How to read a topographic map

Consider this a language lesson, but in a language quite easy to learn and one that pays immediate rewards to any wilderness traveler. Some of this language is in words, but most of it is in the form of symbols drawn on a map. The best way to follow the lesson is to study it along with an actual USGS topographic map. Any one will do.

Each map is referred to as a quadrangle (or quad) and covers an area bounded on the north and south by latitude lines that differ by an amount equal to the map series (7.5 minutes or 15 minutes) and on the east and west by longitude lines that differ by the same amount. Each quadrangle is given the name of a prominent topographic or human feature of the area.

What the colors mean

Colors on a USGS topographic map have very specific meanings. This is what the different colors stand for:

Red: Major roads and survey information.

Blue: Rivers, lakes, springs, waterfalls, and other water-related features. The contour lines of glaciers and permanent snowfields are in solid blue, with their edges indicated by dashed blue lines.

Black: Roads, trails, railroads, buildings, and other features not part of the natural environment.

Green: Areas of substantial vegetation. Solid green marks a forested area while mottled green indicates scrub vegetation. The lack of green doesn't mean there's no vegetation in the area, simply that it is too small or scattered to show on the maps. Don't be surprised if a small, narrow gully with no green color on the map turns out to be choked with slide alder or vine maple.

Brown: Contour lines and elevations.

Purple: Partial revision of an existing map.

Translatins contour lines

The heart of a topographic map is its overlay of contour lines, each line indicating a constant elevation as it follows the shape of the landscape. A map's contour interval is the difference in elevation between two adjacent contour lines (usually 40 feet on 7.5-minute maps and 80 feet on 15-minute maps). Every fifth contour line is printed darker than other lines and is labeled periodically with elevation.

One of the most important bits of information a topographic map reveals is whether you will be hiking uphill or downhill. If the route crosses lines of increasingly higher elevation, you will be going uphill. If it crosses lines of decreasing elevation, the route is downhill. Flat side-hill travel is indicated by a route that crosses no lines, remaining within a single contour interval.

This is only the start of the picture that contour lines paint of your actual wilderness route. They also show cliffs, passes, summits and other features (fig. 4-1). You will get better and better at interpreting these lines by comparing actual terrain with its representation on the map (fig. 4-2). The goal is to someday glance at a topographic map and realize you have a rather sharp mental image of just what the place will look like. The following listing shows the main features sketched by contour lines.

Flat areas: No contour lines at all.

Gentle slopes: Widely spaced contour lines.

Steep slopes: Closely spaced contour lines.

Cliffs: Contour lines extremely close together or touching.

Valleys, ravines, gullies, couloirs: Contour lines in a pattern of "U's" pointing in the direction of higher elevation, if the valley or gully is gentle and rounded; a pattern of "V's," if the valley or gully is sharp and narrow.

Ridge or spur: Contour lines in a pattern of "U's" pointing in the direction of lower elevation, if the ridge is gentle and rounded; a pattern of "V's," if the ridge is sharp.

Saddle, pass, col: A low point on a ridge, with higher contour lines on each side and often with a characteristic hourglass shape.

Cirques, bowls: Patterns of contour lines forming a semicircle (or as much as three-quarters of a circle), rising from a low spot in the center of the partial circle to form a natural amphitheater at the head of a valley.

Peak: A concentric pattern of contour lines with the summit being the innermost and highest ring. Peaks often are also indicated by X's, elevations, benchmarks (BM), or a triangle symbol.

The margin of a USGS map holds important information, such as date of publication and revision, names of maps of adjacent areas, contour

Nearly flat area

Usgs Map Symbols Checked Spot ElevationsTopo Map Most Beautiful

Fig. 4-1. Basic topographic features

Summit Cirque or bowl Saddle, pass, or col

Fig. 4-1. Basic topographic features interval, and map scale. The margin also provides a diagram showing the area's magnetic declination, which is the difference between true north and magnetic north.

Keep a couple of cautionary thoughts in mind as you study a topographic map, because they do have certain limitations. The map won't show all the terrain features you actually see on the climb because there's a limit to what can be jammed onto the map without reducing it to an unreadable clutter. If a feature is not at least as high as the contour interval, it may not be shown, so a 30-foot cliff may come as a surprise in an area mapped with a 40-foot contour interval. Check the date of the map

Fig. 4-2. Photograph of a mountainous area; keyed features are represented on topographic map.
Rock Buttress Foot Slope

