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
Grid Systems
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

GeoSTAC Home

Field Exercises
April 7, 2008

Vertical Scale

The scales discussed before only deal with the relationship between horizontal distances on the map and horizontal distances in the real world. Because topographic maps incorporate the third (vertical) dimension of the earth’s surface, they also have a vertical scale.

This scale is listed on a topographic map as the contour interval. The contour interval is the vertical distance represented by consecutive contour lines on the map. In general, the smaller the scale of the map (remember, small scale maps show a larger area of the earth’s surface) the larger the contour interval will be. For example, the contour interval on a 7.5 minute quad is commonly 40 feet, while on a one or two degree sheet it will often be 100 feet. In order to make topographic maps more useful, there are exceptions to this rule of thumb.

In very flat areas, such as the plains of the midwest or the Snake River Plain, contour intervals of one hundred, or even forty, feet may not be very useful as they will be very widely spaced. In areas such as these, supplemental contours are often added at five or ten foot intervals (supplemental contours appear on USGS topographic maps as dashed lines). Similarly, in very steep mountainous areas the contours may be more widely spaced to avoid clustering of lines into unreadable masses. The contour interval used on a topographic map is printed below the scale in the map legend.

Contour Interval Descriptions

Regardless of the contour interval chosen, you will notice that there are at least two types of contour lines on a topographic map. Thick contour lines, called index contours, have elevations printed on them periodically over their length. Between each index contour are four intermediate contours that are thinner lines than the index contours. The elevation change between the intermediate contours is what is given in the map legend. So, if the contour interval listed in the map legend is forty feet, each intermediate contour represents forty feet and the elevation change between index contours is 200 feet. On many topographic maps these will be the only types of contour lines shown.

However, as mentioned above, some maps will have supplementary contour lines representing smaller vertical distances. If supplementary contour lines are used, they will be dashed lines and the supplemental contour interval will be listed below the regular contour interval in the map legend. A final type of contour that may appear on a topographic map is a line representing a closed depression (such as a sinkhole or a crater at the top of a volcano). These contours will be hachured (they will have small tic marks perpendicular to the main contour line), with the tic marks pointing downslope.


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