Much Biological Work at the seashore is conceptually linked to tidal levels. The degree of achievement of real correlation varies from subjective assessment (i.e. guesswork) to a professional levelling survey. The latter is tedious, involves some expense, and adds heavy gear to that carried in fieldwork: its accuracy is beyond the requirements of much biological work.

The method described here is simple, accurate enough for most purposes and more desirable than guessing. The gear is cheap, easily made and carried. The method was first described by Morgans (1959).

The fundamentals of a levelling survey are a horizon and a datum point, or level.

This method uses the natural sea-sky horizon and assumes that lines of sight from the littoral zone to the marine horizon is horizontal. The method is only applicable where the shore transect can be related to a distant horizon by lines of sight.

Equipment

Two ranging rods are made from one-inch diameter poles. A thick nail is driven into one end of each and is beheaded to leave a spike projecting about one inch from the end of the rod or pole so that it mav perch well on rocky surfaces. The rod is cut to an exact length in feet (7ft. is most suitable, 6ft. is adequate). measured from the tip of the spike. The rod is painted with red primer. It is then marked exactly at one foot intervals and alternate segments are then enamelled white. A two-inch square plate of thin metal is screwed to the end opposite the spike, this plate being used as the bottom of the rod on sandy beaches.

In addition to the two ranging rods one needs a foot-rule (opaque white plastic is ideal) and a one-chain measuring tape which should be of best quality plastic. Since most tape spools corrode and become clogged it is best to buy merely a re-fill tape. An additional foot-rule is handy.

The Method

The observer holds his rod vertically on the survey station and views from any convenient level on the rod towards the ‘ruler page 188

Fig. 1: Sighting on a Rocky Transect.
Reading of Station 1 —3ft. level on rod. Reading of Station 2 — 6ft. 6.lin. on rod. Therefore drop from Station 1 = 3ft. 6.1in. Horizontal distance from Station 1 = 5ft. 7.7in.

man’ who holds the second rod vertically on the other marked survey station. The ruler-man holds the ruler horizontally against his rod and moves it at the observer's commands until its upper edge intersects the line of sight to the horizon (see Fig. 1). This ruler (or the spare ruler) is then used to measure the distance from the mark of its upper edge to the relevant foot-mark on the rod: observer and ruler-man work out the difference in levels between the stations and record them.

Sightings are repeated to check their accuracy. It should be possible to read levels consistently to the nearest 1/10th inch, even on beaches.

The distance between stations is measured by tape and it is carefully noted whether this is the horizontal or the sloping measurement (the former is most convenient but is only practical across short distances).

The observer moves to the newly surveyed station and the ruler-man takes up the next station in obvious continuation of the method.

The spike on the rod is useful for scratching station identity marks on rocks or sand. On very soft substrata the metal plate may be inadequate to prevent sinking and it is helpful to bury page 189 a flat stone (or small plank) to provide a base that maintains its position during work.

There are obvious refinements that can be introduced such as slides and spirit-levels on the rods.

Establishing Datum

Each transect must be related to a defined point (called the bench-mark) that is easily established in relation to the permanent surroundings. It is usually necessary to relate the bench marks of transects to land survey datum by means of professional surveying: after which, of course, one may relate transect levels to the tidal levels of the nearest tide guage.

It is sometimes adequate for one's purpose merely to relate the transect bench marks to one another (by surveying) and then to observe extreme low and high water levels at the transects as often as possible, interpreting these levels in terms of predicted tide levels for the district.

In certain cases (e.g. expeditions to remote places) it may be difficult to survey one's bench marks in relation either to one another, or to land survey datum. The only resort is to observe low and high water levels at the transects simultaneously (through co-operative assistants) and so to fix transect heights in relation to water levels.

Acknowledgment

The subject coding published with my recent article on ‘A Punched-Card Indexing System …’ is the version currently in use in this Department. I am reminded that it differs from my own final version in that a number of vacant codes have been filled and that there has been some rearrangement of the listing. For this I must acknowledge particularly the work of Dr. E. Young: also of Mr. P. Johns and others of this department who provided their preferred classifications in certain groups after discussion that was often considerable. I apologise for omission of these acknowledgments in my article.