Class: Osgb::Gridref

Inherits:
Object
  • Object
show all
Defined in:
lib/osgb/gridref.rb

Overview

Implementation derived from the Ordnance Survey guide to coordinate systems in the UK www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/guidecontents/ with help from CPAN module Geography::NationalGrid by and © P Kent

Constant Summary collapse

OS_TILES =

maps OS letter codes onto their coordinates in the master grid

{
  :a => [0,4], :b => [1,4], :c => [2,4], :d => [3,4], :e => [4,4],
  :f => [0,3], :g => [1,3], :h => [2,3], :j => [3,3], :k => [4,3],
  :l => [0,2], :m => [1,2], :n => [2,2], :o => [3,2], :p => [4,2],
  :q => [0,1], :r => [1,1], :s => [2,1], :t => [3,1], :u => [4,1],
  :v => [0,0], :w => [1,0], :x => [2,0], :y => [3,0], :z => [4,0],
}
FALSE_ORIGIN =

the offset makes all coordinates positive and <1000km

{:e => 2, :n => 1}
SQUARE_SIZE =

a shorter grid ref denotes a larger square

[nil, 10000, 1000, 100, 10, 1]
@@default_datum = 
:osgb36
@@iteration_ceiling = 
1000
@@defaults = 
{
  :projection => :gb,   # mercator projection of input gridref. Can be any projection name: usually :ie or :gb
  :precision => 6,      # decimal places in the output lat/long
}

Instance Attribute Summary collapse

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(string, options = {}) ⇒ Gridref

Returns a new instance of Gridref.

Raises:

  • (ArgumentError) 


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# File 'lib/osgb/gridref.rb', line 38

def initialize(string, options={})
  raise ArgumentError, "invalid grid reference string '#{string}'." unless string.is_gridref?
  options = @@defaults.merge(options)
  @gridref = string.upcase
  @projection = Osgb::Projection[options[:projection]]
  @precision = options[:precision]
  @ellipsoid = @projection.ellipsoid
  @datum = options[:datum]
  self
end

Instance Attribute Details

#ellipsoid ⇒ Object

Returns the value of attribute ellipsoid.



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# File 'lib/osgb/gridref.rb', line 23

def ellipsoid
  @ellipsoid
end

#gridref ⇒ Object

Returns the value of attribute gridref.



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# File 'lib/osgb/gridref.rb', line 23

def gridref
  @gridref
end

#options ⇒ Object

Returns the value of attribute options.



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# File 'lib/osgb/gridref.rb', line 23

def options
  @options
end

#precision ⇒ Object

Returns the value of attribute precision.



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# File 'lib/osgb/gridref.rb', line 23

def precision
  @precision
end

#projection ⇒ Object

Returns the value of attribute projection.



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# File 'lib/osgb/gridref.rb', line 23

def projection
  @projection
end

Class Method Details

.iteration_ceiling ⇒ Object 



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# File 'lib/osgb/gridref.rb', line 33

def iteration_ceiling
  @@iteration_ceiling
end

Instance Method Details

#digits ⇒ Object 



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# File 'lib/osgb/gridref.rb', line 53

def digits
  @digits ||= gridref[2,10]
end

#easting ⇒ Object 



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# File 'lib/osgb/gridref.rb', line 74

def easting
  @east ||= offsets[:e] + digits[0, resolution].to_i * SQUARE_SIZE[resolution]
end

#lat(datum = nil) ⇒ Object 



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# File 'lib/osgb/gridref.rb', line 82

def lat(datum=nil)
  to_latlng(datum).lat
end

#lng(datum = nil) ⇒ Object 



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# File 'lib/osgb/gridref.rb', line 86

def lng(datum=nil)
  to_latlng(datum).lng
end

#northing ⇒ Object 



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# File 'lib/osgb/gridref.rb', line 78

def northing
  @north ||= offsets[:n] + digits[resolution, resolution].to_i * SQUARE_SIZE[resolution]
end

#offsets ⇒ Object 



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# File 'lib/osgb/gridref.rb', line 61

def offsets
  if tile
    major = OS_TILES[tile[0,1].downcase.to_sym ]
    minor = OS_TILES[tile[1,1].downcase.to_sym]
    @offset ||= {
      :e => (500000 * (major[0] - FALSE_ORIGIN[:e])) + (100000 * minor[0]),
      :n => (500000 * (major[1] - FALSE_ORIGIN[:n])) + (100000 * minor[1])
    }
  else
    { :e => 0, :n => 0 }
  end
end

#resolution ⇒ Object 



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# File 'lib/osgb/gridref.rb', line 57

def resolution
  @resolution ||= digits.length / 2
end

#tile ⇒ Object 



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# File 'lib/osgb/gridref.rb', line 49

def tile
  @tile ||= gridref[0,2]
end

#to_latlng(datum = nil) ⇒ Object

Returns an Osgb::Point corresponding to this grid reference and lying on the specified datum. We default to WGS84 since that is the representation most likely to be useful.



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# File 'lib/osgb/gridref.rb', line 97

def to_latlng(datum=nil)
  datum ||= :wgs84
  point.transform_to(datum)
end

#to_s ⇒ Object 



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# File 'lib/osgb/gridref.rb', line 90

def to_s
  gridref.to_s
end