Maps
There are many kinds of maps. The
most common is the road map showing transportation routes and
cities. Soil maps tell what kinds of soils are found in an area.
Geological maps show the structure of the earth's crust, types
of rocks, and forms of life found as fossils in a given area.
Land use maps show how a portion of the earth's surface is being
used. Topographic maps show the physical and cultural characteristics
of an area which includes: elevation; rivers, streams, and reservoirs;
roads and hiking trails; cities and villages; and a multitude
of other features.
What
is a map? It's more than a sheet of paper with lines drawn on
it. A map represents a portion of the surface of the earth as
it would be viewed from a vertical perspective. This representation
is reduced to a usable size.
There are many kinds of maps. The most common is the road map showing transportation routes and cities. Soil maps tell what kinds of soils are found in an area. Geological maps show the structure of the earth's crust, types of rocks, and forms of life found as fossils in a given area. Land use maps show how a portion of the earth's surface is being used. Topographic maps show the physical and cultural characteristics of an area which includes: elevation; rivers, streams, and reservoirs; roads and hiking trails; cities and villages; and a multitude of other features.
For general outdoor use, the United States Geological Survey (USGS) topographic maps are considered best (see bibliography). These maps come in several sizes and scales, but the one most often used is the 1:24,000 scale or 7.5 minute quadrangle map. (These terms will be explained shortly.)
Compare the features of a road map with those of a topographic map. Both are graphic representations of a portion of the earth's surface at a reduced scale. The road map plots position and distance of cities and roads, but it tells nothing about the terrain. The topographic map shows cultural features such as towns and highways too. But natural features also are included and, perhaps most important, contours which show the shape and elevation of the land.
A topographic map contains more information than a road map, so you need more skill to read it. The first step is to study the map's margin.
Items
printed in the map's margins are called descriptors. The map
name appears in the top and bottom margin and is often determined
by the main feature in the area covered. Words in parentheses
in the margins are names for topographic maps of adjacent areas.
In order to locate places, features, or areas on a map, ancient mapmakers devised a system of lines-a grid that fits the earth. There are many different grid systems used by professional geographers and cartographers but by far the most common grid system used is latitude/longitude.
The
latitude/longitude grid system is one of several systems used
on USGS topographic maps. Numbers in the margin, such as 38 '22'30"
indicate either the latitude or longitude of the area covered
by the map. These numbers pinpoint the area's location on the
globe.
Lines of longitude (also called meridians) run from the North Pole to the South Pole and completely circle the globe. The prime meridian which runs through Greenwich, England, is given a value of zero degrees. All other meridians are given a value based on their positions east or west of the prime meridian.
Longitude values on topographic maps are found along the top and bottom edges of the map. For the continental U.S. these values will range between approximately 70°W on the coast of Maine and 125 °W on the west coast of California.
A series of horizontal lines that run in an east-west direction around the globe are called lines of latitude. The equator is identified as zero degrees. The North Pole is 90°N and the South Pole is 90°S. Lines of latitude are often called parallels because each line is equal distance from the other and are assigned values based on their position north or south of the Equator.
Latitude values on topographic maps are located along the sides of the map. The lower 48 states lie between 48 °N latitude (Maine) and 26°N latitude (southern Texas).
The
values assigned to lines of latitude and longitude are expressed
in degrees of a circle and are further broken down into minutes
and seconds. A degree has 60 minutes, and a minute has 60 seconds.
Latitude lines range from zero degrees at the Equator to 90 degrees
north or south at the respective poles. Longitude lines have
values ranging from zero degrees at the prime meridian to 180
degrees at the International Date Line. Thus a latitude measurement
of 37° 15'30"N appearing in a map margin would be read
as "37 degrees, 15 minutes, 30 seconds, north latitude."
A longitude value appearing in the upper or lower map margin
of 92°15'W would be read as "92 degrees, 15 minutes,
west longitude." You can pinpoint any location on earth
by finding the point where the two coordinates, latitude and
longitude, intersect.
Topographic maps are sometimes identified using the area of longitude covered by the map sheet. Thus a USGS "15 minute" topographic map covers 15 minutes of longitude or approximately 18 miles, while a "7.5 minute" topographic map covers approximately 7 miles.
Other descriptors are also contained in the map margins. Dates given at the bottom of the map indicate when the map was made and field-checked. These dates are important because new features may not be shown. A highway, lake, or subdivision may have appeared since the map was made and field-checked.
