Surveying - The Science of Navigation - page 72

FIGURE 3.14. Measurement of distance using a stadia rod. In this example, the stadia
hairs in a sighting scope are separated by angle a = 0.01 radians, and a 5-m stadia
rod is placed a distance d away. We observe through the scope that 2.5 m of the rod
appear between the stadia hairs, and so the distance d is 250 m.
emerge from a comparative study, as instruments developed over the cen-
turies: increasing accuracy and reduced size. Thus, a fifteenth-century
cross-sta√ might be 90 cm long, while a mariner's quadrant from 1680 has
a radius of 75 cm. A backsta√ from 1740 has a maximum dimension of
63 cm. An octant from 1785 has a radius of 41 cm.
Old designs, and old instruments, depended upon size for accuracy
(more space to graduate the scale). An octant from 1943 has a maximum
dimension of 20 cm, though both it and its much larger predecessor from
1785 are calibrated to one minute of arc. A sextant from 1790 with a one-
minute Vernier scale has a maximum dimension of 49 cm, while one from
1940, also with a one-minute Vernier, has maximum dimension of 24 cm.
Smaller-size instruments are easier to use; thus, there is an incentive to
reduce dimensions, so long as accuracy is not compromised. Miniaturiza-
tion predates the electronic age.
BY DISTANCE MADE MORE SWEET
You will have noticed that most of the development in navigation and
surveying instruments we have considered thus far concern increasingly
accurate angle measurements. Accurate measurement of distance lagged
(it was not so important for maritime navigation) and would not really
catch up until the modern era. Before laser rangefinders came on the scene
in the 1970s, for many generations distances were measured via stadia rods
(fig. 3.14). A sighting telescope contained stadia hairs as well as cross-