150 Years of Tides on the Western Coast: The Longest Series of Tidal Observations in the Americas

Theberge, Albert

The same year as the acquisition of California by the United States Government, the United States Coast Survey had been authorized to begin surveying the coast of Oregon Territory from the northern border of Alta California to the Puget Sound area of Washington Territory. Discovery of gold in 1848 and the subsequent gold rush added urgency to the requirement to chart our western coast including California. Small crews of Coast Surveyors headed west, either around Cape Horn or to Chagres, Panama, thence overland to Panama City, and then by ship to California. Accurate astronomic latitudes and longitudes of prominent points and landmarks on the Coast were first determined, magnetic declinations at these points observed, thence topographic surveys of critical areas including suggested locations for lighthouses were conducted, and finally reconnaissance hydrographic surveys based primarily on astronomic positioning and dead reckoning were run. Once this critical first work was accomplished, local triangulation schemes tied into the established astronomic positions were observed, and then topographic and offshore hydrographic surveys controlled by the triangulation were conducted. After years of work, an arc of triangulation parallel to the coast tied together all of the local triangulation networks and a coherent set of charts of our western seaboard based on a common geodetic datum were produced. In the early years, a final step prior to conducting hydrographic surveys was the installation of a tide staff in the local area of a survey so as to be able to determine the stage of tide during the time of survey work and relate the observed soundings to an arbitrary zero point. This arbitrary zero point could be the mean tide level, mean high water, mean low water, or, as in the case of the western shores of North America, mean lower low water. As technology improved with the development of self-recording instruments and a network of permanent gauges was installed, many serendipitous geophysical and oceanographic discoveries were made over and above the primary mission of providing safe passage for mariners. Remarkably, the self-recording gauge installed at Fort Point at the entrance to the Golden Gate and its successors have subsequently survived storms, earthquakes, the potential for human error and intervention, and produced the longest running series of tidal observations in the Americas. Since the early surveys, the study of tides has matured and accurate long-term tide predictions have been developed and coupled with real time water-level and meteorological observations to guide the shipping of America into its ports.

150 Years of Tides on the Western Coast: The Longest Series of Tidal Observations in the Americas

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