In the absence of a Coast and Geodetic Survey “markers” at or near the site of a proposed fish farm, a secondary bench mark may be established. The following steps describe this process:
The time and height of the high and low waters occurring during the day of the field work at the proposed site is determined in advance (before field work day).
During the day of the field work and about an hour before the calculated time of high (or low) water, establish on shore or at the location of the proposed main gate, a tide pole to measure the high (or low) water level.
At the computed time for high (or low) water, mark on the tide pole the water surface elevation of high (or low) water.
Somewhere at halfway distance between the tide pole and the proposed BM, or at same strategic location, set up the engineer's level.
Set the rod at the high water mark on the tide pole and take backsight reading (BS).
Calculate the height of the instrument (HI) by adding the elevation of high (or low) water with the backsight reading, thus:
Elevation + BS = HI
Establish bench marks by driving wooden stakes onto the ground or by driving nails in permanent trees. Distinguish the bench marks by painting the wooden stakes or paint a ring around the tree where the nail is located.
Set the rod on top of the bench marks and take foresight readings (FS).
Calculate the elevation of the BM by subtracting the foresight from the HI, thus: HI - FS = Elevation. The computed elevation is the elevation of the bench mark.
Bench marks are set up at different strategic locations. Inasmuch as these bench marks are future references for elevations during construction, they should be properly described in the survey notes and identified in the field by dabbing the tops with a coat of paint. Permanent physical structures or objects such as stump of trees and top of big stones make a good permanent bench marks.
Care should be exercised when marking the high (or low) water mark on the tide pole, especially when watches are not set at the correct time. The highest water mark (for high tide) may be registered on the tide pole as indicated without regard to time.
