th 
  
cter- 
sible. 
ition 
used 
each 
ders: 
n^. 
culty 
rious 
:cted 
tered 
10to- 
such 
  
  
  
    
    
   
  
   
  
  
  
  
  
  
  
  
   
  
   
   
  
  
  
  
  
   
  
  
  
  
  
  
   
   
  
    
     
REPORT OF COMMISSION VII 
  
] o 3 4 5 6 7 83 ? 10 
FiG. 10. A "Gray Scale" useful in determining or describing the photographic tone of an 
object. (Courtesy of Eastman Kodak Company.) 
In estimating the size of regularly shaped objects on photography flown to 
rigid scale specifications, photo-interpreters sometimes refer to object size scales, 
such as that shown in Figure 11. When using such scales to measure the crown 
diameter of a tree, for example, the scale is moved back and forth over the 
photo until a circle of comparable size lies alongside the image of the tree crown. 
The tree crown diameter in feet is then read directly on the scale, or interpolated 
in the case of intermediate sizes. The primary advantage of such a scale is the 
speed with which a large number of photo measurements can be made to the 
desired order of accuracy. Alternatively micrometer wedges may be used for 
quickly measuring the size of objects as seen on vertical aerial photographs 
(Spurr, 1948). 
SIZE SCALE 
  
1:10.000 
10 20° 30 40' 50' 60" 70 80“ go 100‘ 
| + || *+|e |. |. MU | M NM 
e e e e e e e e e @ 
  
  
  
  
  
  
  
  
  
  
  
  
FiG. 11. A “Size Scale” useful in determining the size of circular, square or rectangular 
objects at a specified scale. (Slightly reduced in reproduction.) 
(3) Density Scales. As used by photo-interpreters, the term “density” 
usually refers to the proportion of the total ground area under consideration 
which is obscured in the vertical view by the objects in question (e.g. buildings, 
tree crowns, waterfowl, etc.). In estimating densities, photo-interpreters have 
found a density scale, such as that shown in Figure 12 to be quite helpful 
(Jensen, 1945; Moessner, 1947). When comparing an area on the photograph 
with such a scale, the density of the population being interpreted will be found 
either to match closely that of one of the ten squares, or to be of an intermediate 
value which can be interpolated. 
DENSITY SCALE 
  
  
  
10% 20% 30% 407 50% 60% y 80% 90% 100% 
. m n n" n = : : : 
lan LP a. m | 
"og wo". |", a8] 
m mgm nly RS . 
  
  
  
  
  
  
  
  
  
FIG. 12. A “Density Scale” useful in determining or describing the density of objects in a 
given area from vertical aerial photographs. (After Jensen, 1945.)