6.0 



5.0-f 



4.0 



0) £ 



I i J 



W 2.0 



1.0 



0.0 















Jetty Construction 





V 





















\ 







Polygons 1-18 





















P 



olygons 4-9, 



11-13 



















\ - 





i 



i i 



i ii 



1 1 1 



i i i 



I I 1 



I 1 1 



Jan-67 Jan-71 



Jan-75 Jan-79 



Jan-83 



Figure 49. Growth of the ebb-tidal shoal at East Pass, FL. Areas used in the computations 

 are shown in Figure 47. Growth rates are dramatically different depending upon 

 which polygons are included in the volumetric computations 



This section outlines a basic procedure that can be used to calculate 

 volumetric errors, provided that estimates of vertical (AZ) accuracy are 

 available. If AZ values are unavailable for the specific surveys, standard 

 errors of ± 0.5, + 1.0, or + 1.5 ft, based on the class of the survey, can be 

 used (Table 6). For coastal surveys close to shore, this method assumes that 

 errors in positioning (AX and AY) are random and have an insignificant effect 

 on the volumes compared with possible systematic errors in water depth 

 measurements, tide correction, and data reduction. For older historic surveys, 

 positioning error may be important, requiring a much more complicated analy- 

 sis procedure. Positioning accuracy of hydrographic surveys is discussed in 

 HQUSACE (1991) and NOAA (1976). 



The error in volumetric difference between surveys can be estimated by 

 determining how much the average depth in each polygon changes from one 

 survey to another and then calculating an average depth change over all 

 polygons. Maximum likely error (MLE) is: 



2 x AZ 



AZ(ave) 



118 



Chapter 5 Analysis and Interpretation of Coastal Data 



