How To Read a Topographic Map
​
​​​​
​Among the most essential items for any outdoor adventurer to bring along, a Topographic Map (Topo Map for short) offers a wealth of information to the user, without requiring significant navigational experience. In this article, we'll break down the map symbology, as well as important aspects like contour lines and map shading. We will also demonstrate how to determine the Latitude and Longitude of any location on the map.
Map Symbols and Shading
Many different symbols appear on a topographic map to depict landmarks, man-made structures and other points of interest. However unlike many maps, topographic maps generally don't come with a legend to explain these symbols. It is therefore necessary to learn the symbols separately. The full map symbol guide from USGS can be seen below. Symbols may change slightly over time, depending on what map you look at, but the differences tend to be fairly minor.
Shading offers information unaffected by the shape of the terrain itself. Green shading represents forested areas and blue represents water. White areas are generally not densely forested, but may still contain shrubs or other smaller vegetation that impede movement, so don't assume they're all grassy fields.
Contour Lines
​
The Contour Lines on a topographic map allow for a visual representation of the elevation and shape of terrain, by marking the elevation of the terrain in set increments. Contour lines are generally brown except in certain circumstances where they may be other colors, such as blue around mountain peaks and glaciers to indicate permanent snow cover. Every fifth line is bolded to make it easier to differentiate, and marked with it's elevation above sea level. In the map below, the contour interval (the elevation change between each line) is 50 feet. The closer together the contour lines are, the steeper the incline. The extreme example is a cliff, which results in multiple lines that converge into a single line along the face of the cliff. So here, it is easily seen that the south slope of the ridge is significantly steeper than the north slope. In this way contour lines offer a simple way for the navigator to identify a variety of terrain features including but not limited to:
Peaks & Hilltops: These are represented by the central contour line of a given feature. If the hill or mountain comes to a distinct peak it is generally marked with an X and it's elevation, as is the case with the middle and right hand hilltops in this image.
Ridges: A continuous crest of elevated terrain running between peaks. Other than the peaks themselves, a ridge is the most elevated terrain in the area along it's entire path.
Saddles: A low, often fairly flat area along a ridge between peaks which forms a saddle shaped dip along the profile of the ridge.
Valleys: A V shaped depression in the terrain with some amount of level ground and usually a stream or creek running along the bottom. Valleys are identifiable by contour lines that curve into a U like shape. Valleys often devolve into draws at their upper reaches.
Draws: Essentially a narrow valley, a draw is a V shaped cut in the terrain, without any level ground to speak of at the bottom and usually, steep sides running up to higher ground. Draws are identifiable by V shaped, pointed contour lines with little or no rounding at their point.
BLM Land Survey Grids
​
Found on virtually all topographic maps, but not familiar to most people. BLM public land surveys, divide land up into grids using what is called the Rectangular Survey System.
Each grid is called a township (these have nothing to do with actual towns) a 6 x 6 mile block, that is then divided into smaller blocks which are each 1x1 mile. The result is 36, 1 square mile sections in each township, which are numbered on the map. This image shows a complete township highlighted against the surrounding townships. These township grids often end up overlapping and cutting each other off on the map when different surveys are combined to make larger maps, so you often won't see the whole township. For the navigator, the most useful thing to know about these grids, aside from being a useful distance scale, is that north-south township lines run to True North, offering an improvised True North lines every mile on the map.
Map Margin Information
The white area around the actual map (called the margin) contains data that cannot be discerned from the map itself. Depending on the map source, a given topographic map may have slightly more or less data than the example here, but all the important data points are shown on this map. This map also has Latitude and Longitude lines printed on it. This high resolution image may be enlarged to see finer details as you go through this section.
Vicinity Map
This section is fairly self explanatory, simply showing what area of a state the map covers.
Map Source Information
Topographic maps are stitched together using data from different surveys, taken at different times across years, or sometimes even decades. This guide shows the year each section of the map was made, as well as the name assigned to each map sheet in the USGS database. The guide also indicates the contour interval (the interval between contour lines) of the map for each section.
The Declination Diagram
The declination diagram is one of the most important data points in the map margin for navigation. This tells us the degree of difference between Magnetic North (where a compass needle points), Grid North (if the map had a UTM Grid, the direction of the lines running up the map) and True North (the direction to the North Pole).
The declination diagram on this map shows that Magnetic North (MN) is 16 degrees east of True North (designated here with TN and a star, but often with only a star symbol). Grid North (GN) on this map is 0 degrees off of True North. In many locations Grid North and True North are not the same and can be several degrees apart. The difference between Grid North and True north is due to the difficulties of projecting a spherical surface (in this case the surface of the earth Earth) on a flat pice of paper which introduces errors. Oftentimes however, there is no error so True North and Grid North are the same on the map.
