How to Read Contours on a Topographic Map
Knowing how to read a topographic map is the foundation of any backcountry hazard. Through the use of contour lines, topo maps bring a iii-dimensional chemical element to a ii-dimensional paper map. Topo maps permit you to visualize the rise and fall of the land and "see" the depths of canyons, the location of boggy meadows, and the summit and shape of mountains.
But, make no mistake most it, learning to read and understand topographic maps takes fourth dimension and practice. Whether you're new to topo maps or a seasoned wilderness navigator, this article will help explain the bones features found on topo maps, like lines, numbers, symbols, and colors, so that you can read them with ease and confidence.
This article covers:
- Contour Lines
- Identifying Features with Contour Lines
- Map Calibration
- Distance Scale
- Map Legends
- Orienting the Map
- Declination
- Map Grids
- Find Topo Maps
Contour Lines
Contour lines are the primary and well-nigh important characteristic on a topo map. They bear witness the shape of the terrain, including its hills, slopes, and depressions, past tracing a constant line of elevation on the map that corresponds with the mural in the real globe. Think of contour lines every bit imaginary horizontal planes sliced through the terrain surface.
Visualizing Profile Lines
Information technology may be helpful to visualize profile lines as stacked "layers" of the mural, similar to a layer cake. A large mountain appears as a dumbo group of lines with a minor circle in the center that represents the peak—just picture looking downwards at a wedding cake from above. Areas with few contours appear relatively apartment—more like a 2-tier birthday cake. A topo map provides a bird's center view of those concentric circles, assuasive yous to "see" both the acme and shape of the mountain on the map.
From the USGS Topo on Gaia GPS, the tiptop of Pu'umakanaka, Hawaii is a near-perfect cone as indicated past the evenly spaced and round contour lines. Notice the tick marks on the profile lines in the centre of the map. Those marks signal a crater or depression at the summit.
Knuckle Mountain
Try this quick exercise at home to get a better understanding of contour lines:
- Brand a fist with your paw, taking note of the "features" on your fist. There are 4 knobs (knuckles), a gentle slope (back of your manus), and four pocket-sized ridges (fingers) separated by ravines (space betwixt the fingers).
- With your other hand, mark an "x" on the knuckle that sticks up the highest to mark the "summit." Using a pen, drop downwards from the superlative a quarter-inch and brand a contour line around the elevation of "knuckle mount." Follow the peak around the mount, without dropping or climbing, until you complete the contour line and close the circle. Driblet down another quarter of an inch and repeat. Do that once more and once more, until you've mapped out your entire fist.
- Next, lay your manus apartment with your palm on the table. Find the spread out contour lines on the back of your hand, indicating a gentle gradient. Notice the "v" shaped topo lines where the ravines were betwixt your fingers. The areas by your fingers accept contour lines that are shut together, indicating steepness. Find an hourglass-shaped topo line for the saddle betwixt your knuckles.
Reading Gradient Steepness on a Topo Map
Contour lines present in very predictable means on the map. When the slope is gentle, the profile lines are spread far autonomously. Conversely, when the slope is steep the contour lines pack closely together. On a cliff, the contour lines are stacked on top of each other. A flat meadow is void of profile lines.
California'south Mount Whitney, at 14,505 anxiety, the tallest mountain in the lower 48 states, is steep on its east face every bit indicated by the closely spaced profile lines. Whitney's west confront is marked by low-bending slopes, as depicted by the wide-spaced contour lines.
Identifying Features with Contour Lines
Contour lines testify more than just the steepness of a slope or the elevation of a peak. You tin can option out major state features on the map by the way the contour lines are shaped.
- Peak: A mountain or hill that is marked past concentric profile lines. The summit may exist marked with a proper name, an "X", or an acme.
- Valley: A landform with a depression in which water, if nowadays, would flow down. Likewise know as gullies, drainages, ravines, and couloirs, valleys are indicated by "V"or "U" shaped contour lines that "point" uphill, toward college elevation.
