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

Map Projections

What is a Map?

Once a reference datum has been determined the elevation of any point can be accurately determined, and it will correlate to the elevation of any point on the earth's surface that has the same elevation and is using the same datum. But…how do you accurately represent the X and Y coordinates of that point? This question leads to one of the fundamental problems of mapmaking…how do you represent all or part of an ellipsoid object on a flat piece of paper? The answer to this question is a bit complicated, but understanding it is fundamental to understanding what maps actually represent (this statement will become clearer shortly).

planar projection

In order to represent the surface of the earth on a flat piece of paper, the map area is projected onto the paper. There are many different types of projections, each with its own strengths and weaknesses.

The simplest (and easiest to visualize) example of a projection is a planar projection. To understand this type of projection, imagine inserting a piece of paper through the earth along the equator. Now imagine that the earth is semi-transparent and you could shine a flashlight oriented along the (geographic) polar axis through the earth.

The resulting outline on the paper would be a map created using this type of projection (known as a transverse azimuthal or planar projection).

There are three main types of projections, based on the shape of the 'paper' onto which the earth is projected. The example above used an azimuthal (planar) piece of paper.

conical and cylindrical projections


The other main types, illustrated to the right, are cylindrical and conical projections. These three types of projections can be further modified by the way the 'paper' is oriented when it is inserted into the earth.

In the example above, the plane was oriented along the equator, known as a transverse orientation (hence the 'transverse azimuthal' projection). Projections may also be equatorial (oriented perpendicular to the plane of the equator) or oblique (oriented at some angle that is neither parallel nor perpendicular to the plane of the equator.

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