Geodesy, Coordinate Systems, and Control Surveys
What this domain tests
Geodesy, Coordinate Systems, and Control Surveys tests the reference surfaces, projections, and control networks that let a survey tie legally to a recognized coordinate system.
Datums and projections
- NAD83 is the horizontal datum for current New York work; NAD27 coordinates are legacy and differ by tens to hundreds of feet.
- NAVD88 is the vertical datum; NGVD29 is legacy. A geoid model converts ellipsoid heights to orthometric heights.
- The New York State Plane Coordinate System (NYSPCS) is a transverse Mercator system with four NAD83 zones: East (3101), Central (3102), West (3103), and Long Island (3104). Zones are published in meters and U.S. survey feet; do not mix.
- Grid scale factor and elevation factor combine: combined factor = grid factor × elevation factor. Ground distance × combined factor = grid distance.
- Convergence is the angle between grid north and true north; it varies with longitude and must be applied to bearings.
Control
- NGS monuments, CORS/OPUS solutions, and published NAD83(2011) coordinates are the backbone; check a second known point before accepting a base.
- Leveling: run loops, close on a known benchmark, and use the order of accuracy appropriate to the work.
Worked example
A ground distance of 1,000.000 ft has a combined factor of 0.99992. Grid distance = 1,000.000 × 0.99992 = 999.920 ft. If you record the ground value as a state-plane coordinate difference, you introduce 0.080 ft of error over 1,000 ft, which grows to 0.40 ft over a mile.
Traps
Mixing feet and meters; applying only the scale factor and forgetting elevation; using NAD27 control with NAD83 data; and treating a geoid model as a projection.