Geodetic and Control Surveying
Datums and projections
Ohio horizontal control uses NAD83, current realization, and vertical control uses NAVD88. The Ohio State Plane Coordinate System has North and South zones and uses a Lambert conformal conic projection; Ohio law recognizes state-plane coordinates for describing locations when properly computed. GNSS-derived ellipsoid heights must be converted with a geoid model, such as GEOID18, to NAVD88 orthometric heights.
Scale factors and accuracy
State-plane grid distances differ from ground distances. The combined factor is the projection scale factor times the elevation factor; grid distance equals ground distance times the combined factor. Control surveys need redundant observations, adjusted networks, and documented accuracy at a stated confidence level. Ties to NGS CORS or published passive control should state epoch and datum.
Worked example
A ground distance is 1,000.000 ft. Projection scale factor = 0.99994; elevation factor = 0.99996. Combined factor = 0.99994 × 0.99996 ≈ 0.99990. Grid distance = 1,000.000 × 0.99990 = 999.900 ft. A crew that records 1,000.000 ft as the state-plane distance creates a 0.100-ft error before any other uncertainty.
Traps
Using ground distances directly in state plane; omitting the elevation factor; mixing NAD27 and NAD83; treating ellipsoid height as orthometric; and failing to check the geoid model or epoch. Exam questions often test the direction of the correction and the difference between scale and combined factors.