PS Geodetic Datums & Coordinate Systems 3 — Questions and Answers
Question 1: What is the approximate horizontal shift between NAD27 and NAD83 coordinates in the contiguous United States?
- Less than 1 meter
- 10 to 100 meters (Correct answer)
- 500 to 1000 meters
- More than 1 kilometer
Correct answer: 10 to 100 meters
NAD27 to NAD83 datum shifts in CONUS typically range from about 10 to 100 meters depending on location.
Question 2: What does the acronym ITRF stand for in the context of global geodetic reference frames?
- International Terrestrial Reference Frame (Correct answer)
- Integrated Topographic Reference Framework
- International Tidal Reference Format
- Inertial Tracking Reference Field
Correct answer: International Terrestrial Reference Frame
ITRF (International Terrestrial Reference Frame) is maintained by the IERS and represents the most precise global geodetic reference system.
Question 3: In SPCS 83, the false easting assigned to the central meridian of a Transverse Mercator zone is typically:
- 500,000 meters
- 200,000 meters (Correct answer)
- 0 meters
- 1,000,000 feet
Correct answer: 200,000 meters
SPCS 83 TM zones are assigned a false easting of 200,000 meters to keep all coordinates positive.
Question 4: Which of the following best describes an orthometric height?
- Height above the WGS84 ellipsoid
- Height above the geoid measured along the plumb line (Correct answer)
- Height above the local mean sea level gauge
- Height above the GRS80 ellipsoid
Correct 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).
Question 5: The Transverse Mercator projection is conformal, which means it preserves:
- Area
- Distance
- Shape (angles) locally (Correct answer)
- Volume
Correct 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.
Question 6: NGS OPUS (Online Positioning User Service) processes GPS/GNSS data and reports positions referenced to which datum?
- NAD27
- WGS84 (G1762)
- NAD83 (2011) epoch 2010.0 (Correct answer)
- ITRF2014
Correct answer: NAD83 (2011) epoch 2010.0
OPUS reports positions in NAD83(2011) epoch 2010.0, the current realization of NAD83 maintained by NGS.
Question 7: What is the primary reason surveyors apply an elevation factor (sea-level factor) when working in a State Plane coordinate system?
- To correct for atmospheric refraction
- To reduce ground distances to the ellipsoid surface (Correct answer)
- To account for the geoid-ellipsoid separation
- To convert from metric to US survey feet
Correct 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.
PS Geodetic Datums & Coordinate Systems 3
What is the approximate horizontal shift between NAD27 and NAD83 coordinates in the contiguous United States?