mate the expected correspondence between the 

 mapped positions of the depths and their true posi- 

 tions. This expected correspondence is expressed 

 as a probability that the true position falls be- 

 tween certain limits. For example, if we assume a 

 normal distribution of position errors in an area 

 where the standard deviation of the position error 

 is 0.2 nautical mile then the probability is 99 per- 

 cent that a given depth will be found within ±0.5 

 nautical mile of its indicated position (i.e., will be 

 found within a circle 1.0 nautical mile in diameter 

 centered on the given depth), 90 percent that it 

 will be found within ±0.3 nautical mile of its 



indicated position, or 50 percent that it will be 

 found within ±0.1 nautical mile of its indicated 

 position. The position of any isobath as shown 

 on the maps should be thought of as the center of 

 a probable range of positions rather than as a 

 single exact position. 



The above limits were computed by the same 

 formula as was the probable range of depths, ex- 

 cept that here /■ is the expected range of positions, 

 and (T is the standard deviation taken from the 

 position error diagram (fig. 3). 



This computation also applies only to map 

 errors. To find the expected range when the 



Figure 3.— Standard deviation of tlie isobath position error. (1) 0.a>-0.09 nautical mile. (2) 0.10-0.14 nautical mile. 

 (3) O.lfi-0.19 nautical mile. (-1) 0.20-0.24 nautical mile. (.5) 0.2.5-0.2!> nautical mile. ((>) O..30-0..39 nautical mile. 

 (7) 0.40-0.49 nautical mile. (8) 0.50-0.59 nautical mile. (9) 0.60-0.09 nautical mile. (10) 0.70-0.79 nautical mile. 

 (11) 0.80-0.89 nauUcal mile. 



46 



U.S. FISH AND WILDLIFE SERVICE 