1. Basin: moderate slope, camp spots

2. Snow or ice line: dashed line ends on cliffs, rock

3. Buttress: change in features of wall may provide approach to ridge

4. Twin summits: which is higher?

5. Gendarmes, aiguilles, or pinnacles

6. Gully or couloir

8. Rock face

9. Summit: highest point on map

10. Ridge or arete

11. East slope: note shadows and ice accumulation

12. Cirque wall: glacier occupies this cirque

13. Moat

14. Crevasses: indicated by irregular contours, not smooth as near buttress, 3, above

15. Bergschrund: not seen on map but inferred possibility when rock and snow are steep

SCALE 1:24000 o

1000 0 1000 2000 3000

CONTOUR INTERVAL 40 FEET

DOTTED LINES REPRESENT20-F00T CONTOURS DATUM IS MEAN SEA LEVEL

because topographic maps are not revised very often and information on forests and on roads and other manufactured features could be out of date. A forest may have been logged or a road either extended or closed since the last updating. Although topographic maps are essential to wilderness travel, they must be supplemented with information from visitors to the area, guidebooks, and other maps. As you learn about changes, note them on your map.

Sometimes a trip runs through portions of two or more maps. Adjoining maps can be folded at the edges and brought together, or you can create your own customized map by cutting out the pertinent areas and splicing them with tape. Include plenty of territory so that you have a good overview of the entire trip, including surrounding area. Photocopies, good for marking on, don't show colors and may distort, meaning they should be used only in addition to the real thing.

As the precious objects they are, maps deserve tender care in the wilds. A map can be kept in a plastic bag or map case. You can also laminate the map with plastic film or coat it with waterproofing, though these coatings are difficult to mark on. Some maps come already waterproofed. On the trip, carry the map in a jacket pocket or some other easily accessible place so you don't have to take off your pack to get at it.

ORIENTING A MAP

During a trip it sometimes helps to hold the map open so that north on the map is pointed in the actual direction of true north. This is known as orienting the map, a good way to gain a better feel of the relationship between the map and the countryside.

It's a simple process (fig. 4-3). Place your compass on the map, near its declination diagram. Turn the map and compass together until the north-seeking end of the compass needle is aligned with, or parallel to, the magnetic-north arrow on the diagram. The map is now oriented to the scene before you. (This orientation can give a general feel for the area but can't replace the precise methods of

Fig. 4-3. Using the compass to orient a map

orientation and navigation that we will cover later in this chapter.)

ROUTEFINDING WITH THE MAP Before the trip

Most wilderness orientation, navigation, and routefinding is done by simply looking at your surroundings and comparing them with the map.

One useful technique is to identify a navigational "handrail," a linear feature on the map that lies in the same direction you are traveling. The handrail should be within frequent sight of the route, so it can serve as an aid to navigation. Features that can be used from time to time as handrails during a trip include roads, trails, power lines, railroad tracks, fences, borders of fields and meadows, valleys, streams, ridges, lake shores, and the edges of marshes.

A handrail helps in staying on route. Another map technique can help in finding the way home if you've gone off track. This is the "base line," a long unmistakable line that always lies in the same direction from you, no matter where you are during the trip. Pick out the base line on the map during trip planning. It does not have to be something you can see during the trip. You just have to know that it is there, in a consistent direction from you. A base line (sometimes called a catch line) can be a road, the shore of a large lake, a river, trail, power line, or any other feature that's at least as long as your climbing area. If the shore of a distant lake always lies west of your climbing area, you can be sure that heading west at any time will get you to this identifiable landmark. It's not the fastest way to travel, but it saves you from being truly lost.

Also before the trip, anticipate specific route-finding problems. For example, if the route traverses a glacier, you may consider carrying route-marking wands, especially if the weather outlook is marginal. Make a note of any escape routes that can be used in case of sudden bad weather or other setbacks.

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Responses

  • IDRIS
    How to read a cirque on a topo map?
    8 years ago
  • senja wuori
    How to read a climbing topo map?
    8 years ago
  • LUCA
    How to study a topographic map without color?
    8 years ago
  • cian
    How to read a topographic map?
    8 years ago
  • selma
    How to read climbing topographic maps?
    8 years ago
  • brooklyn
    How to read a quadrangle map?
    8 years ago
  • kevin
    Are topography maps revised often?
    8 years ago
  • gavino
    What colors mean on topographic maps?
    8 years ago
  • Zofia
    How to read a topographic map publication?
    7 years ago
  • Saradoc
    What does a cirque look like on a topographic map?
    7 years ago
  • MARY
    What does a cirque look like on a topography map?
    3 years ago
  • Pimpernel
    How to read topo map contours with pictures?
    3 years ago
  • Marcho Burrowes
    How to read topographic overlay?
    5 months ago
  • Rudibert Noakes
    How to know were climbing uphill on a topographic map?
    3 months ago
  • KIFLE
    How to know if you are going uphill or downhill on a topographic map?
    27 days ago

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