Reading contour lines on a topo map is a lot like looking down at a wedding cake.
Like cake layers, each contour line represents a different height, or elevation, of the land.
Contour lines are always printed in brown, with every fifth line (aka index line) indicating the elevation.Waterways are blue. Forests and other vegetation are green. Contour line V's point upstream when they straddle creeks, streams and waterfalls.
A saddle is a low spot between mountains that hikers always look for in the high country. The topography of it on a map resembles two eggs in a fry pan.
The black dashed line indicates a maintained trail. The red broken grid lines indicate historical township designations. Each township represents a 36-square-mile unit of land. Surveyors, firefighters and search and rescue teams still use townships in dividing up the terrain. You will sometimes find the old stone markers in the terrain.
Section of the Grand Canyon reveals steep topography on either side of the Colorado River. On most topo maps, north is always straight up. You can use the black grid lines to orient your compass or taking a bearing from the map.
On this map, the Bright Angel Trail descends as a black dashed line down into the canyon on the left upper side. Because the trail drops so quickly in elevation, there are lots of switchbacks to make the trek less treacherous.
An old USGS map legend.
Sample chart for converting slope grade percentage to an angle in degrees.
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Like road maps, topographical maps provide a birds’s eye view of the landscape and identify roads, rivers, forests, parks, dams and other manmade structures. Unlike road maps, topo maps include many more details about the geography in question, including slopes, elevation (i.e. height above sea level) and the contours of the terrain.
It's not uncommon for hikers in unfamiliar topography to box themselves into a canyon or get overwhelmed by a steep grade. You can take a wrong turn on a poorly marked trail, or the trail may suddenly end without warning. Another pitfall of wilderness travel is trailblazing into an area too densely packed with foilage to manuever around. Using a topo map is the equivalent of sending out a scouting expedition. It helps you identify the easiest path through changing terrain or the fastest way back to civilization if you get lost.
A topo map's best-known feature is its brown contour lines. Much like peering down onto a wedding cake, contour lines cut the landscape into different layers based on height of the land, or elevation. Imagine each of these lines connecting points of equal height, just like the cake in the photo above right. Of course, no terrain looks like a layer cake, so most contour lines aren't round and uniformly spaced apart. The area between the lines is called the contour interval. This interval represents the rate of altitude (i.e. elevation) change. It's actually a scale, sort of like "1 inch = 1 mile". Most maps are calibrated in feet, but outside the United States, meters are more often used.
It's important to remember that contour lines on a map are read much differently than distance. For instance, if two contour line are four inches apart, that means the elevation changes 50 feet across the 4-inch gap. If two contour lines on the same map are only 1/4-inch apart, then the elevation changes 50 feet in that quarter inch. Therefore, if the lines are bunched up together, the land is steep. If the lines are far apart, then the land is relatively flat.
Always check the scale of the contour interval on a topo map, which is listed below the map proper. The most common intervals used are 20, 50 and 100. Every fifth contour line is thicker than the rest and is known as an index line. This allows you to quickly calculate the sum of multiple contour lines, which can give you the total elevation change between two locations. Index lines also help you to more easily follow one contour line as it wraps around a map. In addition, each is numbered with an exact elevation above sea level. When you look at the numbers as they change from index line to index line, you can thus deduce whether the terrain is climbing or receding. In the case of undulating hills, the elevations will rise and fall repeatedly.
Interpreting the shape of terrain from contour lines
Whenever you map out a route through the wilderness, the general rule of thumb is that if you hike parallel to a contour line, you'll neither be climbing nor descending in altitude. If your path crosses contour lines in succession, then you'll be changing elevation more rapidly and the hike may be a much tougher slog. Maintained trails, incidentally, are identified by a dashed black line on topo maps. If the dashed black line zig-zags, this indicates switchbacks, which are commonly found in steep terrain.
Contour lines that wrap around as a set of tight loops may indicate a mountain, plateau, butte or peak in the center of the innermost circle. Conversely, they could signify a canyon or depression. Knowing which is which depends on whether the index lines show the elevation rising or falling. Most topo maps use either an X or miniature triangle to designate a mountaintop and generally identify the location by name and its exact altitude in feet or meters.
The V and U shapes you see in the contour lines reveal the direction of a dry slope or stream. For dry land, the closed side of a V or U points in the direction of downhill. However, in the case of streams (and waterfalls), the closed end of a V indicates the upstream direction. In other words, the water is falling down from above where the V points. Rivers flow along more of a flat course, so you'll generally see them tucked in between two sets of closely bunched contour lines.
Streams and washes may be marked by a solid blue line if they flow yearround, or a dashed line if they flow only seasonally. Keep in mind that a dry wash or can be used as a trail during the dry months, although the going may be steep, with lots of loose rock to navigate through. Lakes and rivers are represented by solid blue lines or patches of blue on a map. Glaciers are also marked in blue lines, with their interiors containing line fragments.
A ridge or cliff is identified by a series of bunched up lines that don’t curve or loop around. Long-haul trekkers in the high country are always on the lookout for a saddle, or pass, which indicates the easiest way to cross through mountains. A saddle is indicated on a topo map as the uncontoured space in between two summits. Likewise, hikers must also avoid getting stuck in a canyon bowl, or a gully packed with dense spring growth. Areas on the map colored in light green indicates vegetation. A solid green usually signifies a forest, while a mottled green indicates shrubs or other vegetation.