- Ridge: A landform with an elevated crest that slopes downward on the sides. Ridges are also shown by "5" or "U" shaped contours, but these "point" downhill, toward lower elevations. Recall, water never runs along ridge tops.
- Saddle: A low spot between two peaks marked by hourglass-shaped contour lines.
Contour Intervals and Index Marker
The profile interval is the corporeality of elevation change between each profile line. Contour intervals vary from map to map. Intervals ready at twoscore-feet are mutual on 1:24,000 calibration maps. But many maps, especially modest calibration maps, have l-foot or 100-foot intervals. Alphabetize contours are the more prominent, bold colored lines with the elevation marked on them.
Find the contour interval in two ways:
- Observe the contour interval on the map margin or fable, or
- Calculate the contour interval between index contours, which are are the more prominent, assuming-colored lines with the elevation marked on them. First, subtract the lower number from the higher number and and then split up the effect by the number of contour lines. For example, if the index contours elevations are 8000 and 7500 anxiety as shown in the picture above, the difference is 500 feet. If every fifth line is assuming, so split by 5. The contour interval is 100 feet.
Topo Map Scales
Map scale is important because information technology tells y'all how detailed a topo will be. Map calibration is divers as ane unmarried unit of measurement of measurement equal to a definitive number of the same number of units in the real earth. You'll detect the map calibration written as a ratio in the bottom margin of a topo map.
Map scales vary greatly across different topo maps, simply the most mutual scale is 1:24,000 for topo maps created in the Us. On a 1:24,000 scale map, one inch on the map is representative of 24,000 inches, or 2000 feet, on the ground. The United States Geological Survey quadrangle maps are scaled to 1:24,000. These are large scale maps and bring a lot of detail into focus.
A map scale of 1:63,360 is mutual in Forest Service maps, and maps throughout Alaska. These are smaller-scale maps, with less intricate detail. Some National Geographic maps use a scale of one:65,000, which allows a large area of land to be packed onto a single topo map.
What you need to know is that the bigger the number on the bottom of the ratio, the smaller the scale and the less detail:
- 1:24,000= a large scale map that depicts a smaller region of land in intricate item
- 1:63,360= a small-scale scale map that depicts a larger area of country in less detail
Altitude Scale
The distance scale in the margin identifies the distance on the ground in relation to a straight line on a map. For example, in a i:63,360 map, one inch on the map equals one mile on land. On a larger scale 1:24,000 USGS quadrangle map, every bit referenced in the photo above, one inch on the map equals ii,000 anxiety on the ground. This is important when you're out in the field considering it places context to distances on the map's representation of the landscape.
The bar scale doesn't have into account altitude added by switchbacks and twists and turns of a trail. Just you can use the lanyard of your compass, a guy line from your tent, or the shoelace from your boot to trace the bends of the trail and so measure out the lanyard against the linear bar scale. This volition give y'all a more accurate assessment of distance than a direct-line measurement on the map'southward surface.
Map Fable
A map legend explains what the map'south symbols, lines, and colors represent. Look for the legend in the margin or in the corner of a map. On gaiagps.com, click on the map's thumbnail in the layers menu to access the fable.
Large calibration USFS recreation maps oft include a robust legend with keys for recreation symbols, like campgrounds and restrooms. There may be a listing of points of interest, such as prominent peaks and glaciers, and a fable defining roads, trails, and manmade features similar gates and ability lines.
Colors on the Map
Some legends define what different colors stand for on the map:
- Blue represents water in the form of creeks, rivers, and lakes.
- White areas outlined by a sparse blue line point a year-round snowfield.
- Dark-green areas are sections of land covered with vegetation, similar trees.
- White, or the color of the base map, represents land that lacks tall vegetation.
The absenteeism of vegetation can mean many things, including a desert mural, a meadow, a scree field, a gentle grassy slope high higher up treeline, or a fire area. Apply the contour lines to accept a approximate at what the surface characteristics will be on the basis without vegetation. If the area is flat with no profile lines and a river flows through the non-vegetated expanse, then the white-shaded zone is likely a meadow. If the not-vegetated area is high higher up tree-line, steep, and below a rocky mountain, there is a expert chance it's a field of talus.
