Control / GPS Surveys

Control surveys establish a common, consistent network of physical points that are the basis for controlling the horizontal and vertical positions of transportation improvement projects and facilities. Corridor control surveys ensure that adjacent projects have compatible control. Project control surveys provide consistent and accurate horizontal and vertical control for all subsequent project surveys -- photogrammetric, mapping, planning, design, construction, and right of way.

Global Positioning System (GPS) is a space-based, microwave, 24-hour, all-weather, global military navigation system designed, deployed, financed and managed by the U.S. military authorities.
GPS has had a tremendous impact on surveying. In fact, the application of GPS for control surveys (or so-called "geodetic" surveys) was the first civilian use of GPS which was well beyond that for which GPS was originally intended by its military designers.
Nowadays GPS surveying techniques have completely replaced technologies such as Doppler satellite positioning and even long-range EDM for traditional first (and lower) order control surveys. However, the adoption of GPS is not restricted to control survey applications. More and more GPS is being used for cadastral, topographical and engineering surveying. The constraints that have previously restricted the application of GPS technologies, primarily those of cost (capital and running) and productivity (measured in terms of the numbers of points which can be coordinated in a day) are being aggressively addressed by the manufacturers, and it is confidently predicted that GPS will shortly be used by the majority of surveyors and geomatic engineers.


USGS NAD'83 Control


State, County and City Control Surveys


Benchmarks and vertical control

Benchmarks are the fixed elevation markers against which the zero setting of the gauge is checked during its operation, from which hydrographers may recover chart datum for future surveys, and through which surveyors and engineers may relate their surveys and structures to chart datum. Hydrographic benchmarks landmark the elevation of the benchmarks above chart datum and this procedure is basic to charting and gauging procedures.
As part of the installation procedure of any water level gauge, a minimum of three benchmarks are established in the immediate vicinity of the gauge, with no two in the same feature or structure. The elevation difference between the preliminary gauge zero and each of the benchmarks is then determined by accurate spirit leveling. When the elevation of chart datum is finally chosen with respect to the preliminary gauge zero, the benchmark elevations are converted and recorded in the benchmark descriptions as elevations above chart datum. If the water level gauge is to continue in operation, its permanent zero would be set to chart datum. The benchmarks provide for the recovery of chart datum in future surveys and for consistency in the setting of gauge zero for all water level measurements at the same site.