Most manmade structures are indicated pictorially in black. Dams are one exception. Here, the color blue is used because of the connection to water. Primary highways are indicated in red, other paved roads in black. Survey lines are also printed in red. UTM grid lines are printed in black and are used in conjunction with GPS units to plot travel coordinates.
Geographical North is almost always featured straight up on a topo map. Small black crosses at the map’s borders indicate latitude and longitude. In order to determine the coordinates of an exact spot on the map, draw lines from the latitude/longitude crosses so they intersect near locations. You can also take bearings by setting your compass down on the map with one edge on the trip starting point and the other, in the direction of travel, astride your destination. Turn the compass dial so that North lines up with the top of the page. Then take the bearing and write it down.
Since maps use Geographical North, be sure to factor in declination when transferring bearings to a compass in the field. Topo maps include the correct declination value for the local area. However, it the map is more than ten years old, the amount may be off one or more degrees.
To measure the length of a winding trail on a map, lay a string or your compass lanyard over the route so that it curls with all the bends. Then grab both ends of the string, straighten it out along a ruler (or the map scale), and measure the distance.
Measuring a Slope
You can also use a topo map to blaze your own trail. When you follow the path of a contour line, for instance, your elevation won’t change. When you start crossing multiple lines, then you have to consider the steepness of the slope. You do this by comparing the linear distance between any two points to the amount of elevation change along that corridor. To do this:
As a rule, any grade over fifty percent is considered somewhat treacherous. (Under snow, it has avalanche potential.) Any slope with less than a twenty percent grade is manageable. In between the two, you’ll have to make a judgment call, based on the stability of the rock or soil, the weather, the size of your backpack and your physical stamina. Any grade above 100 percent generally requires belays and other rock climbing gear. To make a more accurate assessment of the grade, you can convert the percentage into an angle using trigonometry, or consult a slope conversion chart.
Another method would be to draw a right triangle using the Pythagorean Theorem of C2 = A2 + B2 where A is the distance, B is the elevation, and C is the slope. Once you know the angle, you can visualize how difficult the climb or descent will be.
Keep in mind that the grade or angle you calculate is the average for the entire distance measured. Parts of the route may have a ten percent grade, while in other locations it might be fifty percent. Examine the contour lines carefully to see how they bunch up. Measure the area where they’re mostly tightly packed to determine the steepest grade in the climb.
In many cases, you'll find there’s a trail that wraps around the steep section you want to cross. While a straight vertical shot may seem faster at first glance, after calculating the slope you may decide otherwise.
Like regular maps, topo maps are produced in different sizes, covering either tiny or vast amounts of territory. The U.S. Geological Survey and National Geographic Society are the primary publishers for maps of the United States. USGS mostly produces the popular 7.5 minute maps, called quadrangles, which cover 40 to 70 miles square miles. 7.5 minutes is the amount of latitude and longitude that the map encompasses. The distance covered varies, depending on how close the location is to the equator.
7.5 minute maps are also referred to by the scale of proportion, which is 1 to 24,000. Alaska maps come in the 15 minute size, where one inch equals one mile, but otherwise the distance scale is 2 1/2 inches equals one mile. Log onto the store portion of the USGS.gov website to buy maps for areas you’re interested in. Many outdoor sports retailers also sell maps for nearby wilderness areas. A company called AdventureStation.com sells a waterproof version of the maps and can also create a custom map for your specific theatre of travel. You can also download sections of a USGS topo map free at Topoquest.com. Several mapping software applications sold online add a three-dimensional look to the maps (otherwise known as a relief map).
Topo maps include the compass declination value for the area covered. See Navigating with a Compass for more on declination. If the map is more than ten years old, this value may be off several degrees. That's because the magnetic north pole is drifting at a rate of 6 minutes to the west each year, or 1/10th of a degree.
Maps carried into the backcountry must be stored in a waterproof container. Veteran hikers typically fold the map so the relevant section is visible and pack it in a see-through Ziplock bag or plastic sheet holder sold at stationery stores.
A staple of airplane navigation, the altimeter is also used by rock climbers to measure altitude. The instrument is essentially a barometer, measuring air pressure, but is calibrated to tell you your elevation. As you climb in altitude, air pressure increases. So the instrument can help you pinpoint your position in relation to the summit.
Keep in mind that in bad weather, or when a storm’s approaching, an altimeter's readings may become inaccurate, since a storm causes the air pressure to change. At least you’ll be able to hunker down before the tempest hits your location. It’s a dead giveaway that something’s brewing in the sky when an altimeter reading rapidly drops a couple hundred feet or more as you sit tight in camp between legs of your journey.
Temperature can also affect the pressure sensor that produces altimeter readings. To avoid this, keep the device close to your body along the trail. For best results, take a reading at a trailhead or parking lot where the elevation is either identified on a sign or listed on your map. This has the affect of calibrating your instrument based on the local conditions.
Wrist-worn digital altimeters are preferred by hikers since they incorporate a clock, thermometer, altitude gain, compass and other features. However, the traditional analog model requires no battery and is more reliable in extreme conditions.
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