Topographical Maps

Topographical maps are detailed representations of a part of Earth’s surface drawn to scale using lines, symbols and colors to represent changes in elevations.

Topographical maps are used extensively by a variety of people including geologists, engineers, field biologists, military, developers, hunters and hikers.

The features on a topographical map can be divided into four groups: Relief – changes in elevation Vegetation – Dense vegetation or open areas Water features – creeks, rivers, lakes, waterfalls Cultural features – man-made features such as roads, bridges, dams, buildings

Relief Relief is the difference in elevation between any two points. Relief is represented on maps using contour lines. Contour lines are imaginary lines on Earth’s surface that connect points of the same elevation.

Contour Lines Usually, every 5th line is printed darker than the others and bears the elevation above sea level. These lines are called index contour lines. The contour interval represents the change in elevation between two adjacent contour lines.

Contour intervals are designated on the map legend. A contour interval of 10 feet would mean there is a 10 foot change in elevation between each adjacent contour line.

Scale Scale expresses the relationship between distance on the map and the true distance on Earth’s surface. Scale is usually expressed as a ratio. USGS maps generally always use 1:24,000 scales. 1 = 1 inch map distance 24,000 = 24,000 inches or 2,000 feet ground distance

In this case, the 1 represents 1 cm and the 25,000 represents meters. Scale Countries that use the metric system use topographical scales of 1:25,000. In this case, the 1 represents 1 cm and the 25,000 represents meters.

Contour Lines When the contour lines are spaced far apart, the elevation is changing very slow and the terrain has gentle slopes. When the contour lines are spaced close together, the elevation is changing quickly and the terrain has steeper slopes.

How high is the top of Breeds Hill? Hills Contour lines near the top of hills form circles and the elevation of the hill is higher than the elevation of the highest circle. How high is the top of Breeds Hill? Between 820 ft. and 840 ft. X

Depressions Depressions without outlets, such as a sinkhole, are shown as closed, hatched circle and the elevation of the hollow is lower than the lowest closed circle.

Water Flow Water always flows downhill, from higher elevation to lower elevation. The path water tends to take can be found on topographical maps by looking for a group of V’s in the contour lines. Water flows out of the open side of the V. What way does the water flow in Deadman Gulch? Right to Left

Ridges Ridges on topographical maps can be found by looking for a group of U’s in the contour lines. The map, on the left, shows water flowing (V’s) between to fairly steep ridges (U’s).

Each color on a topographical map has a specific significance. Blue = Water features Green = Vegetated area with shrubs and trees White = Open areas such as meadows or beaches Red = Urban areas and important roads Black = Man-made works Brown = Contour lines Purple = Features seen by planes but not verified in the field.

Symbols Significant features are represented on the map by symbols that correspond with a symbol key.

Using a Map with a Compass In order to really use a topographical map in the field, you also need to know how to use a compass.

On a map, north is always at the top of the map. All compasses will have an arrow that orientates itself to the magnetic north. On a map, north is always at the top of the map.

Using a Compass 1. Point the base plate of the compass to your destination. Orient the map to north and place your compass on the map with the edge along the desired line of travel.

Using a Compass 2. Set your compass heading by turning the compass dial until “N” and the north arrow point to the North on your map. Your direction in degrees is read at the Index Line on the Dial.

Using a Compass 3. Follow your heading. Remove the compass from the map and hold it level, so the Magnetic Needle is free to turn. Turn your body until the red end of the Needle aligns with the Orienting Arrow and “N” on the Dial..

Using a Compass Follow your heading using the direction of the travel arrow to sight a distant landmark and then move to it. Repeat this process until you reach your destination.

Finding Your Location on a Map 1. In order to determine your position, you will need to choose two visible landmarks, that you can see from where you are standing that are also visible on your map. Label these two points L1 and L2.

Finding Your Location on a Map Place the compass on your map with base plate edge touching the first landmark (L1). Pivot the compass around on L1 until the orienting arrow or orienting lines align with the magnetic north lines on your map. Draw a line from the landmark (L1) along the side of the base plate across your map. Repeat this process for the second landmark (L2).

Finding Your Location on a Map 2. Where the two lines intersect is your exact location.

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