LSIT Geodesy and Control Surveys 4 — Questions and Answers
Question 1: Which geoid model is the official vertical datum reference for the conterminous United States?
- GEOID18
- GEOID12B (Correct answer)
- EGM2008
- NAVD 88
Correct answer: GEOID12B
GEOID12B is the NGS model used to convert between ellipsoidal heights (GPS) and orthometric heights relative to NAVD 88.
Question 2: A surveyor measures an EDM slope distance of 1,523.47 ft at a zenith angle of 88°14'30". What correction must be applied first before reducing to horizontal?
- Sea-level correction
- Scale factor correction
- Slope reduction using the zenith angle (Correct answer)
- Atmospheric refraction correction
Correct answer: Slope reduction using the zenith angle
Slope reduction (H = S × sin(zenith angle)) is applied first to obtain the horizontal distance before grid or sea-level corrections.
Question 3: In a geodetic survey, the 'deflection of the vertical' refers to:
- The angle between true north and grid north
- The angular difference between the normal to the ellipsoid and the direction of gravity (Correct answer)
- The difference in elevation between benchmarks
- The error in astronomic azimuth due to gyroscope drift
Correct answer: The angular difference between the normal to the ellipsoid and the direction of gravity
Deflection of the vertical is the angle between the ellipsoidal normal and the local plumb line (direction of gravity), affecting astronomic vs. geodetic positions.
Question 4: What is the primary purpose of a Helmert transformation in geodesy?
- Convert between geodetic and Cartesian coordinates
- Transform coordinates between two different datums using scale, rotation, and translation parameters (Correct answer)
- Project geographic coordinates onto a plane surface
- Correct GPS observations for ionospheric delay
Correct answer: Transform coordinates between two different datums using scale, rotation, and translation parameters
A Helmert (seven-parameter) transformation shifts, rotates, and scales a 3D coordinate set to align it with another datum.
Question 5: When performing a GPS survey to establish vertical control, the orthometric height (H) is derived by:
- H = h + N
- H = h − N (Correct answer)
- H = N − h
- H = h × N
Correct answer: H = h − N
Orthometric height H equals ellipsoidal height h minus the geoid undulation N (H = h − N).
Question 6: Which map projection is used by the US State Plane Coordinate System for states with a predominantly east-west extent?
- Transverse Mercator
- Lambert Conformal Conic (Correct answer)
- Albers Equal-Area Conic
- Azimuthal Equidistant
Correct answer: Lambert Conformal Conic
Lambert Conformal Conic is used for east-west oriented states/zones because it minimizes distortion along parallels.
Question 7: A NGS first-order horizontal control monument has a positional accuracy standard of:
- 1:10,000
- 1:50,000
- 1:100,000 (Correct answer)
- 1:1,000,000
Correct answer: 1:100,000
NGS first-order horizontal control requires a relative accuracy of at least 1 part in 100,000 between adjacent monuments.
LSIT Geodesy and Control Surveys 4
Which geoid model is the official vertical datum reference for the conterminous United States?