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Map Scale

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									Map Scale
                  Scale
• Ratio between distances on a map or globe,
  and their actual ground distances.
        Expressions of scale
1) Graphic scale (scale bar)
  visual expression
  line marked off in units
  Good for making rough estimates of distance
   Estimating Distance using Graphic Scale

• Mark off map
  distance using
  piece of paper
• Estimate distance
  using the scale
  bar.
    Expressions of scale, cont.
2) Statement of scale
  – Verbal expression
     • Ex. One inch represents ten miles
  – Easy to understand
    Expressions of scale, cont.
3) representative fraction (RF)
  – mathematical expression
  – written as a ratio (sometimes as a fraction)
     • ex. 1: 24,000        (same as 1/24,000)
     • map distance unit : ground distance unit
     • 1 : (some value)
  – independent of units
  – Good for more precise calculations of distance
  – But you usually have to convert your answers
    for them to be meaningful
     • Ex. How far is 24,000 inches?
   English System Conversions
• Inches to miles divide by 63,360
• Feet to miles: divide by 5,280
• Inches to feet: divide by 12

• Miles to inches: multiply by 63,360
• Miles to feet: multiply by 5,280
• Feet to inches: multiply by 12
Distance Calculations using Scale
1) Set up ratio
                   md      md
                  ---- =   -----
                   gd       gd
• md is map distance; gd is ground distance
• one represents scale and the other is the
  distance between two places.
   Solving scale problems, cont.
• Rules
   a) three values are known, insert these values and their
      units of measure (if given)
   b) units for numerators must be the same
   c) units for denominators must be the same

2) Solve for the unknown value
   – not necessarily the final answer
   – Convert into appropriate unit of measure, if required
     Example Scale Problems
1) The map distance between two towns is 10
  inches on a map whose scale is 1:24,000.
  How far apart are the towns?
                  md/gd = md/gd
                 1/24000 = 10”/x
                1”/24000”=10”/x”
         1*x = 24,000*10, or x = 240,000”
         240,000”/63,360”per mi = 3.78 mi
2) Two towns are 15 miles apart and shown
  four inches apart on a map.
a) what is the statement of scale?
                  4”/15mi = 1/x
                4”/15mi = 1”/x mi
              4x=15, so x = 3.75 mi
          one inch represents 3 3/4 miles
b) what is the RF?
                  1”/3.75mi = 1/x
        3.75 mi * 63,360 in/mi = 237,000 in
                  RF is 1:237,000
             Relative Scale
• The larger the scale, the closer the map
  comes to reality.
•
Scale      Mapped Area              RF denom
large      little, yet detailed     small
small      great, yet general       large
              Relative Scale
Small Scale                       Large Scale




    Example: 1:250,000 map vs. 1:24,000 map
Homework

								
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