Topographic map ridge
|
Show
Knowing how to read a topographic map is the foundation of any backcountry adventure. Through the use of contour lines, topo maps bring a 3-dimensional element to a 2-dimensional paper map. Topo maps allow you to visualize the rise and fall of the land and “see” the depths of canyons, the location of boggy meadows, and the height and shape of mountains. But, make no mistake about it, learning to read and understand topographic maps takes time and practice. Whether you’re new to topo maps or a seasoned wilderness navigator, this article will help explain the basic 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 LinesContour lines are the primary and most important feature on a topo map. They show the shape of the terrain, including its hills, slopes, and depressions, by tracing a constant line of elevation on the map that corresponds with the landscape in the real world. Think of contour lines as imaginary horizontal planes sliced through the terrain surface. Visualizing Contour LinesIt may be helpful to visualize contour lines as stacked “layers” of the landscape, similar to a layer cake. A large mountain appears as a dense group of lines with a small circle in the center that represents the peak—just picture looking down at a wedding cake from above. Areas with few contours appear relatively flat—more like a 2-tier birthday cake. A topo map provides a bird’s eye view of those concentric circles, allowing you to “see” both the height and shape of the mountain on the map. From the USGS Topo on Gaia GPS, the summit of Pu’umakanaka, Hawaii is a near-perfect cone as indicated by the evenly spaced and round contour lines. Notice the tick marks on the contour lines in the center of the map. Those marks indicate a crater or depression at the summit. Knuckle MountainTry this quick exercise at home to get a better understanding of contour lines:
Reading Slope Steepness on a Topo MapContour lines present in very predictable ways on the map. When the slope is gentle, the contour lines are spread far apart. 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 contour lines. California’s Mount Whitney, at 14,505 feet, the tallest mountain in the lower 48 states, is steep on its east face as indicated by the closely spaced contour lines. Whitney’s west face is marked by low-angle slopes, as depicted by the wide-spaced contour lines. Identifying Features with Contour LinesContour lines show more than just the steepness of a slope or the elevation of a peak. You can pick out major land features on the map by the way the contour lines are shaped.
Contour Intervals and Index MarkerThe contour interval is the amount of elevation change between each contour line. Contour intervals vary from map to map. Intervals set at 40-feet are common on 1:24,000 scale maps. But many maps, especially small scale maps, have 50-foot or 100-foot intervals. Index contours are the more prominent, bold colored lines with the elevation marked on them. Discover the contour interval in two ways:
Topo Map ScalesMap scale is important because it tells you how detailed a topo will be. Map scale is defined as one single unit of measurement equal to a definitive number of the same number of units in the real world. You’ll find the map scale written as a ratio in the bottom margin of a topo map. Map scales vary greatly across different topo maps, but the most common scale is 1:24,000 for topo maps created in the United States. On a 1:24,000 scale map, 1 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 common 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 1:65,000, 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:
Distance ScaleThe distance scale in the margin identifies the distance on the ground in relation to a straight line on a map. For example, in a 1:63,360 map, one inch on the map equals one mile on land. On a larger scale 1:24,000 USGS quadrangle map, as referenced in the photo above, one inch on the map equals 2,000 feet on the ground. This is important when you’re out in the field because it places context to distances on the map’s representation of the landscape. The bar scale doesn’t take into account distance added by switchbacks and twists and turns of a trail. But 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 then measure the lanyard against the linear bar scale. This will give you a more accurate assessment of distance than a straight-line measurement on the map’s surface. Map LegendA map legend explains what the map’s 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 legend. Large scale USFS recreation maps often 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 legend defining roads, trails, and manmade features like gates and power lines. Colors on the MapSome legends define what different colors represent on the map:
The absence of vegetation can mean many things, including a desert landscape, a meadow, a scree field, a gentle grassy slope high above treeline, or a burn area. Use the contour lines to take a guess at what the surface characteristics will be on the ground without vegetation. If the area is flat with no contour lines and a river flows through the non-vegetated area, then the white-shaded zone is likely a meadow. If the non-vegetated area is high above tree-line, steep, and below a rocky mountain, there is a good chance it’s a field of talus. DeclinationTopo maps are oriented to true north, while your compass needle is drawn to magnetic 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, as seen in the photos below.
These diagrams show 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 south of true north. Declination varies with location on the earth, and in one part of the United States may be several degrees different than in another part of the country. Also, declination changes over time as magnetic north responds to the earth’s ever-changing magnetic field, so it’s important to check the map’s publish date as older maps may not accurately reflect the current declination. You can also look up magnetic declination by using this nifty calculator. Orient the MapTopographic maps are oriented to true north. If you’re holding the map in front of you, and the letters and numbers are right-side up, then the direction of true north is located at the top of the map. This is important because when you are in the field, you’ll need to orient the map to true north to bring the map in line with the land features around you. 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 will line up with the contour lines on the map. This is the time to practice picking out ridges and peaks on the skyline and matching them up with distinctive contour lines on the map. Map GridsTownship and RangeMany 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 square on the map represents one-square-mile on the ground, conveniently giving you the ability to assess distance without having to use the distance bar. UTM Coordinates Some maps, like National Geographic Trails Illustrated, display the Universal Transverse Mercator (UTM) coordinate system, which includes markings on the map for every 1,000 meters on the map’s north-south line and east-west line. The measurements on the north-south lines are “northings” and represent the number of meters that point on the map is from the equator. The numbers on the east-west lines are “eastings” and they depict the number of meters east or west of a particular reference line. You can find these measurements along the margins in the USGS quadrangle maps. Some maps display UTM grid lines, and others don’t, leaving you to eyeball the exact measurement or use a tool to help calculate the exact point. Latitude and Longitude
Latitude and longitude coordinates are also listed in the margins of most topographical maps. They present in the form of degrees, as shown in the picture below. Find Topo MapsThe USGS has been producing topographic maps since 1879 and has produced some 54,000 maps that cover the topography of the entire lower 48 states and Hawaii. These maps are referred to as the USGS quadrangle maps, with each map covering a 7.5-minute section of latitude and longitude on the globe. USGS quad maps, scaled at 1:24,000, are still considered the industry standard. The U.S. Forest service also makes topo maps for its lands. The FSTopo is the same large-scale map as the USGS quadrangle map and is available in hard copy 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 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 idea of which topo maps people use most. With a premium membership, you can access and download the entire Gaia GPS map catalog, including the best backcountry maps and layers. Check out all the classics: USGS TopoGaia TopoUSFS 2016You can also print and customize the maps from Gaia GPS and take them with you on the trail. Mary Cochenour works on the marketing team at Gaia GPS. She enjoys backpacking, skiing, and mountain biking in the mountains near her home in Montana. |
