Vertical Control Benchmarks
Purpose and Importance of Benchmarks
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. Only the Hydrographic Service of Canada may assign or alter the elevation quoted for a benchmark above chart datum.
As part of the installation procedure of any water level gauge, a minimum of three benchmarks are established in the immediate vicinity (½ km) 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 levelling. 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.
The Tide and Water Levels Benchmark Data Base
The benchmark database is a warehouse of information on benchmarks related to DFO stations from around the country and was built for the regional tidal officers of the Canadian Hydrographic Service. The data and information resides on a server at the Integrated Science Data Management (ISDM). The data is stored at the ISDM but maintained by the regional CHS tidal officers. The data can be viewed by everyone but the maintenance access (Edit/Add/Delete) is password protected.