.

A geodetic survey finds a height difference of +2.352 m between two benchmarks using spirit leveling. This is best described as a difference in:

Answer: Orthometric heights

Spirit leveling directly determines orthometric height differences referenced to the geoid (gravity field), not ellipsoidal heights.

Which map projection property is sacrificed in the Lambert Conformal Conic projection?

Answer: Equal area

As a conformal projection, LCC preserves angles and shapes but distorts area, which grows increasingly toward the poles and map edges.

The t-T correction (second term correction) in State Plane coordinate computations accounts for:

Answer: The difference between the geodetic azimuth and the grid azimuth for long lines

The t-T correction adjusts for the angular difference between the curved geodesic on the ellipsoid and the straight grid line in SPCS for long survey lines.

What is the primary defining difference between the Clarke 1866 ellipsoid (used by NAD27) and the GRS80 ellipsoid (used by NAD83)?

Answer: GRS80 is defined by four physical constants including the Earth's gravitational constant, while Clarke 1866 was fit to best represent North America only

GRS80 is an internationally defined Earth-centered ellipsoid based on physical geodetic constants, whereas Clarke 1866 was empirically fit to minimize residuals across North American triangulation networks.