Land Navigation · 8 min
Reading a topographic map
Why this matters
Every other navigation skill is downstream of this one. If you can read contours fluently you can see the ground before you walk it, which is how you plan a route, pick a campsite, spot a bail-out and tell a client what the next hour will feel like.
By the end of this lesson you will be able to
- Convert map distance to ground distance at common scales
- Read contour lines for steepness, shape and elevation gain
- Identify the standard terrain features and map colours
Scale
| Scale | One unit on the map | Rule of thumb |
|---|---|---|
| 1:24,000 (USGS 7.5-minute quad) | 24,000 units on the ground | 1 inch = 2,000 ft = 0.379 mi |
| 1:25,000 | 25,000 units | 1 cm = 0.25 km, 4 cm = 1 km |
| 1:62,500 (older 15-minute) | 62,500 units | 1 inch ≈ 1 mile |
| 1:100,000 | 100,000 units | 1 cm = 1 km |
Larger scale, smaller number, more detail. A 1:24,000 quad is the standard for backcountry navigation in New York, and it is what most Adirondack and Catskill trail maps are derived from.
Measured distance is always short. The map is flat and the ground is not, and no map records the small deviations you actually walk. On steep or convoluted ground, add 10 to 20% to any distance you scale off the map.
Contours
- A contour line joins points of equal elevation. Walk one and you neither climb nor descend
- The contour interval is the elevation between adjacent lines. It is printed in the map margin, commonly 10, 20 or 40 feet in New York. Read it every time: a route that looks gentle on a 10-foot map is brutal on a 40-foot one
- Close together, steep. Far apart, gentle. Lines that touch are a cliff
- Index contours, the heavier lines usually every fifth one, carry the elevation figure
- Elevation gain = contour interval × lines crossed uphill. Count every rise, including the ones you climb back out of after a dip
The shapes
| Feature | How it looks |
|---|---|
| Hill or summit | Concentric closed loops, smallest in the middle |
| Depression | Closed loops with hachure ticks pointing inward |
| Ridge | Vs or Us pointing away from the high ground |
| Valley or drainage | Vs pointing toward the high ground. The stream, where there is one, runs through the point of the V |
| Saddle or col | An hourglass between two summits. The natural pass, and the natural place to be blown off your feet |
| Spur | A short ridge running off a larger one. The classic wrong descent |
| Cliff | Contours merged or nearly merged |
The V test is the single most useful contour skill: Vs point upstream. Get that one right and you will not mistake a drainage for a ridge in fog, which is the mistake that puts parties in the wrong valley.
Colours and margin
Black is human-made, blue is water, green is vegetation, brown is contours, red or purple marks major roads and revisions. The margin carries the scale bar, the contour interval, the declination diagram and the date. Check the date: trails move, beaver ponds appear, bridges wash out, and a 1954 quad revised in 1978 is still sold.
From the field
On a 40-foot quad your route crosses 31 contours going up. That is 1,240 feet of gain, which adds 37 minutes to book time. Doing that arithmetic in the planning stage, not at the trailhead, is what separates a timetable from a guess.
This lesson is part of the NYS Licensed Guide Exam course. Open it in the course to answer the retrieval checks that go with it, track what you have read, and quiz the whole unit. Lessons, quizzes and the glossary are free.