Control Surveying and Geodesy
Control and geodesy items check that your coordinates mean something: which datum, which realization, which projection, and what accuracy you can defend. Connecticut items lean on State Plane, GNSS procedure, leveling and the ground-to-grid relationship.
Datums and the Connecticut grid
- Horizontal positions use NAD83 realizations (for example NAD83(2011)); vertical positions use NAVD88 through a geoid model such as GEOID18.
- Connecticut State Plane is a single Lambert Conformal Conic zone: standard parallels near 41°12′ and 41°52′, central meridian 72°45′ W, origin latitude 40°50′ N, published in meters or US survey feet. Confirm the exact constants from the current NGS definition before publishing coordinates.
- Elevation factor = R ÷ (R + h); grid scale factor comes from the projection; combined factor is their product.
GNSS and leveling practice
Static, rapid-static, RTK and network RTK all require redundancy, independent checks and correct antenna heights. Ellipsoid heights converted through a geoid model are weaker than leveled heights. Differential leveling error grows with the square root of distance; balancing sight lengths cancels refraction and collimation error.
Worked example
Reduce a 1,000.00-ft ground distance at elevation 500 ft. With R = 20,906,000 ft, elevation factor = 20,906,000 ÷ 20,906,500 = 0.99997608. The projection scale factor at that location is 0.99998000. Combined factor = 0.99997608 × 0.99998000 = 0.99995608. Grid distance = 1,000.00 × 0.99995608 = 999.96 ft. Note the direction: elevation shrinks distances onto the ellipsoid (factor below 1), while a scale factor above 1 can offset it.
Traps
- Mixing NAD27 and NAD83, or US survey feet with international feet - a difference near 2 ppm that matters at survey accuracy.
- Reporting RTK-derived elevations as though they were leveled.
- Applying the scale factor in the wrong direction, dividing instead of multiplying.
- Using a geoid model built on a different datum realization than the coordinates.