is marked by two flags for alinement on the range and horizontal sextant 

 angles were determined between the range and the marker flag on an adja- 

 cent range (Figure 3). Sextant angles were measured every 20 seconds 

 as the boat proceeded toward shore and were correlated with time marks 

 on the fathometer record. The distances along the range line were then 

 calculated from the angles and plotted with the appropriate sounding. 



3 . Reference Rods . 



The offshore profile measurements made with a fathometer were not 

 accurate enough to detect small changes in sand level on the shelf. Thus., 

 in order to monitor these changes along the profile, reference rods were 

 placed on the bottom on each range using the procedure of Inman and Rusnak 

 (1956). Arrays of reference rods were placed at depths of -16, -24, and 

 -33 feet (-4.9, -7.3, and -10 meters) on each range. Additional refer- 

 ence rod arrays were placed at depths of -45 and -65 feet (-13.7 and 

 -19.8 meters) on Indian Canyon Range (Figure 2). Each reference rod array 

 consists of four 3/8-inch brass rods that were 4 feet long and driven 

 into the sand bottom so that 1 foot of the rod was left exposed. The 

 rods were placed in a "T" pattern with 3 rods arranged in a line paral- 

 lel to shore and 1 rod offshore from the center rod of the line. Fig- 

 ure 2 shows the spacing of the reference rods and their number designa- 

 tion in the array. Some of the shallow reference rod arrays were changed 

 from a "T" to a "+" pattern with the addition of a rod with 2 feet 

 (61 cm) of exposed length placed 10 feet (3 meters) onshore from the 

 center rod of the line. This longer rod was added because sand deposi- 

 tion at the shallow reference rod stations buried the shorter rods in 

 winter. Absolute elevations of the reference rods were determined using 

 a sounding line at the time of installation. A diver held the line on 

 the bottom while an observer in a boat immediately above determined the 

 depth of water. This depth was then corrected to the datum of mean sea 

 level using the tide gage at the end of the Scripps Pier. Each depth 

 measurement was estimated to the nearest 0.1 foot (3 cm), using a grad- 

 uated sounding line. The mean of five measurements was then taken as 

 the depth of the bottom at the site of the rod. Mathematically these 

 measurements usually had a standard deviation of about 0.1 foot (3 cm). 

 However, there are operator biases and wire angle errors in the soundings 

 and additional errors in tide gage corrections that lead us to believe 

 that the probable accuracy of the absolute elevation is about 0.3 foot 

 (9 cm) . 



The reference rods were measured with each monthly survey of the 

 beach profiles. Measurements were made by divers who located the station 

 by range and horizontal sextant angle, marked it with a buoy, and found 

 the rods by underwater search from the buoy position. Positioning on 

 the surface was usually accurate enough to place the buoy anchor within 

 20 feet (6.1 meters) of the array edge. Often the buoy anchor was 

 placed within the margins of the array so that no underwater search was 

 required. The exposed length of each rod was marked on a piece of 

 plastic and measured at the surface. The reference rod elevations in 



