Lessons Index:
6. CHART MAKING FOR NAVIGATORS
Osher Map Library
University of Southern Maine
From Classical Mythology to Satellite Imagery
An exhibition at the Osher Map Library and Smith Center for Cartographic Education, University of Southern Maine, Portland, 4 April 2000 to 11 January 2001
Donald S. Johnson, guest curator
Lesson 1b: Background Information
MAP AND CHART
Osher Map Library Lesson
Charting Neptune's Realm
Lenora Liebowitz, Peter Rice, Andy Alley
B. Background Information
Today we think of a map as a graphic representation of a portion of the earth's surface drawn to scale. Road maps of the areas of the United States are the most common maps, and we can use them to travel across the country on the Interstate Highway System. Equally common and useful are the street maps of cities. These maps show street names, points of historical and cultural interest, the one-way streets, and public parking lots. A good navigator in the passenger seat of the car can make travel across the country and around a big city significantly easier than it would be without that person. In many cars today, the Global Positioning System and voice recognition computer have taken over the function of the navigator. No matter who or what is doing the prompting, a good navigator will keep us from going the wrong way on a one-way street. Good navigation avoids disaster.
Maps of land areas, as opposed to charts of the oceans, were among the first graphic representations used by man. Man is a land animal, and getting across the land was more important than crossing the trackless and dangerous ocean. When mankind began to make maps, he chose to place on the maps the pertinent information to guide the user to where he wanted to go. Safety from bogs, wide rivers, dangerous people, and killer beasts was important. If there were a dragon hiding in the caves along the trail, that kind of information was of vital importance. If there were a place where the deer came to drink water in the morning (a good place for hunting), this was also important.
We do not know who the first, intrepid person was who pushed the log out into the deep water that stretched to the horizon and set off in search of some unseen point. It must have taken incredible courage to make the first voyage. The courageous traveler probably knew where he was going because the coast line had been mapped. What was unknown were the dangers and directions for making that journey across the uncharted sea. We do know that the voyage was made, though we do not know when or by whom. The most important thing is that it began a tradition of sea travel that enabled cultures to explore distant places more easily.
As sea travel became more technologically advanced, trips became faster. In addition, the traveler was able to carry back in his boat much more than he would have been able to portage on his back. Also, he did not have to deal with a balky mule that constantly required feed and water. There was no packing and unpacking at night. Because the journey was faster, there was less chance of spoilage of the cargo.
So man set out across the seas in search of commerce. All, however, was not perfect. Journeying by water was fraught with danger. Unseen rocks, shoals eddies, currents and sand bars could wreck the unwary or unaware, and the entire enterprise could be lost as well as the lives of the travelers. As journeys by sea increased in frequency and value, collisions with other vessels and attack by pirates became significant problems. Navigating a busy channel in a thick fog became dangerous. A system was needed for the mariner to tell where he was and where he should be going. A straight line between two points might be the shortest distance, but it could also be the most dangerous.
As
we look at the United States Coast and Geodetic Survey Chart of Casco Bay
we see that there is a significant difference between it and the United
States Geological Survey Maine Portland Sheet. The map contains the names
of streets. One way streets are marked with tiny arrows. Parking lots are
shown. All these items allow us to navigate the city. If we know the street
address of a building, we can find the street. By knowing the cross streets,
and by looking at the numbers on the buildings, we can determine the exact
location to which we are going.
The
chart of Casco Bay is much different. The familiar land form is still there,
but all of the streets and buildings seem to have vanished. Instead, on
the land that does exist, chimneys and church spires have been located.
The rest of the chart is just blank white. That apparent blankness, however,
is filled with all kinds of notes that allow the mariner to travel into
Portland Harbor with as much ease and safety as the truck driver does when
he traverses the streets of the city.
The harbor is covered with numbers. These numbers represent the depth of the water at mean low water; that is, the lowest depth of the water at low tide. In areas where there are significant tides, the depths are listed as Mean Low Water Spring. In the harbor the numbers are given in feet of depth. In ocean charts the depths (soundings) are given in fathoms.
The bottom of the harbor is also shown with the type of material found there. A mariner, lost in the fog, can raise a sample of the bottom, and by knowing the depth and what the bottom is composed of, he can approximate his location.
On the chart numerous floating aids to navigation called buoys are depicted. These buoys appear as either black rectangles or red diamonds. They represent buoys of the same shape. The black rectangles are called cans (and they look like that), and the red diamonds are nuns because in the water they are seen as cones that look like the top of a spinning top which is also called a nun. The nuns and cans all have numbers. A closer examination shows a pattern for the numbers. As we enter the harbor from the sea, we can see that all of the buoys on the right (starboard) are red while those on the left (port) are black. In addition, the numbers of the black buoys are all odd, while the red buoys are all even. Mariners remember this system with 'BPOE' that stands for Black, Port, Odd, Entering. (Some mnemonic devices that will help you remember the acronym BPOE are 'Brotherhood of Paternal Order of Eagles' or 'Best People On Earth.') The other acronym is RRR for Red, Right, Returning.
The buoys mark the safe channel for ships and are placed at the intersection of channels or in places where accurate navigation is important. They are like street signs and hazard markers. The vessels themselves operate within these channels (the streets and roads of the water) under a set of rules called The Rules of the Road. To become a licensed operator of a vessel, one must take a 'driver's test' in the same manner in which the driver of a car must take a test. The Rules of the Road cover all types of conditions that affect safe passage on the water. As an example, when powered vessels are approaching head on, both vessels will pass each other on the port (left) side just as cars approaching each other pass left to left. This rule stems from the fact that the port side of the bridge (command deck) is the place where the commander traditionally stands.
Some of the buoys and markers have additional recognition signals such as lights, bells, or horns. Lights have the height above the water listed on the chart as well as the frequency of the light. In this way a mariner can identify the light (by the frequency of the flashes) and the distance from the light by the height above the horizon (level). Bell buoys and horn buoys warn a sailor that he is very close to danger. The general rule with an audible buoy is that if you can hear it, you are too close!
The last item on the chart is called a compass rose. These are representations of the compass and are used by navigators when they plot a course. The direction of the course is drawn on the chart from point to point, and then that line is transferred to the nearest compass rose with a pair of parallel rules. The same concept can be achieved by placing a straight edge on the course and then carefully keeping that direction as the straight edge is moved to the nearest rose.
On the Portland Harbor chart, the rose shows both true north (the point with the star) and magnetic north (the point with the arrow). The difference between the two is called variation, a subject that will be covered in detail in 'Latitude, Osher Map Library Lesson 15'.
In this lesson, students will be given the chance to draw a map and a chart of a port. A blank port sheet suitable for duplication is included with this lesson. Copied enough to give each student (or pair of students) two. The first exercise will be to draw a map of the port with as much detail as desired. The second exercise will be to draw a chart of the port, placing on the land only those items that would be of interest to a mariner (chimneys, spires, heights of land). The chart should also show depths (See 'Chart Making for Navigators, Osher Map Library Lesson 6') buoys and bottom types.
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