During field-checking, surveyors may visit the area represented by the map. They will note the accuracy of map measurements and examine the area for any new features that may have appeared since the map was made. Most frequently, topographic maps are field-checked by taking an aerial photograph and examining it for changes or additions. For example, a river may be dammed to form a lake, a subdivision might be constructed in a meadow, or a hill might be leveled to make way for a new highway. These changes can be quickly and accurately identified by using aerial photography
When
using maps it is essential to know how far it is from one location
to another. If a secluded lake or reservoir is located on a map
and you would like to hike into it to go fishing, you need to
know if you will be walking 1 mile, 5 miles, or 50 miles-especially
if you are carrying a canoe and fishing gear! When you need information
on distance, it usually is impractical to measure the distance
on foot. The easiest method is to understand the concept of map
scale. Scale is the proportion between distance on the map and
distance on the earth (or in the field). Since a map is smaller
than the area it represents, a ratio can be established between
map distance and actual ground distance.
If your fishing lake were located on the map one inch from the end of the road and the scale on the map were l:lOO, you could accurately say that the lake was actually 100 inches from the end of the road (1 inch on the map = 100 inches on the ground). This could then be translated into an understandable distance of about 8 feet (100 inches divided by 12 equals 8). In this case you could not only drag a boat or canoe to the lake, but take along a cooler, lawn chairs, motor, and other necessary gear.
To show why a complete understanding of scale is important, let's decrease the scale of the fishing lake map to 1:250,000. In this case, the one inch from the road end to the lake on the map would be 250,000 inches on the ground-250,000 inches is much farther than the original 100 inches. In fact, the distance to the lake increased to about 4 miles (250,000 inches divided by 12 equals 20,833 feet and 20,833 feet divided by 5,280 ft./mile equals 3.9 miles).
Dragging a canoe or boat, cooler, and other gear would be quite difficult over this distance-in fact, you probably would limit your gear significantly!
Common
scales used on USGS topographic maps are: 1:250,000; 1:62,500;
and 1:24,000. This means that one inch on the map represents
250,000 inches; 62,500 inches; or 24,000 inches in the field,
respectively. These scales represent convenient measurements
for conventional maps. They are designed to show adequate detail
without being excessively large. The smaller the scale ratio,
the larger the area covered. The larger the scale ratio, the
greater the detail shown. (See map examples of scales.)
At first glance, these ratios may seem rather arbitrary. But actually, they figure out to scales that people can more easily understand. This removes some of the mystery. A scale of 1:250,000 represents one inch on the map to about four miles in the field. A scale of 1:62,500 represents one inch to about one mile, and 1:24,000 represents one inch to about 2,000 feet or a third of a mile. The last scale shows the greatest detail but covers the smallest area.
Another common type of scale showing map distance to ground distance is called a graphic or bar scale. This is a small ruler type of scale that appears on the bottom of most USGS maps. The bar scale shows graphically the distances represented on the map. Usually they are read from left to right with units such as inches, feet, yards, or miles identified for the map user. All one needs to do is measure the distance from one point to another on the map and compare it to the bar scale to easily compute how far it is between the two points,
A working knowledge of both ratio scales and bar scales is essential for map users. Both scales are useful in particular instances and can be adapted to a variety of uses. Sometimes using a small hand calculator can help in doing the mathematics and computations necessary to compute scale. But remember, often a calculator is not available when you are backpacking, fishing, or hiking-so be sure to familiarize yourself thoroughly with scale before attempting to go off on your own. Try several of the exercises in the lesson plans.
Maps are
constructed with a variety of symbols. Some look like what they
represent, while others may be confusing and abstract. Symbols
must be used on maps to show features because it is impossible
to realistically identify objects such as buildings, trees, and
highways as they appear to us. Learning to identify common symbols
is easy and as you begin to familiarize yourself with them, your
mind often will immediately transfer the symbol on the map to
what you really see when you look over the terrain.
Light
brown lines on the map are called contour lines. Contour lines
connect points which have the same elevation. Contour lines were
first used on land maps in the eighteenth century; however, they
did not become popular until recently because they require that
elevations be precisely determined and located on a map.
Two
kinds of contour lines appear on topographic maps: index lines
and intermediate lines. Numbers spaced periodically along an
index line show its elevation (in feet) above sea level. An index
line is darker in color. The lighter brown lines between index
lines are the intermediate lines.
