Introduction to Map Use

study outline – distance measurements

You should be able:

1. estimate the distance between two points in the real world using a map with any of the three scale representations

2. estimate the distance between two points in the real world along a curved path

3. convert a distance measurement to different units within the English and metric systems and between the English and metric systems

4. describe at least two problems that may cause an error in your distance estimates

A. 1^st step – measure the distance on the map

1. ruler

2. paper

3. curved paths must be broken up into straight line segments

4. computer mapping programs often have a measurement tool

B. 2^nd step – use the scale to convert map units to real world units

1. graphic scale

i. hold the measured distance against the scale and read the distance directly off the scale

ii. use interpolation skills to estimate the distance between marked units

2. text scale

i. convert map measurement to the given units if needed

ii. multiply map measurements in the given units by the scale

iii. example: scale is 1 in = 4 mi., distance on map is 2.5 in, 2.5 in X 4 mi/in = 10 mi.

C. problems

1. measurement accuracy

i. any measurement has errors due to the measuring device and your ability to accurately determine the end points.

ii. especially when measuring short distances on the map these errors may be substantial

2. slope

i. distance measured on a planimetric map doesn’t include distances up and down.

ii. the slope of most roads is gentle enough that this is usually not a problem measuring distance along a road

iii. in mountainous terrain, measurements along a hiking path or straight line may be substantial

3. smoothing may cause an estimated distance to be too short

i. measurements along a curved path using an insufficient number of segments

ii. measurements of a feature that has been generalized by simplification