There
are two major considerations in determining the accuracy and value
of the depths printed on a nautical chart. These are the accuracy
of depth measurement and the accuracy with which latitude/longitude
values of each respective depth are known. This second element
is constrained by the accuracy with which the surveying vessel has
been navigated.
The
Coast and Geodetic Survey has always been concerned with assuring
that its surveying vessels have been navigated to the highest standards
of accuracy. For most of the first one hundred years of its existence,
all determinations of ship position were made by visual means. Deep
ocean surveys and oceanographic studies required using classical astronomic
means to position the vessel. However, for the inshore hydrographic
surveys that were the basis of coastal and harbor charts, the primary
means of navigation were by the use of the three-point sextant fix
or by means of shore observers simultaneously obtaining azimuths with
theodolites to the survey vessel.
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