Special Properties of a Map Projection

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Special Properties of a Map Projection
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• 1. Conformality (e.g. Mercator)
• deformation increases regularly in all
  directions
• small circle on spherical surface represented by
  small circle on map
• no angular deformation
• map can be used to measure angles

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Mercator
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• Conformality (cont.)
• shapes of small areas are preserved
  (orthomorphic)
• large areas distorted e.g.Greenland
• absence of angular deformation is most valuable
• navigation charts, military mapping, topographic
  mapping
• area always distorted
• larger areas near margins

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• 2. Equivalence or Equal area (e.g. Mollweide,
  Albers Equal Area)
• areas are preserved
• small circle on spherical surface represented by
  ellipse on map
• all regions have correct relative size

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Mollweide
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• Equivalence (cont.)
• useful for statistical maps (number of symbols
  per unit area)
• dot densities, choropleth mapping
• visual impression of density must not be
  contradicted
• cannot be conformal (shapes distorted/angles
  deformed)

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• 3. Equidistance (e.g. Azimuthal Equidistant)
• no projection can preserve all distances
• equidistant projections preserve distances from
  one or two points or lines only
• circles on sphere are ellipses on map
• Normal aspect of cylindrical, conical and
  azimuthal
• scale preserved along the meridians (parallels
  equally spaced)

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Azimuthal Equidistant
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• Equidistance (cont.)
• not an important property in itself - can be used
  to plot airline routes from a point
• balances all errors (not excessive area or
  angular distortion
• compromise when conformality or equivalence are
  not essential
• used for general reference maps in atlases for
  contries and continents

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Robinson
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Azimuthal (Zenithal) Equal Area
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IMPLEMENTING A NATIONAL REFERENCE SYSTEM

• GREAT BRITAIN

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DAVIDSON COMMITTEE (1938)

• need for national referencing system
• re-triangulation needed
• problems with existing projection
• existing projection
• multiple central meridians (County cased)
• distortion at E and W edges of counties (1m in
  1km)

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Projection Implemented

• Transverse Mercator
• suited to country with N-S extent
• true scale on central meridian
• meridians parallels intersect at right angles
• scale varies equally in all directions away from
  central meridian
• angles, shape preserved over small areas
  (conformal)

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• Origin
• 49oN 2oW
• Modifications
• reduce scale of central meridian by 0.04
• scale true 180km E W of central meridian
• apply local scale factors

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National Grid

• international metre adopted
• grid not aligned with graticule
• N gt S axis aligned with central meridian
• false origin 49o46'N 7o43'W (avoids negative
  coords)

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Uses of National Grid

• sheet numbering system
• computerisation
• thematic mapping
• constant
• present on O.S. maps
• facilitates automation