A 



^ 



2 3 4 5 



2 3 4 5 



Flffure 8. "Picture of an object vltb one end Ijlng b«tv»en 

 grid lliMt. 



Figure 9-— Picture of ■□ object irtth both ends lying 

 betveen grid llnas. 



To obtain the length of any other object, 

 say C in figure 9, we determine the length of 

 G lying between grid lines 4 and 5 and between 

 grid lines 2 and 3 by the technique used in de- 

 termining the length of A in figure 8. The 

 length of C lying between grid lines 3 and 4 is 

 determined by inspection . The total length of 

 C then is equal to the sum of the lengths lying 

 between grid lines 2 and 3, 3 and 4, and 4 and 5. 

 The length, therefore, of any object lying 

 (perpendicular to the grid lines) anywhere on 

 the base plane (or appearing to lie on the base 

 plane so viewed in a photograph) can be deter- 

 mined free from the error caused by perspec- 

 tive. 



Thus, if the foregoing techniques are 

 used to eliminate errors due to parallax and to 

 perspective, theory indicates that longitudinal 

 measurements of fish can be made accurately 

 by means of photography. 



DESCRIPTION OF DEVICE V 



Since the design of the photographic 

 measuring device may be altered as further 

 tests dictate, only a general description is given 

 here. 



features, and (5) the wiring. All similar parts 

 are numbered so that the machine can be as- 

 sembled the same way each time . 



Base and framework 



The "base and framework" (figs. 10 and 

 11) consists of four sub-assemblies: (1) the 

 base, (2) the legs, (3) the cap, and (4) the cross 

 members #1 and #2. 



Base. --The 20" X 48" base was made of 

 a material called "sandwich board", consisting 

 of two, tliin sheets of aluminum between which 

 is a core of honeycombed metal. This type of 

 material was chosen because it combines great 

 flexural strength (i.e., resistance to bending) 

 with light weight. 



The base was spray -painted with a yel- 

 low, metal-etching zinc chromate primer. 

 Parallel grid lines 1.5 centimeters apart and 

 an orientation line perpendicular to the grid lines 

 were then scribed on the base, deep enough to 

 score the metal. TTie grid lines were then filled 

 with black Indi^ ink, and each was numbered 

 according to the total distance in centimeters 

 between it and the zero line . 



Photographic Measuring Device 



The photographic measuring device con- 

 sists of five major units: (1) the base and the 

 framework, (2) the strobe light and power pack, 

 (3) the camera assembly, (4) the automatic 



Legs. --The legs are hollow aluminum 

 tubes with an outside diameter of .628 inches, 

 1/ The commercial components described in 

 this paper were chosen only on the basis of local 

 availability. There may be many other types 

 that will serve as well. 



