• In the picture above, the scale is 1:24,000 (common for USGS topographic maps) and the contour interval is 20 feet.

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• Scale is given in a ratio format: “Map Measurements : Real Life Measurements”.

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• “1:24,000” means that 1 inch on the map = 24,000 inches (or 2,000 feet) in real life. It can also mean 1 cm on the map is 24,000 cm in real life. It’s a ratio of sizes between the things seen on the map vs the size in real life. Any length units can apply.

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• A full-scale map would have a scale of 1:1; for one inch on the map, it would be one inch in the real world.  For one mile on the map, it would be one mile in the real world.​​

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• Scale determines how much detail a map can have. It’s a form of magnification. The smaller the number after "1:" is, the more detail the map can provide.

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• The terms "small", "medium", and "large" are used sometimes to describe map scale. 

  • When one says a map is "small scale", it means there's very little detail.  The ratio may be something like 1:1,000,000. 

  • A "large scale" map would be one that has much more detail.  The ratio for a "large scale map" might be something like 1:24,000.​

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• When photocopying maps, the scale can often be magnified or demagnified, so statements like "1 inch = 1 mile" or numeric scale ratios like "1:24,000" may not be correct after photocopying.  As long as there's no distortion in the map, the bar scale is a valid way of determining distance between two points.

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• BE CAREFUL!  Notice on the bar scales where ZERO (0) is! Pay attention to that when using scales to measure distances! The part of the scale to the left of 0 has more detail for smaller measurements and is referred to as the "Extension Scale."  The part to the right is called the "Primary Scale."

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• Measuring the distance between two points can be done in several ways.  Most commonly used:

  • Use a flexible but non-stretchable material laid out between the two points.  A copper wire will work.  Twist the wire along the object or path you're measuring.  Mark the beginning and end of the path.  Then extend the wire and compare it to the bar scale.

    • If you're measuring "as the crow flies" or direct distance, you only need to measure between the two points.​

    • If you are measuring a walking distance, you must be sure to follow all the turns that the intended path follows!

  • You can use your compass lanyard if it doesn't stretch.  Carefully lay the lanyard out on the path you want to follow. 

    • Some compass lanyards have scales on them.  BE SURE TO USE A SCALE THAT MATCHES THE MAP if you have them (and remember the precaution about magnified photocopies).​

  • You can use the edge of a piece of paper and mark with a pencil the distance between the two points, then transfer the paper marks to the bar scale at the bottom.

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• For measuring distances along curved paths, you will need to use a flexible (but NOT stretchable) length of material and lay it on the map matching the outline of the path, then extend the material and compare it to the bar scale.  Or you can use a piece of paper progressively marking the edge along multiple straight segments along the path.

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• Know that the scale/distance you read from a map is line-of-sight or "as the crow flies" distances.  Measuring distances on topo maps may NOT be the same as ground distance walked!  That's because the measurements on maps do not take into account elevation changes with hills.  The distances measured on these maps is only accurate on flat terrain and will be an under-estimate on sloped terrain (you will have to walk further on ground than you measure on the map.)

 

For a review on units, look here: 

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Other methods

Suppose you have no bar scale and the numeric ratio is in question.  What's a good way to deal with that?

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If you have UTM grids, then you have a scale.  Suppose your UTM grid is a typical one for USGS topo maps and the map gives 1 km (1,000 m) grids.  If you take a ruler and read 4.15 cm (halfway between 4.1 cm and 4.2 cm) between those grid marks,  then you know that 1 cm read on your ruler is 241 meters in real life:

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1,000 m in real life / 4.15 cm on the map = 241 meters in real life for every cm on the map

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If you know how to convert meters to feet, you can determine that 1 cm = 241 m = 790.7 feet.

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If you print a caltopo map (see free map resources) with a scale of 1:800, then you know that means 1 cm on the map is 800 cm in real life.  Since 800 cm is 26.2 feet, then for every cm you read on the map, you would walk 26.2 feet in real life.  So a leg that measures 7.9 cm on the map is 207 feet in real life (7.9 cm x 26.2 feet per cm).

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