(686) 
direction and probable strength of current, the type of beach materials, the 
character of exits, the trafficability of inland terrain, etc. 
  
Fig. 5. Offshore data derived from metrical Fig. 6. Onshore data derived from metrical 
analysis. analysis. 
Metrical Analysis. It is not enough to discover the nature of features seen 
in the coastal zone. One must also determine exactly where they are located and 
what their dimensions are. Hence, measurements are required. A sand shoal may 
be recognized from a study of stereoscopic photography, but it is not completely 
analyzed until its distance offshore and location relative to the landing beach 
are known, and its depth and extent have been measured. Frequently, measure- 
ment is an important step in the identification itself; for example, a dark line 
behind the beach on the photograph may be a drainage ditch if 3 feet wide, and 
anti-tank ditch if 15 feet wide. 
The following are some of the important types of amphibious intelligence 
derived largely from metrics: 
In the offshore area: 
Underwater depths and gradients. 
Planimetric location of shoals, bars and other hazards. 
And above the waterline: 
Beach dimensions (widths at high and low water stages, overall and usable 
lengths, and onshore beach gradients). 
Whdths and gradients of natural and man-made beach exits. 
Dimensions of specific obstacles to movement such as seawalls, embank- 
ments, etc. 
Sufficient other metrical data in the beachhead area to permit the prepa- 
ration of large-scale (1/5,000) beach landing charts of resonable accuracy. 
Measurements in the horizontal plane are usually made simply and directly 
with a measuring scale (when vertical photography is available). The scale of 
the photography is first determined from comparison with available maps, 
from objects of known size in the photo, or by the focal-length altitude ratio. 
Usually no correction for minor tilts or changes of scale due to relief is made 
since a high order of accuracy is not usually required. 
Vertical measurements (other than water depth determinations) are nor- 
mally made by means of standard photogrammetric procedures and parallax 
measuring equipment. When vertical or horizontal measurements must be made