Declination
Topo maps are oriented to true n, while your compass needle is drawn to magnetic due north. Declination is the angle of deviation between the magnetic north and true north. This angle is often depicted in the bottom margin of the map or on the map itself, equally seen in the photos beneath.
These diagrams bear witness the declination for the area on the map. Map and compass navigators use the declination angle to adjust their compass to account for the difference between true north on the map and magnetic north, which lies some 400 kilometers s of true north.
Declination varies with location on the earth, and in one part of the U.s.a. may be several degrees unlike than in another part of the country. Besides, declination changes over time as magnetic north responds to the earth'south e'er-changing magnetic field, so it's important to check the map's publish appointment as older maps may not accurately reverberate the current declination. You can also look upwards magnetic declination by using this nifty calculator.
Orient the Map
Topographic maps are oriented to true north. If you lot're property the map in front of you, and the letters and numbers are right-side up, then the direction of true n is located at the superlative of the map. This is important because when you lot are in the field, you'll need to orient the map to true n to bring the map in line with the land features around yous. Learn how to orient the map from backpacking guide Andrew Skurka.
When the map is oriented to true north in the field all the surrounding features volition line upward with the contour lines on the map. This is the fourth dimension to practice picking out ridges and peaks on the skyline and matching them upwardly with distinctive profile lines on the map.
Map Grids
Township and Range
Many maps have grid lines with the numbers 1-36 written in the middle of each box. These numbered boxes are called sections and originate from the township and range survey method that was used throughout the western United States, some midwestern states, and Alabama and Florida. Each foursquare on the map represents one-square-mile on the ground, conveniently giving you the ability to assess distance without having to employ the distance bar.
UTM Coordinates
Some maps, similar National Geographic Trails Illustrated, display the Universal Transverse Mercator (UTM) coordinate system, which includes markings on the map for every one,000 meters on the map's n-south line and east-west line. The measurements on the n-south lines are "northings" and correspond the number of meters that signal on the map is from the equator. The numbers on the due east-westward lines are "eastings" and they depict the number of meters east or west of a particular reference line. Y'all can discover these measurements along the margins in the USGS quadrangle maps. Some maps display UTM filigree lines, and others don't, leaving you lot to eyeball the verbal measurement or utilise a tool to help calculate the exact point.
Latitude and Longitude
Latitude and longitude coordinates are besides listed in the margins of most topographical maps. They nowadays in the form of degrees, equally shown in the picture below.
Find Topo Maps
The USGS has been producing topographic maps since 1879 and has produced some 54,000 maps that comprehend the topography of the unabridged lower 48 states and Hawaii. These maps are referred to equally the USGS quadrangle maps, with each map covering a 7.5-minute department of latitude and longitude on the globe. USGS quad maps, scaled at 1:24,000, are still considered the manufacture standard.
The U.Southward. Woods service as well makes topo maps for its lands. The FSTopo is the aforementioned large-calibration map as the USGS quadrangle map and is available in hard re-create at some of the Forest Service offices. Its wilderness maps cover an entire wilderness area and come in a small scale to fit on a single page.
Private companies have delved into publishing topo maps too. National Geographic Trails Illustrated offers more than than 250 titles for recreation areas in America. Some smaller companies make regional topo maps, such as Beartooth Publishing in Bozeman, Montana, and Tom Harrison Maps, based in San Rafael, California.
View the nine most popular hiking maps at Gaia GPS to get an thought of which topo maps people utilize nearly. With a premium membership, you lot tin can access and download the unabridged Gaia GPS map catalog, including the all-time backcountry maps and layers. Check out all the classics:
USGS Topo
Gaia Topo
USFS 2016
Yous tin can also print and customize the maps from Gaia GPS and accept them with you on the trail.
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Source: https://blog.gaiagps.com/how-to-read-topographic-maps/
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