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Special Properties of a Map Projection

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Mercator. Conformality (cont.) shapes of small areas are preserved (orthomorphic) ... Transverse Mercator. suited to country with N-S extent. true scale on ... – PowerPoint PPT presentation

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Title: Special Properties of a Map Projection


1
Special Properties of a Map Projection
2
  • 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

3
Mercator
4
  • 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

5
  • 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

6
Mollweide
7
  • 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)

8
  • 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)

9
Azimuthal Equidistant
10
  • 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

11
Robinson
12
Azimuthal (Zenithal) Equal Area
13
IMPLEMENTING A NATIONAL REFERENCE SYSTEM
  • GREAT BRITAIN

14
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)

15
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)

16
  • Origin
  • 49oN 2oW
  • Modifications
  • reduce scale of central meridian by 0.04
  • scale true 180km E W of central meridian
  • apply local scale factors

17
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)

18
Uses of National Grid
  • sheet numbering system
  • computerisation
  • thematic mapping
  • constant
  • present on O.S. maps
  • facilitates automation
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