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Maps and Navigation- Chapter 3-
About Maps


ABOUT MAPS

Map Scales

United States Geological Survey topographic maps come in many scales, the most common being 1:24,000, or 7.5’ series and 1:62,500, or 15’ series. These are read as “one over twenty four thousandths” or “seven and a half minute series” and “one over sixty two thousand five hundred” or “fifteen minute series”. These scales represent the ratio of map distance to the distance on the Earth’s surface that corresponds to it.

The 1:24,000 map, referred to as a large scale map, is more detailed. One inch on this scale of map represents 2000 feet on the ground. This scale covers less ground area and is a better choice for more accurate navigating. The 1:62,500 map, a smaller scale map, covers more area but is less detailed. One inch on this scale of map represents about 1 mile on the ground.

Other map scales available from the USGS include 1:100,000, 1:250,000 and 1:1,000,000 scales, each covering more ground in less detail as the scale ratio increases.

Marginal Information

The marginal information is printed on the margin around the map itself. Included in the marginal information you will find a sheet name, bar scales, the contour interval note, road classification legend, declination diagram, and information on the age of the map and its updates.

Use the sheet name when ordering additional mapsheets. You will also find along the margin, or at the bottom of newer maps, the sheet names of adjoining map sheets.

Bar scales are used to convert map distances to actual ground distances. They are found in feet, miles, and kilometers.

The contour interval note tells the vertical distance between contour lines in relation to the ground. If the contour interval is twenty feet this means the distance between each contour line on the map is twenty feet. Using the contour interval note during planning allows you to avoid steep terrain. Note that contour interval can change from one map to another, even when they are adjoining.

The road classification legend tells the sizes and capacities of the roads you will find on the map.

The declination diagram tells the amount of declination, in degrees as well as mils, to either add or subtract depending on your location on the earth, in order to convert grid (map) azimuths to magnetic (ground) azimuths or the reverse. The amount of declination can change from map to map. Always be sure to check the declination diagram on the map that you are using.

Map Colors

There are five basic color representations on topographic maps. They are:

green- vegetation

blue - water features

black - man made features

brown- relief features and elevation

red- populated areas, roads

Another color you will find on updated maps is purple. Purple represents new man made features that have been added since the original map was made.

Basic Terrain Features

The following diagrams are included to help familiarise the reader visually with basic terrain features. The size and shape of the features may vary tremendously from map to map. If you have access to a map sheet it would be helpful at this point to locate the various features as you familiarise yourself with them.

Terrain features are represented on maps by contour lines. The distance between the contour lines relates to the amount of slope of the land being represented. Lines that are further away denote land that is flatter. Lines that are closer together denote land that is steeper. Index contour lines are numbered, the numbers denoting the actual elevation. Between the index contour lines are intermediate contour lines. These lines are not numbered with elevations. To find the elevation of an intermediate contour line simply count up or down your slope from the nearest index contour line, adding or subtracting the contour interval for each line (see contour interval note, above).

Hill

The final closed contour line in the series indicates the hilltop. Ground slopes down in all directions.



Saddle

A saddle is a low point between two areas of higher ground, here shown between two hilltops.



Valley

Contour lines slope down and away from higher ground, “V” of the valley points up towards higher ground. Valleys often contain streams or other water courses.



Ridge

A ridge is a line of high ground. A ridgeline is a series of high areas, often hilltops, running together.

Depression

A depression is a low point in the ground. It is represented by a closed contour line with tick marks pointing in toward the lower ground.



Draw

A draw is a small valley. It generally has a less developed, or intermittent, stream course than a valley.



Spur

A spur is a short, continuous, sloping line of ground, usually jutting out form a ridge. (Also referred to as a finger.)



Cliff

A cliff is a vertical or near vertical feature. Contour lines converge to become one heavy line or become very close together. A cliff may be represented by tick marks toward the lower ground.



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