Geodetic Datums & Coordinate Systems Flashcards

What is the approximate horizontal shift between NAD27 and NAD83 coordinates in the contiguous United States?

Answer: 10 to 100 meters

NAD27 to NAD83 datum shifts in CONUS typically range from about 10 to 100 meters depending on location.

What does the acronym ITRF stand for in the context of global geodetic reference frames?

Answer: International Terrestrial Reference Frame

ITRF (International Terrestrial Reference Frame) is maintained by the IERS and represents the most precise global geodetic reference system.

In SPCS 83, the false easting assigned to the central meridian of a Transverse Mercator zone is typically:

Answer: 200,000 meters

SPCS 83 TM zones are assigned a false easting of 200,000 meters to keep all coordinates positive.

Which of the following best describes an orthometric height?

Answer: Height above the geoid measured along the plumb line

Orthometric height is the distance from the geoid to the point, measured along the curved plumb line (direction of gravity).

The Transverse Mercator projection is conformal, which means it preserves:

Answer: Shape (angles) locally

Conformal projections preserve local angles and shapes, meaning small figures on the map are geometrically similar to those on the ellipsoid.

NGS OPUS (Online Positioning User Service) processes GPS/GNSS data and reports positions referenced to which datum?

Answer: NAD83 (2011) epoch 2010.0

OPUS reports positions in NAD83(2011) epoch 2010.0, the current realization of NAD83 maintained by NGS.

What is the primary reason surveyors apply an elevation factor (sea-level factor) when working in a State Plane coordinate system?

Answer: To reduce ground distances to the ellipsoid surface

The elevation factor reduces ground-level distances to the ellipsoidal surface before applying the grid scale factor.