portability, automatic features, efficiency, and 

 accuracy. 



Portability 



The base of the photographic measuring 

 device used in the tests was not designed to be 

 disassembled. Consequently, the device cannot 

 be considered fully portable . Since making the 

 base in more than one part may introduce fur- 

 ther errors, it was decided to test the device 

 in its present form first to have a basis of com- 

 parison for any modifications made later. 



Automatic Features 



Since the device has not been operated 

 without the automatic features, there is no way 

 of deterinining the degree of added efficiency 

 for which they are responsible. In general, 

 these features were added to accomplish two . 

 things: (1) To save time when fish are being 

 measured in the field and (2) to eliminate pos- 

 sible sources of mistakes on the part of the 

 operator. From our experience in using the 

 machine, we feel that all of these features are 

 valuable aids in these two respects. 



Efficiency 



Taking Measurements 



The efficiency of the device under field 

 conditions remains to be tested. However, it 

 can be operated in the field or under laboratory 

 conditions at a greater speed than the methods 

 now in use. Under certain conditions in the 

 field, a greater speed of operation will mean a 

 larger sample of fish measured. In canneries 

 especially, where small boatloads qf fish are 

 processed immediately after they are received, 

 time becomes invaluable; 



Reading Film and Calculating Longitudinal 

 Distances 



Reading the film with the microscope and 

 calculating the longitudinal distances with these 

 readings is tiring and time consuming. The fact 

 that this must be done at all is a disadvantage 

 unique to this photographic method. In methods 

 now used by the Fish and Wildlife Service, the 

 measurements are read directly from the measur- 



ing device or from a paper tape upon which the 

 measurements have been stamped or recorded 

 by the device . In the photographic method, four 

 film measurements that have been corrected 

 for perspective are needed to calculate each 

 longitudinal distance. 



Accuracy 



Design of Tests 



The primary objective of these tests 

 was to determine the accuracy of the device in 

 its present form. At the inception of this work, 

 we decided that a tolerance of -I- 0.05 centimeters 

 would be our goal . 



A solid aluminum bar was used as the 

 subject instead of a fish, since a subject of in- 

 variable length was necessary so that a com- 

 parison of the computed length to the actual 

 length could be made . 



Since tlie length of the subject and the 

 position of the subject on the base may have 

 some effect on the accuracy of the results, tests 

 were made using subjects of different lengths 

 placed at different points on the grid. A bar 

 25.20 centimeters long was placed in nine posi- 

 tions on the grid for each of three tests - Aj, 

 A2, and An. Figure 17 shows the general loca- 

 tion of eacn of these positions. Similarly, a bar 

 76.11 centimeters long was placed in six positions 

 on the grid in test B; two positions above the 

 orientation line, two positions on the orientation 

 line, and two positions below the orientation line. 



Since the constants used in formula 10 

 depend upon the positions of the cameras, it fol- 

 lows that the positions of the cameras should 

 always be the same after the device is assembled. 

 This was tested by disassembling and assembling 

 the device prior to each of the three tests - A,, 



A , and A„ 

 ^2 3- 



Tests A, , A,, and Ao, therefore, con- 

 sisted of 9 negatives from the left camera and 

 9 negatives from the right camera; one left and 

 one right negative for each position. Test B con- 

 sisted of 6 left and 6 right negative." , 



11 



