Geographical perl modules

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Geographical perl modules
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Some etymology
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Some etymology

• geo'graphy
• Drawing the Earth

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Some etymology

• geo'graphy
• Drawing the Earth
• geo'metry
• Measuring the Earth

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The interest in geography
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The interest in geography

• The Open Guide to London
• Geocache, MUD-London and other web based mapping
  ideas

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The interest in geography

• 'Grubstreet' had map links with OS grid
  coordinates (www.streetmap.co.uk)
• We can use the X and Y to plot a map.

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The interest in geography
Y
X
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Find by distance

• We know the location of A (X1, Y1)
• We know the location of B (X2, Y2)
• The distance between them is
• Sqrt ( (X2 - X1)2 (Y2 - Y1)2)
• And OS eastings and northings work in Metres

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BUT everyone else uses Latitude and Longitude

• The standard for GPS
• Works worldwide

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The problem

• The world is flat

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The problem

• The world is flat
• round

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Latitude and Longitude are angles
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Mercator's Projection
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Mercator's Projection

• Was designed for nautical use
• Preserves angles (azimuth, heading)
• Distorts large distances
• Works well over short range distances
• The Mercator projection is geared to temperate
  latitudes (e.g. Europe)

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Transverse Mercator
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Ordnance Survey Grid

• Is a transverse Mercator, with false (offset)
  Easting and Northing.
• A perl module exists to convert between OS Grid
  and Lat/Long
• GeographyNationalGrid

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GeographyNationalGrid

• Object Orientated interface
• Each object is a location
• As parameters to new specify one of the
  following
• Lat/Long
• OS Grid reference e.g. TQ 123456
• 6 digit Easting and Northing (i.e. X and Y)

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GeographyNationalGrid

• Method calls include
• latitude, longitude
• gridReference
• easting, northing
• deg2string( degrees )
• Converts an angle to degrees, minutes and seconds

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GeographyNationalGrid

• Subclassable
• Subclasses are used to implement grids for
  individual countries.
• The module comes with
• GeographyNationalGridGB
• GeographyNationalGridIE

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Back to OpenGuides

• Location and find_by_distance are based on the
  Ordnance Survey grid

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Back to OpenGuides

• Location and find_by_distance are based on the
  Ordnance Survey grid
• The OS charge licence fees to use their data
  and maps

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Back to OpenGuides

• Location and find_by_distance are based on the
  Ordnance Survey grid
• The OS charge licence fees to use their data
  and maps
• We want a system that will work outside the UK
  and Ireland

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Why don't we do it ourselves?
Radius of a circle of parallel R E cos
A where E radius of Earth A latitude
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The radian approximation

• For small ?
• sin ? lt ? lt tan ?
• ? must be in RADIANS

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The radian approximation

• For small ?
• sin ? lt ? lt tan ?
• ? must be in RADIANS
• For a small distance on the ground
• The conversion between lat/long and X/Y is linear

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Transverse Mercator revisited

• There is an emerging standard, UTM
• Universal Transverse Mercator
• It is not UK-centric

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Problem 2

• The world is flat
• round
• squashed

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The Earth is an oblate spheroid

• More like the shape of an apple than a ball
• Instead of projecting onto a cylinder, we project
  on to an ellipsoid

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To use UTM

• You need to specify a datum
• This includes an ellipsoid and offsets
• (false easting and false northing)

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GeoCoordinatesUTM

• Takes an ellipsoid, not a datum
• Hence no internal facility for false easting or
  false northing.
• Non OO interface.

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latlon_to_utm
my (zone,east,north) latlon_to_utm (
'clarke 1866', 98.251, 2.562)
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utm_to_latlon
my (lat,long) utm_to_latlon ('clarke
1866', '30V', 12554, 41562)
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Plug-ins for CGIWiki and OpenGuides

• CGIWikiPluginLocatorUK
• CGIWikiPluginLocatorUTM