| |
The Nautical Chart
Background
A nautical chart is like a road map for the world’s oceans and inland
waterways. The nautical chart is designed especially for navigation. A
chart is a printed reproduction of Earth’s surface showing a plan view of
the water and land areas. It contains parallels and meridians to use
when plotting a position, locating aids to navigation, and much more.
Chart
Projections
The task of putting the round Earth on flat paper is a complex one. This
text will not go into great detail on chart projections. More information
on this subject may be found in Dutton's Navigation and Piloting.
We
will discuss the two projections most widely used in today’s Navy and
by mariners in general.
Mercator
Projections
Mercator projection charts are the most commonly used navigational
charts. Therefore, it is important that you understand the characteristics
of these charts. The first thing to understand is that no navigational
chart is perfect.
Example:
Cut a hollow rubber ball in half and try to flatten it out, you
cannot do so without tearing or stretching the rubber. In fact, no
section of the hemisphere will lie flat without some amount of
distortion. No system of projection has yet been devised that preserves
the exact true proportions of the original sphere.
Mercator projections almost always display meridians and parallels.
Meridians run from the top to the bottom of the chart, parallels run from
the left to the right. Due to distortion in high latitudes, this projection
rarely exceeds 70° north or south.
Advantages:
The Mercator projection shows a rhumb line as a straight
line. A rhumb line is nothing more than a compass course or direction
plotted by the navigator to show that he will follow from his point of
departure to his destination.
Gnomonic
Projections
The gnomonic projection’s chief advantage is that it plots a great circle
as a straight line. This is most useful when planning long ocean
passages.
It is always best to take the shortest route from point A to
point B. This projection will be covered in greater detail in chapter 12.
1-12
|
