Survey Benchmarks (Updated)

Note: This post updated on 1 Jan 2019 with some material from the DFO website that previously eluded me.

Sometimes, you find interesting things when you look down. Before boarding the ferry to the Magdalen Islands, I walked out onto the pier in Souris, PEI, to watch the C.T.M.A Madeleine arrive. Looking down at my feet, I spied what remained of a piece of bronze that was cast into the concrete deck may years previous.

Although damaged, I recognized (more or less) what this was. Way back in 1997, while taking my engineering degree at the Technical University of Nova Scotia (TUNS, now part of Dalhousie University), I spent a work term with the Canadian Hydrographic Service (CHS) - the organization whose name is partly obscured on the top of the bronze pin above. In my time with CHS, I "assisted" in the production of some of their charts that were being renewed at that time - I'm not sure that anything I produced ever actually made it into print.

After graduation and my return to Canada after two years abroad, I joined an engineering consulting firm in Halifax, and I spent several weeks inspecting the installation of a new brow stand on the south dolphin pier at Nova Scotia Power's Tuft's Cove generating station in Dartmouth, NS. A similar pin was cast into the concrete deck of this dolphin, and during the work, the concrete deck of the dolphin was jackhammered away. I took possession of the pin, and called one of my former co-workers at CHS to let them know that pin number 6444 was no longer in its original position and could no longer be relied upon for survey purposes. 

CHS benchmark #6444, removed from the pier in front of the Tuft's Cove Generating Station.

The bottom of CHS benchmark #6444, removed from the pier in front of the Tuft's Cove Generating Station.

CHS and other similar organizations, including the Canadian Geodetic Survey, use a system of these pins to provide geodetic benchmarks, or known points, to aid in their survey work and the maintenance of their maps and charts.

From the Fisheries & Oceans website (http://www.waterlevels.gc.ca/eng/info/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 website continues with the following regarding Vertical Datums (http://www.waterlevels.gc.ca/eng/info/verticaldatums/):

"All surveyed features on a navigational chart are positioned on some horizontal datum system such as NAD27 (North American Datum of 1927) or NAD83 (North American Datum of 1983). In addition to a horizontal datum reference, all charts also require a vertical datum reference.

For navigational safety, depths on a chart are shown from a low-water surface or a low-water datum called chart datum. Chart datum is selected so that the water level will seldom fall below it and only rarely will there be less depth available than what is portrayed on the chart. The following three criteria place somewhat more restriction on its choice: chart datum should be:

1. so low that the water level will but seldom fall below it,
2. not so low as to cause the charted depths to be unrealistically shallow, and
3. it should vary only gradually from area to area and from chart to adjoining chart, to avoid significant discontinuities.

On most Canadian coastal charts the surface of lower low water, large tide, or LLWLT , has been adopted as chart datum, but the term " lowest normal tide, " or " LNT, " has been retained on the charts since it encompasses a variety of other choices for chart datum on some older charts.

The choice of a chart datum is usually more difficult on inland waters than on coastal waters because inland waters lack the stabilizing influence the huge ocean reservoir exerts on the mean water level. Whereas a 2-month water level record at a coastal location provides sufficient tidal information to determine a reasonably accurate chart datum, many years of record may be necessary to provide the information on seasonal and secular fluctuations in mean water level required to determine chart datum on lakes and rivers. In non-tidal waters, chart datum is set so that the water level will be above datum approximately 95% of the time. A preferred guideline is that the daily mean water level should " never " fall more than 0.2 m below the chart datum during the navigation season.

The water level of a lake or river is always changing due to variations in supply and discharge or in meteorological conditions. Dry and wet periods in many drainage basins such as the Great Lakes seem to occur in several-year cycles, causing corresponding periods of low and high water. Chart datum must be set with the low-stage years in mind and may appear pessimistically low during high-stage years. On most lakes a single, level surface is adopted as chart datum over the whole lake. Along a river, chart datum is a sloping surface that approximates the slope of the river surface at low stage.

In non-tidal waters, chart datums are often assigned an elevation on some vertical reference system. On the Great Lakes, water level and chart datum elevations are presently referenced to International Great Lakes Datum 1985 (Fig. 1. IGLD 1985). Some of the previous vertical reference systems used for charting on the Great Lakes are IGLD 1955, USLS 1903 (U.S. Lake Survey 1903 Datum) and USLS 1935."

Interestingly (to me, anyway), while standing high above the harbour on a lookout over Cap-aux-Meules Harbour, I spied a similar survey benchmark pin. 

The lookoff in Cap-aux-Meules really does have a commanding view.

The benchmark pin on the lookoff in Cap-aux-Meules. 

Almost unreadable, I notice that there does not appear to be anything on this particular pin indicating it belongs to CHS. 

In the case of the three examples here, all were located in horizontal surfaces. Others may be found in walls, and may have horizontal lines through them to indicate a known elevation - and may have the letters BM, for bench mark, stamped into them.

Being many years beyond the single survey course that I took in 1995 or so, I'm not up on the finer points of how these benchmarks are located or used, or for that matter whether they are still used to any great extent - modern electronic survey technology may have rendered them superfluous. At the very least, I was unable to find any reference in the CHS online database to stations 6234 or 6444 - with the former damaged, and the latter removed, this may not be surprising - but the database seems to have a limited number of stations listed, and CHS may have culled their full inventory due to technological advances or simply the cost of maintaining them all. If I figure it out, I will update this post again.