Datums Reference¶

Geodetic::Datum¶

A datum defines the reference ellipsoid used for geodetic calculations. The Geodetic::Datum class provides access to 16 pre-defined geodetic datums.

Constructor¶

datum = Geodetic::Datum.new(name: 'WGS84')

The name parameter is case-insensitive and must match one of the 16 available datum names.

Raises NameError if the datum name is not recognized.

Instance Attributes¶

Attribute	Type	Description
`name`	String	Uppercase datum name
`desc`	String	Human-readable description
`a`	Float	Semi-major axis (equatorial radius) in meters
`b`	Float	Semi-minor axis (polar radius) in meters
`f`	Float	Flattening (computed as `1.0 / f_inv`)
`f_inv`	Float	Inverse flattening
`e`	Float	First eccentricity (computed as `sqrt(e2)`)
`e2`	Float	First eccentricity squared

All attributes have both reader and writer accessors.

WGS84 Constant¶

The most commonly used datum is available as a pre-built constant:

Geodetic::WGS84
# => #<Geodetic::Datum name="WGS84", a=6378137.0, f_inv=298.257223563>

This constant is the default datum for all conversion methods throughout the gem.

Class Methods¶

`Datum.list`¶

Returns an Array of strings describing each available datum.

Geodetic::Datum.list
# => ["AIRY: Airy 1830", "MODIFIED_AIRY: Modified Airy", ..., "WGS84: World Geodetic System 1984"]

`Datum.get(name)`¶

Returns a Datum instance for the given name. The name is case-insensitive. Raises NameError if the datum is not found.

datum = Geodetic::Datum.get('WGS84')
# => #<Geodetic::Datum name="WGS84", a=6378137.0, f_inv=298.257223563>
datum.name   # => "WGS84"
datum.desc   # => "World Geodetic System 1984"
datum.a      # => 6378137.0
datum.f_inv  # => 298.257223563
datum.e2     # => 0.00669437999014132

Available Datums¶

Name	Description	Semi-Major Axis (a)
`AIRY`	Airy 1830	6,377,563.396 m
`MODIFIED_AIRY`	Modified Airy	6,377,340.189 m
`AUSTRALIAN_NATIONAL`	Australian National	6,378,160.0 m
`BESSEL_1841`	Bessel 1841	6,377,397.155 m
`CLARKE_1866`	Clarke 1866	6,378,206.4 m
`CLARKE_1880`	Clarke 1880	6,378,249.145 m
`EVEREST_INDIA_1830`	Everest (India 1830)	6,377,276.345 m
`EVEREST_BRUNEI_E_MALAYSIA`	Everest (Brunei & E.Malaysia)	6,377,298.556 m
`EVEREST_W_MALAYSIA_SINGAPORE`	Everest (W.Malaysia & Singapore)	6,377,304.063 m
`GRS_1980`	Geodetic Reference System 1980	6,378,137.0 m
`HELMERT_1906`	Helmert 1906	6,378,200.0 m
`HOUGH_1960`	Hough 1960	6,378,270.0 m
`INTERNATIONAL_1924`	International 1924	6,378,388.0 m
`SOUTH_AMERICAN_1969`	South American 1969	6,378,160.0 m
`WGS72`	World Geodetic System 1972	6,378,135.0 m
`WGS84`	World Geodetic System 1984	6,378,137.0 m

Module Functions¶

The Geodetic module provides two conversion functions available as module methods:

Geodetic.deg2rad(180.0)   # => 3.14159265...
Geodetic.rad2deg(Math::PI) # => 180.0

Function	Description
`deg2rad(deg)`	Converts degrees to radians. Multiplies by `RAD_PER_DEG`.
`rad2deg(rad)`	Converts radians to degrees. Multiplies by `DEG_PER_RAD`.

Module Constants¶

Constant	Value	Description
`RAD_PER_DEG`	0.0174532925199433	Radians per degree
`DEG_PER_RAD`	57.2957795130823	Degrees per radian
`QUARTER_PI`	0.785398163397448	Pi / 4
`HALF_PI`	1.5707963267949	Pi / 2
`FEET_PER_METER`	3.2808399	International feet per meter
`FEET_PER_MILE`	5280.0	Feet per statute mile
`INCH_PER_FOOT`	12.0	Inches per foot
`KM_PER_MILE`	1.609344	Kilometers per statute mile
`MILE_PER_KM`	0.621371192237334	Statute miles per kilometer
`SM_PER_NM`	0.868976242	Statute miles per nautical mile
`NM_PER_SM`	1.15077944789197	Nautical miles per statute mile
`NM_PER_DEG`	60.0	Nautical miles per degree of latitude
`SM_PER_DEG`	52.13857452	Statute miles per degree of latitude
`MILES_PER_DEG`	52.13857452	Alias for `SM_PER_DEG`
`GRAVITY_MS2`	9.80665	Standard gravity in m/s^2
`GRAVITY_FS2`	32.174	Standard gravity in ft/s^2
`GRAVITY`	9.80665	Alias for `GRAVITY_MS2`