Global Positioning System Reference
In-Depth Information
FIGURE 3.5. A modern alidade, used for estimating the direction of the object viewed.
U.S. Navy photo by Mass Communication Specialist Seaman Jerine Lee.
comes from the Arabic word for ''ruler.'' Surveyors often used it in conjunc-
tion with a plane table , a flat horizontal surface on which paper can be
placed, to draw a plan of the surrounding area. After placing the alidade on
such a table, the surveyor sighted along the instrument toward a nearby
feature of interest, be it a building, a tree, or a peg driven into the ground;
he then drew a line on the paper guided by the alidade ruler. In this way
many features could be placed on the paper in their correct positions. We
still use alidades today—for a modern one, see figure 3.5—though the tech-
nology has changed considerably. The instrument is usually paired with an
angular scale so that the direction of a feature of interest can be measured.
ASTROLABE
Astrolabes are ancient machines for calculating celestial movements. They
have been around since classical antiquity and were popular in the east
during the Islamic Golden Age, say from the eighth to thirteenth centu-
ries. In Europe they were widely used until the Renaissance. There were
several variants, of which the planispheric astrolabe was the most common
(fig. 3.6). This instrument was applied to solve various astronomical prob-
 
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