Global Positioning System Reference
In-Depth Information
tween two lines extending from the ship to the lighthouse, and from the
ship to the islet, is about 6\. Our pilot knows his geometry and estimates
that he is about 10 km from the coast.
13
Dead reckoning, at least in its early days, was every bit as approximate as
these simple thumb methods.
Dead Reckoning
Navigators had caged latitude since classical antiquity, but as we have seen,
longitude remained wild and free, and dangerous because of that. Longi-
tude would not be tamed until the eighteenth century. At the time that
Spanish and Portuguese mariners first set out into the wider world, longi-
tude was estimated by dead reckoning (DR).
14
Whenever a caravel or a
galleon found itself in open water out of sight of familiar landmarks, the
navigator made use of dead reckoning to estimate the position of his ship
or, more precisely, how far and in which directions it had traveled.
We have seen how Henry the Navigator established at Sagres an institu-
tion that nowadays would be called a navigation research center. This was
necessary because mariners knew perfectly well that their skills, honed
along the coastlines of Europe and particularly in the Mediterranean Sea,
would not be good enough for the open ocean. In the Mediterranean there
were no tidal streams, currents were known, a ship was seldom out of sight
of land for more than a few days, and lines of longitude were separated by
about the same distance because the Mediterranean extends east-west
much more than north-south. Portolan charts for the Mediterranean re-
gion were well developed by the fifteenth century. All of these facts made
for relatively easy navigating. Out in the open ocean, however, it would be
a di√erent story.
Dead reckoning estimated the path of a ship by determining the dis-
tance and direction traveled each hour and then adding up all the distances
and directions. The idea is shown in figure 6.12a. The same method, called
13. Modern mariners can find equivalent tricks—cheap-and-cheerful methods—for
roughly estimating the distance to shore in many of the popular sailing topics. For example,
if you have normal vision and can just discern individual trees on a shoreline, they are about
one nautical mile away. If you are sitting on deck and can just see the line where land meets
sea, you are about three nautical miles away. These two distance-estimation methods are
based on average eye resolution and the earth's curvature, respectively.
14. The odd term, according to the
Oxford English Dictionary
, arises from an old-fashioned
adjectival use of the word
dead
, meaning ''complete, unbroken, or unrelieved,'' as in ''dead
calm.'' Thus, ''dead reckoning'' refers to a method that involves continuous calculation.
