Spatial Referencing Systems


Maps locate and reference information first on a local basis - within the map frame, and on a global basis - on the Earth, using a geographic referencing system which has the following qualities :

  • applicable to any point on the earth, therefore universal.
  • applies to all scales of maps, thus flexible.
  • easy to understand and follow.
  • does not require any previous knowledge of the area.
  • will reference a point to any degree of accuracy which is scale related.

    Coordinates:

    The distances of a surveyed point, north/south and east/west from the origin of survey. Although the word coordinate is usually taken to mean horizontal position (x and y coordinates), the position of a point is not fully defined unless the elevation above the datum, or vertical coordinate (z coordinate), is also provided.

    Spherical Coordinate:

    Latitude, Longitude are known as spherical or geographical coordinates. Excellent for a spherical shape (the Earth), but difficulties arise when used on a flat surface.

  • Round earth to a flat surface.
  • Meridians converge at the poles
  • Parallels are complex curved lines.
  • Angular measurement - degrees, minutes and seconds.
  • Units of measurement - 360 degrees, 60 minutes, 60 seconds.

    Plane Coordinates:

    Plane co-ordinates simple solution of x,y co-ordinates. A rectangular grid consists of a series of straight parallel lines, that are evenly spaced, intersecting at right angles, forming a series of same sized squares.

  • Good for determining absolute and relative positions.
  • Used in digitizing.
  • Hand plotting of map projections using a drafting table and T-square
  • Georeferenced to a common co-ordinate system.

    Polar Coordinates:

    A radius distance and angular measurement to a point referenced from a north/south line of reference.

  • Good for determining relative positions.
  • Used in map construction.
  • Hand plotting of map projections using a drafting table and T-square
  • Georeferenced to a common co-ordinate system.

    Universal Transverse Mercator Grid:

    A plane grid based on the U.T.M. projection, used to locate positions on a map.

  • based on Transverse Mercator Projection.
  • uses a plane grid, squares, superimposed on the irregular shaped graticule
  • from 800 South to 840 North, 1800 West to 1800 East.
  • sixty 60 zones.
  • each zone covers 1 million metres E/W and 20 million metres N/S.

    Block Grids:

    Most common grid type. Simple row and column format to locate information within a block.

  • fast and easy.
  • no fancy mathematics involved.
  • no restrictions to size, shape or scale.
  • not very accurate.
  • reduces search time but not projected reference system.

    Nominal Identification:

    Using proper names to locate an area. Most common method for identifying areas. Not a reference system but works most times.

  • knowledge of area mandatory.
  • not specific.

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