The hook
Every survey rests on a control network — the framework of high-precision points that everything else gets tied to. Modern control work is mostly GNSS; the principle (a sparse, accurate skeleton from which dense detail is densified) is timeless. NGS publishes control with horizontal and vertical accuracy classifications you have to be able to read.
Memorize these
Concepts that show up on the exam
Horizontal control
Network of points with published latitude/longitude (or state plane) coordinates. NGS classifies orders: A (best, sub-cm), B, 1st, 2nd, 3rd.
Vertical control
Network of benchmarks with published elevations on a vertical datum (NAVD88, NGVD29). Classified as 1st, 2nd, 3rd order based on standard error per loop length.
NGS (National Geodetic Survey)
Federal agency maintaining the U.S. geodetic control framework. Publishes the CORS network, OPUS, geoid models (GEOID18), and the data sheets for each control point.
Data sheet
NGS publication for each control point: coordinates, elevation, accuracy, monument description, history. The "birth certificate" of a control point — read before tying to it.
Order of accuracy
Higher order = tighter standard error per unit distance. Class B vertical: 0.7 mm × √k (k = km of leveling). Third-order: 12 mm × √k. The order tells you the budget you have to work within.
Network design
Decide where to put new control + how to tie it. Geometric considerations: avoid long, narrow networks (poor strength); include redundancy; tie to multiple existing points.
Bluebooking
Submitting GPS data to NGS for inclusion in the national network. Uses BLUEBOOK format — extensive metadata + multiple-day occupations.
- 1Recover existing controlVisit known monuments, verify they are intact and undisturbed. Photograph, GPS-pinpoint, and confirm coordinates against the data sheet.
- 2Plan the new network geometrySketch baselines connecting new points to existing control. Aim for redundancy (each point measured from 2+ directions) and good geometry (avoid long thin shapes).
- 3Observe with appropriate methodStatic GPS for high-order; RTK for moderate; conventional total station for high-precision short baselines (rare today).
- 4Reduce + adjustRun a least-squares network adjustment (constrained to the existing control). Inspect σ₀, residuals, and error ellipses.
- 5PublishDocument monument descriptions, photos, coordinates, accuracy. For NGS-eligible work, follow Bluebook submission requirements.
Don't fall for these
What trips people up
Tying to a single control point
One tie gives you NO redundancy and NO check. A typo, a moved monument, or a wrong data sheet all become unrecoverable. Always tie to at least 2 (preferably 3+) control points.
Mixing datum realizations
NAD83(86), NAD83(2011), NAD83(2022) — all "NAD83" but each shifted by cm to dm. Tie to control on the same realization, or apply transformations.
Ignoring vertical when you "only" need horizontal
RTK heights have geoid model uncertainty; vertical control should always be tied to NAVD88 benchmarks for any work that\'ll feed civil design. "Horizontal-only" data sets get demanded for elevation later 90% of the time.
Test yourself
How well did it stick?
A quick 5-question check on Control Surveys. See where you stand and what to review.