28
THE FORESHORE
medium tides in each quarter of the lunar revolution during the year was assumed to delimit the rights of the Crown on the
seashore.
   In the specific case above referred to (“ Pierson v. Burnham, U.D.C. ”), the Board of Trade had granted in 1897 a lease of “the foreshore” to the Burnham (Somerset) U.D.C., using the Ordnance Survey map as indicating its boundary. The Ordnance Survey Department had revised their map in 1897, and again in 1902. The case turned upon the right of the contiguous owner to fence his land, which was above the flooding limit of all save extraordinary spring tides. In answer to inquiry by the plaintiffs solicitor, the Director-General of Ordnance Surveys stated that, in the preparation of Ordnance maps, tide lines were not referred to any datum, but were represented by the contour of mean tidal flow between springs and neaps. In reply to subsequent inquiry he stated that high-water contours were obtained by actual survey taken at the fourth tide before new and full moon. It further appeared that, as must obviously have been the case, the Ordnance surveyors actually took their observations on a “selected tide”. Now unless wind conditions are absolutely still when such survey is undertaken, it is obvious that the range of the tide might be materially raised or depressed from this cause. Again, the summit flow of tidal travel is so transient that its exact definition by survey would be wellnigh impossible.
   From the above considerations it appeared to those engaged in the case that the only practicable method of delimiting tidal flow was that of survey plus calculation. The perpendicular height of the “ medium tide” having been taken from the nearest observation point given in the Admiralty tide-tables of the year, and the necessary correction made for its variation, as ascertained by survey, for the point actually under observation, the contour line on the foreshore would by this method be ascertained by levelling from the Ordnance bench marks. Thus would contour lines indicating high-water level be accurately defined on a plan, and such contours would be based upon the known data of tidal gauges, disregarding the precarious observations of a fleeting line of tideway.