Introduction to Topographic Maps Banner link to home page GeoSTAC Link to Department of Geosciences at Idaho State University
by Jim Riesterer . . . . . . . . . . Edited by Scott Hughes, Dan Narsavage & Diana Boyack

Topographic Maps Tutorial

Introduction & Materials
What is a Map?
Using Topo Maps
Map Scale
Reference Datum
Map Projections
Distortions
Grid Systems
Geographic
UTM
State Plane
Public Land Survey
Vertical Scale
Creating Profiles
Vertical Exaggeration
Calculating Slope
Using a Compass
Magnetic Declination
Get a Bearing
Go from A to B
Find Self on a Map

Topographic Maps Field Exercises

Exercise 1
Exercise 2
Exercise 3
Exercise 4

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Field Exercises


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April 7, 2008

Magnetic Declination

magnetic declination

Magnetic declination is the difference between true north (the axis around which the earth rotates) and magnetic north (the direction the needle of a compass will point). It is usually printed on the map to the left of the scale bar at the bottom of a USGS 7.5' quadrangle. After finding the declination on the map, you need to transfer the information to your compass before you ever take it into the field. If you fail to do this, any readings you get from your compass will be in error and you may wind up far from where you want to be (in other words, LOST ! ! !).

Magnetic north is determined by the earth's magnetic field and is not the same as true (or geographic) north. The location of the magnetic north pole changes slowly over time, but it is currently northwest of Hudson's Bay in northern Canada (approximately 700 km [450 mi] from the true north pole). Maps are based on the geographic north pole because it does not change over time, so north is always at the top of a quadrangle map.

magnetic declination angle

However, if you were to walk a straight line following the direction your compass needle indicated as north, you would find that you didn’t go from south to north on the map. Howfar your path varied from true north would depend on where you started from. The angle between a straight north-south line and the line you walked is the magnetic declination in the area you were walking. In the example figure, if you walked 1.25 miles toward magnetic north(i.e. you followed your compass without adjusting for magnetic declination) you would end up 1/3 of a mile away from where you would be if you walked 1.25 miles toward true north.

Fortunately, magnetic declination has been measured throughout the U.S. and can be corrected for on your compass (see below). This map shows lines of equal magnetic declination throughout the U.S. and Canada.

US Magnetic Declination

The line of zero declination runs from magnetic north through Lake Superior and across the western panhandle of Florida. Along this line, true north is the same as magnetic north. If you are working west of the line of zero declination, your compass will give a reading that is east of true north. Conversely, if you are working east of the line of zero declination, your compass reading will be west of true north. The exact amount that you need to adjust the declination on your compass to reconcile magnetic north to true north is given in the map legend to the left of the map scale.

Setting Magnetic Declination on Your Compass

If you are using a Brunton compass, you set the magnetic declination by turning the declination setting screw on the side of the compass until the reading on the graduated circle in the compass lines up with the index pin at the top of the compass at the proper declination.

For many other types of compasses you can set the declination by simply rotating the graduated circle on the outside of the compass until it lines up with the indicator marker at the top of the compass at the proper declination. If neither of these methods seems to work with your compass, check with the users manual that came with your compass, as it should have instructions on setting the declination.

Once you have set the declination on your compass, any reading you obtain from it will be accurate. In southern Idaho, for instance, the magnetic declination varies from roughly 14.5°E to 17°E. So, after setting the declination at 16°, when you line your compass up with 0° it will be pointing to true north but it will appear to be 16° off from the ‘N’ printed on your compass.

A word of caution here: be sure that you set your declination in the proper direction (east in Idaho). If you set it to 16°W rather than east, you will be off by 32° in all of your measurements, rather than the 16° you would be off if you hadn’t adjusted it at all. To make sure you have set your declination properly, orient your compass so that the north end of the needle is lined up with the 0° mark on the graduated circle.

If you are located west of the line of zero declination, then the index pin or marker on your compass should be west of the 0° marker on the graduated circle (and vice-versa if you are east of the line of zero declination).


----- Useful links for more information -----

http://www.spacecom.com/customer_tools/html/body_mag_dec.htm
USGS site with useful information about magnetic declination and your gps unit... http://rockyweb.cr.usgs.gov/outreach/gps/compass_gps_north.html
http://www.rescuedynamics.ca/articles/MagDecFAQ.htm

 

April 7, 2008

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