The graphs show that, in several of the isotherms, half- 

 minute depth changes as large as plus or minus 30 feet were ob- 

 served within 304 feet, which correspond to a slope angle of 5°40'. 

 However, 24. 3 percent of the adjacent half -minute readings showed 

 changes of less than one-half foot for the shallower isotherm; 16.6 

 percent showed changes of less than one-half foot for the deeper 

 isotherm. 



An example of a single cumulative frequency curve of depth 

 differences (fig. 17) represents data derived from section Q (fig. 

 16). The change in depth of the 19 C C isotherm may be plus or 

 minus with distance. To derive a meaningful value for a large 

 sample of slopes, the 50th and 70th percentiles were chosen. The 

 frequency distribution shows that for the shallower isotherm the 



CENTRAL 70 PERCENT OF DATA 

 |CHANGE| LESS THAN 4.75 FEE 

 I SLOP El LESS THAN 0°54 



TA. 



CENTRAL 50 PERCENT OF DATA 

 ICHANGEl LESS THAN 2.4 FEET 

 ISLOPEI LESS THAN 0°27' 



SECTION Q 

 19°C 



'CUMULATIVE PERCENTAGE' 

 DISTRIBUTION 



FREQUENCY DISTRIBUTION " 

 OF DIFFERENCES IN DEPTH 



40 



30 



20 



10 



-10 10 



DEPTH CHANGE (FEET) 



20 



30 



Figure 11. Cumulative percentage distribution of differences in depth between half- 

 minute or 304-foot-spaced readings of the 19°C isotherm (Section Q). The 25th and 75th 

 percentiles delineate the central 50 percent of data; the 15th and 85th percentiles 

 del ineate the central 10 percent. 



38 



