A map is a representation of what exists on the ground in the area covered by that map. By looking at a map you can determine what you can expect to find at a given location, determine where you are and get guidance on how to get from where you are to where you want to be.
Most of us have used a highway map to determine how to get from point A to point B. A highway map will show roads, cities and other features of interest to people traveling on roads but don't show much detail of use to people traveling away from developed areas.
For planning backcountry travel we use a topographic map. In addition to providing representation of altitude through contour lines, topographic maps give more information to the local area because they are produced at a larger scale. Large-scale maps cover a narrower portion of the Earth's surface in greater detail than small-scale maps.
Scale is represented as a ratio of 1 to another number. The most common maps used in the United States use the English system of measurement; inches, feet, yards and miles. In the English system the 1 represents one inch on the map and the following number represents the number of inches on the ground. Most US Geological Survey (USGS) topographical maps in the Continental US have a ration of 1:24000. This means that if you measure one inch on the map that will represent 24,000 inches on the ground, or 2,000 feet.
Such a map is considered a large-scale map. Since a ratio is also considered a fraction, 1:2, or one half, is larger than 1:4, or one quarter. In the same way a scale of 1:24,000 is larger than a scale of 1:50,000. A large-scale map covers a smaller area than a small-scale map
Smaller scale maps are also produced by the USGS. Scales of 1:50,000, 1:100,000 and greater, including multi-state maps at 1:2,000,000. These maps cover a much larger area in less detail. A small-scale map may be useful for determining areas you might wish to visit and can help you get there. But, since such a map will lack the detail of the smaller scale map it won't help as much in getting around when your ready to put boots to dirt.
When planning the details of your backcountry travel it is important to know exactly what you will be facing. For this purpose a large-scale map should be used. Once you've decided the area you wish to visit you should obtain maps of the area in large scale. The USGS 1:24,000 scale maps are ideal for this purpose.
On such a map even the smallest detail is included. Footbridges, ruins and many landmarks and points of interest will be included. On a small-scale map you will be shown only major features; parks, boundaries, roads and cities for example. But, on a large-scale map you will be able to see fences and gates that may block the path you wish to take. And, to find exactly where that hot spring in which you might wish to spend an afternoon relaxing after a long day on the trail, the large-scale map is just the ticket.
Another consideration is contour interval, or vertical scale. On topographic maps lines called contour lines represent elevation. The spacing between the lines represents vertical changes in elevation. The vertical changes shown by contour lines, the contour interval, depends on many different factors. Larger scale maps, with more room for detail may show smaller elevation changes with each line compared to small-scale maps.
The terrain in an area also affects the contour interval. With steep and rugged terrain it the contour interval will usually be greater than regions with more gentle slopes. With elevation changes in mountainous terrain measured in thousands of feet, compared with hundreds of feet in flatter terrain, using the same contour interval would result in a map with far too much detail to be usable.
When using contour lines in planning your trip the spacing of the lines on the map is an important consideration. Closely spaced contour lines represent steep slopes. A vertical change of 1000 feet in 2000 feet will show many contour lines grouped together in one inch on a standard USGS map. The same vertical change in 4000 feet will show the contour lines spread across two inches of the map.
When planning your trip keep in mind the contour interval of the map you are using. In particularly rugged terrain a 20-foot bluff could be completely lost in a 40-foot contour interval. Being aware of what is represented on your maps, and what might be hidden is an important consideration.
Even with the appropriate large-scale maps it's probably not a bad idea to have a smaller scale map of the area with you on the trail. While the small-scale map is not as useful for finding the perfect spot to camp, it could come in very handy in an emergency. If you get lost and wander off the area covered by your detailed maps, the next larger scale maps can help you navigate back to the trail or to safety.
Each scale of map has a useful purpose to the backcountry hiker. Very small-scale maps can help you find out about the areas you wish to visit and start your initial planning. Once you've identified the general area in which you will be traveling, large-scale maps provide the detail to make the most of your adventure. And, something in between can be very important in finding your way back if something goes wrong.