[akdavis]

6G9. Operation of the stadimeter.

See Figure 6-9 which illustrates the following problem: Given a target vessel whose height from waterline to masthead is known to be 60 degrees feet, find the range. Starting with the periscope in high power and the stadimeter in the observing position, i.e., with the handwheel (12, Figure 4-24). turned counterclockwise to the limit of its travel, the target is brought approximately into the center of the field of view. The observer rotates the handwheel clockwise, causing the duplicate images of the target to separate, until the masthead in one image coincides with the waterline in the other. At this point the scale dials are as shown in the illustration, and the range (2,300 yards) is read on the range scale dial opposite the known height (60 feet) on the height scale dial. The stadimeter is then restored to the observing position by rotating the handwheel counterclockwise to the limit of its travel.
The following hints may be of value: a. Remember that the stadimeter measures only angles, and computes the range on the basis of the estimated height. Hence the range reading


Figure 6-9. Operation of the stadimeter for obtaining the range of an individual problem.

will be no more accurate than the estimate of the heights.
b. The dimension selected for this observation should be one which is known, or which can be estimated with fair accuracy. The reference points should, as far as possible, be definite, easy to see, and widely spaced. The masthead and waterline, for instance, while affording the greatest vertical dimension, might both be invisible at long range.
c. The stadimeter is graduated up to 11,000 yards Longer ranges may be obtained by remembering, that the angle subtended by 80 feet, for instance, at 20,000 yards, is the same as that subtended by 40 feet at 10,000 yards. Thus an object 80 feet high may be set up at the 40-foot line, and the range obtained multiplied by 2. d. The range scale dial is graduated for use with the periscope in high power. The stadimeter may also be used with the periscope in low power, by dividing the range, as read against the correct height of the object, by 4. In the case of high objects, the periscope should be placed in low power, and the object set up on the height scale dial at 1/4 its actual height. In this case the range reading will be correct.

So, observer obtains range (or estimates range) which is displayed on the indicator visible to the plot/TDC operator. Observer calls "mark" to associate this range with the current bearing of the optic (also visible to the plot/TDC). Target can now be plotted and information fed into the TDC. There is no need to call out numbers for range or bearing, as these are observable on the instruments visible to the interested parties.

Remember that none of the instruments visible to you while using the scope in game would be visible to the scope operator while looking through the optic in real life. You are, in a way, wearing several hats in game.