Map Scale and Generalization Concepts

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Map Scale and Generalization Concepts
Chapter 5
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Scale

• Scale is the relationship between distance on the
  map and distance on the ground.
• Selecting the appropriate scale depends on the
  size of the sheet of paper and the accurate
  placement of features.
• http//www.beg.utexas.edu/GIS/tools/scale2.htm

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Type of Map Scale

• Word Statement
• Representative Fraction
• Graphic Scale

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Word Statement

• A word statement gives a written description of
  scale, such as "One centimeter equals one
  kilometer" or "One centimeter equals ten
  kilometers."
• Ineffective if the map is reproduced by a method
  such as photocopying and the size of the map is
  modified (Enlarge or Reduce)

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Representative Fraction

• A ratio or representative fraction (RF) indicates
  how many units on the earth's surface is equal to
  one unit on the map. It can be expressed as
  1/100,000 or 1100,000.
• Good when Enlarge and Reduce complete map

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Graphic Scale

• It is simply a line marked with distance on the
  ground which the map user can use along with a
  ruler to determine scale on the map.
• Good when Enlarge and Reduce complete map
• In the U.S., a graphic scale often includes both
  metric and U.S. common units.

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How to Measure Distances on a Map?

• Find the scale for the map you're going to use -
  it might be a ruler-looking bar scale or a
  written scale, in words or numbers.
• Use a ruler to measure the distance between the
  two places. If the line is quite curved, use a
  string to determine the distance and then measure
  the string.
• If the scale is a representative fraction (and
  looks like 1/100,000 or 1100,000), multiply the
  distance of the ruler by the denominator, giving
  distance in the ruler units.
• If the scale is a word statement (i.e. "One
  centimeter equals one kilometer") then determine
  the distance.
• For a graphic scale, you'll need to measure the
  graphic and divide the scale into the measured
  units on the ruler.
• Convert your units of measurement into the most
  convenient units for you (i.e. convert 63,360
  inches to one mile)
• Tips
• Watch out for maps that have been reproduced and
  have had their scale changed. A graphic scale
  will change with the reduction or enlargement but
  the other scales become wrong.

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Large Scale Small Scale
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Large Scale Small Scale
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Large Scale Small Scale
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Large Scale Small Scale

• Large scaleLarge representative fraction
  (150,000 or Larger)
• More detail
• Covers smaller spatial area of Earths surface
• Intermediate Scale150,000 to 1250,000
• Small Scale1250,000 to 17,500,000
• Less detail
• Covers larger spatial area of the Earths surface

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Scale Determination

• When map dont have scale on map, then
• Use of known features
• Reference features as Football ground, tennis
  court, Baseball diamond
• Use of line of Latitude and Longitude
• Distance of 10 line of latitude is 111.133 km
  (average) can be useful to determine scale on
  ATLAS maps.
• Use of Map Comparison
• Another map from same area, having similar
  features can be helpful to determine scale.

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Scale Conversion

• Map scales can be converted from one form to
  another
• RF to word-statement conversion
• Puts map scale in a form that is more easily
  understood verbally
• 163,360 is that same as saying 1 inch is 1 mile
• Word statement to RF
• 2 inches per mile is the same as 131,680
• RF to graphic conversions
• Graphic conversions to RF

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Scale Conversion
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Generalization
"The process of generalization requires the
selection of those features that are essential to
the map's purpose and the representation of them
in a way which is clear and informative".

• Selection
• Selection and retention of more important area
  eliminating less important area.
• Simplification
• Simplify the Shape of feature retain on maps with
  changes in scale
• Combination
• Combine similar Two or more features in one
  symbol
• Locational Shift
• Road or rail road or river crowded in to each
  other, need shifting (not all features are shown
  at their exact location)
• Size Exaggeration
• Exaggerate the size of the objects to improve
  their visibility (Highway width on US map, to see
  clear need exaggeration)

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Geometric generalization for Simplification
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Simplification

• Simplify the Shape of feature retain on maps

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Exaggeration

• On small scale maps most of the objects that are
  important enough to be shown on the map but are
  small in size in real life.e.g. highway shown
  on a US map with scale 1 10,000,000 with line
  0.5 mm would have to be 5,000 m wide in real life
  to meet the proportional scaling criterion.