PS Geodetic Datums & Coordinate Systems 2 — Questions and Answers
Question 1: What is the ellipsoidal flattening (f) of GRS80, the ellipsoid underlying NAD83?
- 1/298.257222101 (Correct answer)
- 1/297.000000000
- 1/298.257223563
- 1/300.000000000
Correct answer: 1/298.257222101
GRS80 has a flattening of 1/298.257222101, making it nearly identical to WGS84's 1/298.257223563.
Question 2: Which datum adjustment method was used to establish NAD83 and involved over 250,000 control points?
- Minimum-constraint least squares adjustment (Correct answer)
- Helmert transformation
- Bowditch (compass) rule
- Three-parameter Molodensky transformation
Correct answer: Minimum-constraint least squares adjustment
NAD83 was established using a minimum-constraint least squares adjustment holding one point (Meades Ranch area) and one azimuth fixed.
Question 3: In the Universal Transverse Mercator (UTM) system, what is the width of each zone in degrees of longitude?
- 3°
- 6° (Correct answer)
- 8°
- 12°
Correct answer: 6°
UTM divides the world into 60 zones, each spanning 6° of longitude, numbered 1 through 60 eastward from 180°.
Question 4: What is the scale factor at the central meridian of a UTM zone?
- 1.00000
- 0.99960 (Correct answer)
- 0.99980
- 1.00020
Correct answer: 0.99960
The UTM central meridian scale factor is 0.9996, reducing distortion by distributing scale error across the zone width.
Question 5: A geoid height (N) of +35 meters at a point means the geoid is located how relative to the ellipsoid?
- 35 m below the ellipsoid
- 35 m above the ellipsoid (Correct answer)
- 35 m above mean sea level
- 35 m below mean sea level
Correct answer: 35 m above the ellipsoid
Positive geoid height (N) indicates the geoid surface lies above the reference ellipsoid by that amount.
Question 6: Which NOAA geoid model is the current standard used in the contiguous United States for converting GPS ellipsoidal heights to NAVD 88 orthometric heights?
- GEOID03
- GEOID09
- GEOID12B
- GEOID18 (Correct answer)
Correct answer: GEOID18
GEOID18 is the current NGS geoid model for the contiguous US, superseding GEOID12B for NAD83/NAVD88 conversions.
Question 7: In the State Plane Coordinate System, which map projection is used for states or zones that are elongated in the east-west direction?
- Transverse Mercator
- Lambert Conformal Conic (Correct answer)
- Albers Equal-Area Conic
- Oblique Mercator
Correct answer: Lambert Conformal Conic
Lambert Conformal Conic is used for east-west elongated zones because it minimizes distortion along the two standard parallels.
PS Geodetic Datums & Coordinate Systems 2
What is the ellipsoidal flattening (f) of GRS80, the ellipsoid underlying NAD83?