In the declination diagram below, True North is 2 degrees West of Grid North. If you want the magnetic declination from True North, you simply subtract the value of True North from Magnetic North. In declination diagrams West values are negative and East values are positive, so here you would subtract negative 2 from 11 to get 13 degrees East declination.
Map Scale Data
This section shows the map scale which is 1:50,000 (pronounced one fifty thousandth). The scale defines the size of features on the map relative to the real world. So on this 1:50,000 scale map, a distance of one mile, will equate to one fifty-thousanth of a mile (1.267 inches) on the map. Consequently a 1:25,000 scale map of the same dimensions would show an area half the size at double the magnification, a mile would then equal 2.53 inches on the map, offering a more detailed view of the area. Due to the impracticality of putting all available mapped data on a map of limited physical dimensions, maps at 1:50,000 and 1:100,000 scale don't generally show the high degree of detail visible on more zoomed maps like the popular 1:24,000 scale.
Below the map scale ratio, this section also contains the distance scale in both kilometers and miles.
Grid and Datum Information
This section tells us which UTM grid zone the map is in, Zone 6 in this case. The UTM Grid system is a coordinate system like latitude and longitude. While this map doesn't have a UTM grid on it, the Zone information is still given.
This section also tells us which datum this map uses. This map uses the current standard North American Datum of 1983, which is to be superseded in 2025-2026 by the Datum of 2022. The datum designates what specific methods and reference points are used to project the map, and is of little interest to the average navigator.
Defining a Position on the map
​
In order to be able to communicate a location on the map to others, you must have a system to name every random spot on the map. There are two systems in common use, first is the widely known latitude and longitude system, and then there's the UTM (Universal Traverse Mercator) Grid system. In this article, we will demonstrate how to use he better known Latitude and Longitude system.
Latitude and Longitude System
Since any kid in school gets the basic rundown on latitude and longitude, we won't bother with an in depth description here. To use this system to designate a point on your map, you will want a map that has the lines of latitude and longitude printed on it, or you will need to draw the lines over the map yourself. In this example the map has the lines printed.
​
Regardless of wether your map has full printed lines or not, the map will have the latitude and longitude marks printed at set intervals along the maps edges. These will give you a starting point for measurements, and drawing lines if necessary.
Before showing how to take measurements, we will however break down the numbering system in use since it's not the standard 0-100 and decimal system used in daily life.
​We'll use the line of longitude on the left for this example. The numbering system for coordinates is degrees, minutes, seconds and the hemisphere direction. So this line of longitude would be fully written 147°25'00" W. This is read "147 degrees, 25 minutes, 0 seconds West". Since the seconds number here is zero, the seconds are usually omitted, hence it not being written on the map. The W is West since we're in the western hemisphere, but since the map writers assume the user knows what hemisphere they're in, this also isn't specified. There are 180 degrees in each hemisphere, 60 minutes to each degree and 60 seconds to each minute.
​Now we can get to the actual demonstration. In addition to the map itself you will need a ruler, preferably one meant for the task, marked in minutes and seconds at the proper scale for your map. A normal ruler could be used, but would require doing some math to calculate fractions and convert them to coordinates which, while not too difficult, is beyond the scope of this basic demonstration.
We will determine the coordinates of the 1640 foot peak designated by the red arrow in all three images. First, we will determine the latitude of the peak by measuring from the nearest marked line. Here the nearest line is north of the peak indicated by the blue arrow in the center image, avoid confusing these with the BLM survey lines which are the same color here. Since we are measuring from 64 degrees 50 minutes north, the 4 minute mark on the ruler indicates 64 degrees 49 minutes north and we'll count up the seconds from there. If you expand the image you can see this comes to 38 seconds. So the latitude is 64°49'38" N.
Next we will measure the longitude which isn't quite as straightforward. While lines of latitude are separated by a constant distance of almost exactly one nautical mile per minute. Due to the earth being a sphere, lines of longitude converge together the further they get from the equator, eventually touching each other at the north and south poles. This prevents the simple straight line measurements used for latitude. Fortunately, there is a simple trick to manually compute the fractional difference between lines of longitude to take measurements. We will place the ruler at an angle, spanning between the lines of longitude on each side of our target peak. Since our ruler is 5 minutes long and the interval between the printed lines is also 5 minutes, the 0 end of the ruler will touch the lower numbered 146°45' line and the 5 minute end will go on the 146°50' line, indicated by the blue arrows in the image on the right. Now we will slide the ruler up or down as needed to place our target peak directly along the edge of the ruler. Thanks to the geometry involved in this trick, we can now simply read the seconds as normal to get our result of 37 seconds.
So we now have a complete set of coordinates; 64°49'38" N, 146°45'37" W.