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The Complete Natural Scientist -- Field Exercise Module #1

Introduction to Topographic Maps by Jim Riesterer Produced by Scott Hughes and Dan Narsavage with help from Diana Boyack

Required Materials:

• 7.5 minute series U.S. Geological Survey topographic quadrangle map (1:24,000 scale) and perhaps a clipboard or other flat surface on which to write in the field.

• compass (capable of measuring azimuth, borrow one if necessary)

• protractor (cheap)

• graph paper (including a sharp pencil and eraser, not a pen)

• field notebook (optional)

NOTE: Your map must represent a location that is accessible to you by car, truck, llama, or whatever means of transportation is available. Make sure it has a few sites of interest to you, such as lakes, streams, mountains, neighborhoods, etc. (just about anything is appropriate). You can buy these at local outfitter stores, some bookstores, your local BLM or Forest Service office or through the U.S. Geological Survey.

Objective:

Gain an understanding of what a map is, how a map is made, and how to use a topographic map and compass.

What you will be required to do to complete this exercise:

Read through the entire tutorial, using links to visit more extensive explanation in the companion tutorial, then work through the on-line tutorial, visiting any links to outside pages.  Throughout the exercise, questions will be posed and answered.  You will maximize what you get out of this exercise if you work through the questions yourself before reading the answer, but you are not required to turn in your results.  At the end of the on-line tutorial there are a series of questions in the all-important home and field exercises.  You will need to complete all of these exercises, answer the associated questions, and turn in your results.

Questions? Email the instructor:  .

Tutorial (follow the links to more detailed explanations):

What is a map? A map is a way of representing an object’s (or objects’) real-world location on an artificially created two- dimensional surface.  Many maps, such as road maps, only deal with the two-dimensional location of an object without taking into account its elevation.  Other maps take into account the elevation of the object above a ‘reference datum’.  Maps that deal with three dimensions are called topographic maps.  This exercise will look at how topographic maps are created, what information they contain, how you can use them with a compass to get where you want to go, and how to measure the relative positions of points of interest.

The data behind the map

Reference Datum:  A reference datum is a known and constant surface which can be used to describe the location of unknown points. On Earth, the normal reference datum is sea level. On other planets, such as the Moon or Mars, the datum is the average radius of the planet.

Map Projections:  A map projection is a way of representing the 3-dimensional surface of the Earth on a flat piece of paper.

Distortion:  Each of the different types of projections have strengths and weaknesses, and knowledge of the advantages and disadvantages of a particular map projection will often help you to choose what map you want to use for a particular project.

Grid systems:  A grid system allows the location of a point on a map (or on the surface of the earth) to be described in a way that is meaningful and universally understood.

Coordinate systems:  There are several types of grids (AKA coordinate systems) used to divide the earth’s surface.  Four of these are in common use on maps published in the United States: geographic, universal transverse mercator (UTM), state plane, and public land survey coordinate systems.

Topographic maps

Much of the information discussed above is applicable to all types of maps.  Much of what follows is information that can be found on many general maps, but for the rest of this exercise the emphasis will be on what information is contained on a topographic map, and where it can be found.

What is a topographic map?  A topographic map, simply put, is a two-dimensional representation of a portion of the three-dimensional surface of the earth.

Map Scale:  Maps come in a variety of scales, covering areas ranging from the entire earth to a city block (or less).

Vertical Scale (contour interval):  All maps have a horizontal scale.  Topographic maps also have a vertical scale to allow the determination of a point in three dimensional space.

Magnetic declination:  The geographic north pole of the earth varies from its magnetic north pole.  The degree of variation changes, depending on where you are.  As a result, the direction your compass needle points will not always be the same as north on a map.

Additional information contained on a topographic map:  In addition to the information outlined above, topographic maps also contain much additional information.

Interpreting topographic maps

Tips for understanding contour lines

Creating topographic profiles:  A topographic profile is a cross-sectional view along a line drawn through a portion of a topographic map.

Vertical exaggeration:  Vertical exaggeration is the effect that is created when the horizontal and vertical scales on your topographic profile are not the same.

Determining hillslope:  Among other things, a topographic map can be used to measure the average slope of a hill (or hills).

Using a compass with a topographic map (orienteering):  Once you understand the basics of using a topographic map, you can combine that knowledge with the use of a compass to locate yourself on a map and navigate between points.

Setting the magnetic declination:  Before using your compass, you need to adjust its magnetic declination so it is correct for the area you are working in.

Getting a bearing with your map and/or your compass:  A bearing is a measurement of direction between two points.  Bearings can be measured off of your map using a protractor (if you have one) or your compass.   Once you have determined the bearing you want to go on the map, you need to understand how that translates to actually getting from point A to point B in the field using your compass.

Finding yourself on a topographic map:  Getting from A to B is all well and good, but what if you don't know where A is?  You can use your map and compass to triangulate between three points and locate yourself pretty accurately.

Follow these links to other sites to learn more about topographic maps

Once you understand these concepts go to the home and field exercises.

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Contact the webmaster, Dan Narsavage, with any problems.