Page 190 - NAVAL SCIENCE 3 TEXTBOOK

196 NAVAL SK IL LS

The Mercator projection was developed by a Dutch cartographer, Harbor c1wrts are large-scale charts that show harbors and
Gerardus Mercator, ill the sL'Xteenth century. It is the most widely their approaches in detail. Coastal charts are intermediate~scale
used of all chart projections for nm'igation. In this projection, the charts used to navigate a vessel whose position may be determined
spherical globe is projected onto a cylinder-shaped piece of paper, b)' landmarks and lights, buoys. or soundings offshore. For navi-
wrapped around the globe at the equator. Then, the cylindri- gating inside olltlying reefs or shoals, or well offshore in large bays
cal paper is spread flat, after clltting it at convenient meridians. A of siz'lble inland waterways, a coastal or harbor chart may be used.
Mercator projection of the world, for instance, is lIsuall)7 cut verti- General ocean sailing charts are small-scale charts showing the
call)' ncar the International Date Line so the continental land areas approaches to large areas of the coast. These charts show oft-shore
are shown almost unbroken. soundings, principal lights and ollter buoys, and any natural land-
Because of the method of projection, increasing distortion marks visible at a distance.
occurs the farther the area 011 the Mercator chart is from the equa- Scattered all over water areas of any nautical chart arc many
tor. However, this distortion is made uniform in both latitude and tiny italicized figures called sO//Ilrlillgs, each representing the depth
longitude. so the finished chart is usable for navigation even at of water in that particular place. Depths may be given in feet, fath-
high latitudes. The space between parallels increases with latitude, oms (6 feet to 1 fathom), or meters. A not<ltion under the title of
but the distance represented by 1 degree of latitude is alwi.l)7s the the chart is the key; for example. "Soundings in feet at mean low
same, I n<1uticalmile. Because of the distortion at high latitudes. water." Most charts also contain dotted lines called fathom curves,
the island of Greenland appears much larger than the United States marking the limits of areas of certain depths. Most newer charts
on a .Mercator projection. In actuality the reverse is true. will give water depth, heights of lights. and land contours in meters
The meridians on a Mercator chart appear as stmight lines, as well as in feet.
north and south, parallel to and equidistant from one another. Nautical charts are available for purchase at nautical sup-
The}' represent the imaginary curved meridian lines that come pi)' stores in most coastal cities and towns throughout the United
together at the poles on a globe. States. In addition to traditional paper charts, electrollic chnrts on
Scale of charts. The scale of a chart refers to a measurement of computer data storage media that can be projected onto computer
distance. It is a comparison of the actual dist,lI1ce or size of a fea- screens and various types of electronic plotters are coming into
ture with that shown 011 the chart. The scale of a chart or map is widespread use in marine navigation.
normally printed Ileal' the legend ill the form of a ratio, Stich as
I :5,000 (meaning that the feature shown is actually 5,000 times
Plotting
larger than its size 0 11 the chart). Said in another way, an inch or
centimeter or other measurement on the chart represents 5,000 In order to use the nautical chart for navigating, you must know
identical units on the real Earth's surface. The smaller the ratio, the something about how courses, bearings, and lines of position arc
smaller the scale of the chart. A chart with a scalc of I :5,000 is on plotted on it.
a Inllch larger scale than one whose scale is 1:4.500.000. for exam- The basic instruments lIsed ill plotting are parallel rulers, some
ple. Small scales arc lIsed to depict large areas on a chart. and large kind of protractor, and dividers. The parallel ruler consists of two
scales arc lIsed to depict sl11;.111 areas. straightedges connected by pivoted mel'al straps Ileal' each end.
Another wa)' of expressing scale, called the 1IIIIIIerimi smle. is The straps allow the two straightedges to be opened and closed,
in inches, miles, or kilometers to the nautical mile. This is shown while <1lways remaining parallel to e;.tch other. By placing the edge
near the legend as a bar scale (linear scale) with both compared of one straightedge along a line of bearing and "walking" the ruler
measurements shown. one on either side of the bar. By lIsing a pair carefully across the chart to a compass rose (a graphic used as a
of dividers, set to the linear scale desired. YOll can find distances reference for both true and magnetic directions 011 a chart), the
by "w<llking" the dividers across the chart and lIsing simple arith- true bearing of the line may be determined. Dividers are a plot-
metic. For example. if an inch 011 the chart represents 50 miles, 5 ting instrument used for measuring chart distances along a suit-
inches would represent 250 miles. able scale on a chart.
Remember, the larger the scale, the smaller the area shown on a A protractor docs just about the same thing as parallel rulers.
given chart or map. Large-scale charts show areas in greater detail. A simple protractor consists of a gmduated arc on a piece of clear
Features that appe.ll' on a large-sG.lle chart may not show tip at all pl<1stic. Another kind of protmctor has a ruler that pivots on the
on a small-scale chart of the same area. center of the cun'ahlre of the arc. The protmctor's arc is graduated
Types of charts. Nautical charts, as described above, are those like the upper half of a compass rose. Horizontal and vertical refer-
with the necessary information for safe navigation. They have ence lines on the pl<1stic can be lined up with the meridians or par-
standard symbols, figures, and abbreviations that tell the depth of aiJels, and any course or bearing can be easily plotted by swinging
water, type ofbottoll1, location of navigational aids. and so forth. the ruler to the desired degree Illark on the arc.