Reference · 13 pages
Datums & heights
Horizontal and vertical datums, the ellipsoid and the geoid, NAD 83 and NAVD 88, and the modernized reference frames that replace them.
- What a datum isA datum is the agreed reference surface that turns a measurement into a coordinate. Surveying uses two kinds — horizontal and vertical — for two different reasons.
- Three surfacesGeodesy uses three surfaces — a mathematical ellipsoid, the gravity-defined geoid, and the physical ground. Confusing them causes most height errors.
- h = H + NThe single most useful equation in GNSS elevation work. What each term means, which sign convention applies, and where the error in the result comes from.
- GRS 80 and WGS 84GRS 80 and WGS 84 share a semi-major axis and differ in flattening only in the ninth digit. The practical difference is a tenth of a millimeter.
- NAD 27 vs NAD 83The shift from NAD 27 to NAD 83 changed the ellipsoid, the origin and the adjustment. Coordinates moved by tens of meters, by differing amounts.
- Realizations and epochsNAD 83 (2011) is not the same as NAD 83 (1986). A datum has versions because the network is readjusted and the ground itself moves — hence realizations and epochs.
- NGVD 29 and NAVD 88Two national vertical datums built on different principles. The difference varies across the country from roughly −0.4 m to +1.5 m, so no single conversion exists.
- NSRS modernizationNAD 83 is being replaced by NATRF2022 and NAVD 88 by a gravity-based geopotential datum. What is changing, why, and what a surveyor should do now.
- Geoid modelsA geoid model is a grid of geoid heights that turns GNSS ellipsoid heights into elevations. Hybrid and gravimetric models are not interchangeable.
- Height systemsLevel surfaces are not parallel, so a levelling loop does not close on raw height differences. Orthometric, dynamic and normal heights each answer that.
- CORS and OPUSBehind every published coordinate is a chain of observation, processing and adjustment. CORS and OPUS tell you how far to trust the number you hold.
- Datum transformationsThree tools for three problems — horizontal datum change, vertical datum change, and motion through time. Using the wrong one produces confident nonsense.
- Reading a datasheetAn NGS datasheet packs the provenance, accuracy and history of a control mark into a dense text format. Here is how to read it and what to distrust.
Other sections
All reference- Public Land Survey SystemTownships, ranges, sections and aliquot parts as the BLM Manual of Surveying Instructions describes them — how the rectangular system was laid out, how corners were monumented, and how a retracement follows it.
- Coordinate systems & projectionsState Plane, UTM, low-distortion projections, and the grid-versus-ground arithmetic that decides whether a measured distance matches a published one.
- Survey types & practiceWhat an ALTA/NSPS survey is and is not, boundary versus topographic versus construction work, and the vocabulary a client, an attorney and a field crew each use for the same thing.
- FS exam guideHow the NCEES Fundamentals of Surveying exam is structured, what each subject area covers, and how to build a study plan around it.