MARTHA'S VINEYARD MEASUREM NTS 
It is difficult to make a worth-while comparison between the hindcast 
data and that obtained with the pressure-type recorder due to the sporadic 
recordings obtained from the latter. During half of the time under observa- 
tion (September 20 - October 9, 1951) this gage was not functioning properly 
and no records were obtained. In contrast with the visual observations made 
aboard the Nantucket Lightship which numbered 116 (each representing a four- 
hour period), only 58 readings were obtained from the pressure gage records. 
There were no wave length measurements made and no wave directions given so 
that only the wave height and period can be compared. 
with 
A comparison of recorded/hindcast height and period (Figure 7) showed 
good similarity of phase pattern; however the average difference between 
hindcast height and recorded height showed the hindcast height greater by 
3.2 feet. The average period difference showed the recorded period as 
being 2.1: seconds greater than the hindcast. About 76 percent of all waves 
considered had a deviation (hindcast height less recorded height) of h feet 
or less (Figure 9). The height deviation between hindcast and recorded 
data is not as bad as it appears at first glance. The pressure head was 
located in 30 feet of water and waves having periods in excess of 3.5 
seconds would refract before reaching it, resulting generally in smaller 
heights. In the recorded data, none of the waves had a period of less than 
l seconds and 50 percent of the waves had periods greater than 8 seconds. 
An analysis of the hindcast data showed no waves with periods less than 3 
seconds and 50 percent of the waves had periods greater than 5 seconds, 
(Figure 10). Therefore all of the waves under observation would to some 
extent be affected by refraction. Figure 10 shows, as might be expected, 
that the hindcast periods fall between those of the deep water observations 
(where frequently too many waves are counted)and those of the underwater 
pressure head (where lower period components are reduced by pressure 
attenuation, and the method of analysis tends to result in higher periods). 
ACKNOWLEDGEMENT 
The Beach Erosion Board gratefully acknowledges the assistance of the 
Navy Hydrographic Office and the Coast Guard in this study. 
