Topographic mapping

 

Topographic mapping and Interpretation

Topography uses contour lines to show the configuration of the land surface. CONTOUR, TOWNSHIP, and RANGE lines are used to read topographic maps. The information that follows has been taken from the lab manual used in my Introduction to Earth Science course.

 

Image taken from the USGS Learning Web: Working With Topographic Maps.

 

CONTOUR

Contour lines connect points of equal elevation with imaginary lines on the earth. These lines are brown on any standard U.S.G.S topographic map and every fifth contour line is printed with a bold test. Elevations are always given in feet or meters above mean sea level. On these maps the contour interval is of great importance. This interval is the difference in elevation between any two adjacent contour lines and is usually centered at the bottom of the actual map. This interval is consistent throughout the map.

In order to interpret these maps, some basic rules include:

    1. As contour lines cross-streams, they bend upstream in a "V" shape.

    2. A closed contour shown as an ellipse or circle represents a hill.

    3. A closed contour shown with a hachured, short line pointing down slope, is a closed depression without an outlet.

    4. Steep slopes are shown by closely spaced contours and gentle slopes, shown by widely spaced contours.

    5. The maximum relief is the difference in elevation between the highest and lowest point of a given area.

 

TOWNSHIP AND RANGE

 Latitude and longitude are utilized for more precise locations with the township and range system of surveying.  Townships allow for governmental entities in rural areas and help to describe a specific location.   Globes are sectioned by a series of lines that run north to south called meridians and east and west called parallels.  Longitude follows the meredians, while latitude, the parallels. Range lines follow meredians, township lines follow parallels, and the basic block of the system is a section, one mile by one mile or 640 acres.  A Township contains 36 sections.  The Principal Meridian is a zero point for east and west range lines, while the Base Line is the dividing point for north and south township lines.

Topographic Maps don't just provide a location or an elevation.  These maps can also show geomorphic features, such as erosion by streams or glaciers.  

Stream Erosion

There are three different stages dealing with the cycle of stream erosion.  There is a youthful stage, where the stream will be straight and narrow. A mature stage will consist of flat valley floor with some ponds and marshes. Old age occurs where the region has been eroded to such a flat surface that it drains poorly.

Glacial Erosion

There are two types of glacial erosion.  The first type is caused by ice sheets and the second results from mountain glaciers.

 Continental ice sheets are very large and cover an area of at least 50,000 sq. km.  These ice sheets are very thick and change the land surface while leaving erosion or deposition features.  Erosion features include scratching, scouring, streamlining, and leaving grooves in the region.  Depositional features include left debris and uneven and irregular surface from the melting of the ice.  This leaves the land with marshes and lakes because it has poor draining.

Mountain glaciers are always located around valleys and tend to enlarge the valleys and form glacial troughs.  A glacial trough is a groove that is deep, steep walled, and flat bottomed.  These grooves are either straight or gently curved.  The glacier may scoop out the floor of the trough in some places leaving a shallow lake.

If you want to learn more go to http://www.askjeeves.com and type in topographic mapping.  You will find numerous links with additional information.  You can also buy books and maps to help you with visual learning.

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