Map Reading

What is map reading

Map reading is the practice of extracting useful information from a printed map — where things are, how far apart they are, what kind of terrain lies between them, and which routes between them are actually walkable. For wilderness use the relevant map is almost always a topographic map — a sheet that uses contour lines to depict elevation, alongside the planar features (rivers, trails, roads, structures) that other maps would show.

You can read a map at three depths:

Recognizing features. "That's a river, that's a trail, that's a peak."
Reading the geometry. "The trail is 1.4 km long, climbs 200 m, and bends north halfway." Requires Map Scale, and the contour-line conventions.
Picturing the ground. "If I stand at that saddle, the ridge to my right will be steep and rocky and the valley to my left will open up to the west." This is the hardest skill and the one the Wilderness Navigation Fundamentals class builds the most deliberately.
Using your map with your gps Knowing how to relate coordinates from your GPS to positions on your map, will give you fast and accurate answers to the "Where am I?" question. Reading a coordinate from the map, will allow you to create a waypoint in your GPS, which can then provide you with realtime bearing and distance to the point. Requires map-datum, GPS Setup for Map Coordinates, UTM Coordinates or Latitude and Longitude.

Most navigation failures in the field happen because the navigator stalled at depth 1 or 2 and never built the depth-3 picture.

What's on the map

A standard USGS 7.5-minute (1:24,000) topographic map carries, around its edges and across its face:

Scale and distance markers. See Map Scale for the math; the printed bar scale at the bottom is the practical reference.
A grid. UTM, lat/lon, or both. The map's coordinate system controls how you read positions off it. See UTM Coordinates and Latitude and Longitude.
Contour lines showing elevation. The interval is printed near the legend.
A declination diagram showing the angle between true north, magnetic north, and (sometimes) grid north for the map's location and date. See North References (True, Magnetic, Grid) and Declination.
A datum statement. What earth model the map coordinates are based on. Matters when comparing the map to a GPS reading. See Map Datums.
Map projection and zone information. For UTM-grid maps, the zone designation. See UTM Zones.
Date of survey. Maps go stales. Man made features can change over the years, roads and trails move or fade away, and new one are created. Terrain features also change over time, rivers and streams find new courses. The Earth's surface slowly erodes under the influence of wind and water. Landslides, earthquakes, and volcanos may make more rapid changes.

The legend tells you what every symbol means. The first thing to do with any unfamiliar map is read its legend.

Reading contour lines

A contour line connects points of equal elevation — every point along one line sits at the same height above sea level. The contour interval is the constant vertical distance between one line and the next; it's printed in the map's margin near the scale. On a 40-foot-interval map, every line you cross going uphill is another 40 feet of climb, whether the lines are crowded or spread apart.

Contour lines are the topographic map's defining feature and the source of most of its useful information:

Closely spaced lines mean steep ground.
Widely spaced lines mean gentle ground.
Concentric closed loops mean a hilltop (or, less commonly, a depression — look for hachure marks).
A V or U pointing uphill marks a drainage — a valley or draw. (Where contours cross a stream, the V points upstream.)
A V or U pointing downhill marks a spur or ridge — a finger of high ground.

This is the introductory treatment. For the full picture — index vs. intermediate contours, reading slope, the terrain features (ridge, valley, spur, saddle), and spot elevations — see Elevation and Topography.

A navigator who can read contours can see terrain in the printed sheet. A navigator who can't is essentially using a road map.

Map types beyond the USGS topo

A navigator encounters a range of map types: USGS topographic at various scales (1:24,000, 1:62,500, 1:100,000), Forest Service maps, trail maps, road maps, nautical charts, aeronautical sectionals, orthophotos, Google's web maps, and military Joint Operations Graphics. Each has its uses; each fails for some uses. The skill is recognizing which map fits the task.

What can't a map tell you

Maps lie by omission. Common things that aren't on the map:

Whether the trail is actually walkable today (deadfall, washouts, snow).
Whether the land is open to public access — just because a route is on your map doesn't mean you're allowed to travel it.
Whether the contour at 100m intervals has missed a 20m cliff band inside that interval. (For most USGS topos: yes, occasionally.)
Where the parking actually is, if the trailhead has moved since the map was surveyed.

The map is a tool, not a guarantee.