Reports
&
Analyses

Coastal survey methodology

This page describes the coastal survey method we used for surveying a raised beach along Kamishak Bay.

Along the Kamishak coast, we surveyed the uplift recorded by a widespread terrace using a simple horizon-line surveying method. Our goal was to estimate the difference in elevation between the uplifted beach feature and a similar but recently active feature. For example, we might compare the top of the uplifted beach to the top of the modern beach, or the top of uplifted beach ridges to contemporary beach ridges.

Vertical survey method

The ocean horizon can be used to roughly estimate horizontal. If you are near to sea level, the horizon appears at a level that is approximately the same as your eye height. This can be used for surveying the relative elevation of two points, as long as the horizon can be seen beyond one of the points as viewed from the other.

To survey the relative heights of points separated by up to 10's of meters in height, and up to hundreds of meters horizontally, we surveyed the difference in elevation of a series of intermediate points. Selecting a 4.3 meter pole from the beach driftwood, we marked it in 20 cm increments. One person would hold the pole at the lowest position of interest, and the other would walk uphill until the horizon appeared exactly at one of the 20 cm marks, or at the very top of the pole. After this height was recorded, the first person would carry the pole and set it where the second was standing, and the second would move further uphill for another reading. Later the total height could be calculated by first subtracting the eye height of the recorder from each of the recorded heights, and then adding all the resulting changes in height.

In this method there are several sources of error. The first is the ability of the recorder to precisely line up a mark with the horizon. To estimate the basic error introduced by reading, we did two surveys with a total of 28 individual measurements. The closure errors on these two surveys were just 3 and 6 cm. Note that this will not account for the error resulting from the fact that the horizon is not exactly horizontal, it is slightly below horizontal due to the curvature of the earth, which increases as the surveyor gains relative elevation with respect to it. This will introduce cumulative error that is a function of the horizontal distance and range of elevations surveyed and leads to underestimating the elevation of the high point. The horizon can also be distorted by optical effects, none of which were apparent during our survey. The precision with which points on rough ground can be re-occupied by the person carrying the rod can add several cm of error per intermediate measurement. Beyond these errors, there is also a relatively high chance of mis-transcription error, since there is no check on results. All together, it seems unlikely that the error would be greater than +/- 1 m.

Geomorphology

This survey was also dependent on identifying the geomorphology of the preserved beach and correlating it to similar features on the modern beach. Often we had to make correlative leaps, like surveying from a recent high-tide to a preserved beach ridge in one place, and then measuring from that same tide line to a modern beach ridge elsewhere. There were many cases where there was significant uncertainty in this correlation, and so the resulting data likely has additional error that may be greater than the error introduced by our survey method.

We surveyed both uplifted beach plains, and uplifted wave-cut platforms with beaches and bluffs. In two locations there were raised beach-ridge plains where we could measure the crests of individual beach ridges relative to lower modern beach features. In both of these locations, we identified several different levels of beach ridges, surveyed to the highest one, and recorded the crests of lower ridges. In the three areas where we identified uplifted wave-cut terraces, there was only one level terrace, and thus we surveyed the wave-cut platform to the base of the sloping uplifted beach, then up the beach to the break-in-slope at the base of the ancient stranded bluff. This upper measure, where the slope increased to nearly angle-of-repose, we correlated to the modern beach-tops. It is possible that this underestimates the original top of the beach, because the bluff may not have been fully eroded, and also the gradual collapse of the bluff after uplift may have buried the upper edge of the ancient beaches.