.. _ref-measure:

===================
Measurement Objects
===================

.. module:: django.contrib.gis.measure
   :synopsis: GeoDjango's distance and area measurment objects.

The :mod:`django.contrib.gis.measure` module contains objects that allow 
for convenient representation of distance and area units of measure. [#]_ 
Specifically, it implements two objects, :class:`Distance` and 
:class:`Area` -- both of which may be accessed via the 
:class:`D` and :class:`A` convenience aliases, respectively.

Example
=======

:class:`Distance` objects may be instantiated using a keyword argument indicating the 
context of the units.  In the example below, two different distance objects are 
instantiated in units of kilometers (``km``) and miles (``mi``)::

    >>> from django.contrib.gis.measure import Distance, D
    >>> d1 = Distance(km=5)
    >>> print d1
    5.0 km
    >>> d2 = D(mi=5) # `D` is an alias for `Distance`
    >>> print d2
    5.0 mi

Conversions are easy, just access the preferred unit attribute to get a
converted distance quantity::

    >>> print d1.mi # Converting 5 kilometers to miles
    3.10685596119
    >>> print d2.km # Converting 5 miles to kilometers
    8.04672

Moreover, arithmetic operations may be performed between the distance
objects::

    >>> print d1 + d2 # Adding 5 miles to 5 kilometers
    13.04672 km    
    >>> print d2 - d1 # Subtracting 5 kilometers from 5 miles
    1.89314403881 mi

Two :class:`Distance` objects multiplied together will yield an :class:`Area`
object, which uses squared units of measure::

    >>> a = d1 * d2 # Returns an Area object.
    >>> print a
    40.2336 sq_km

To determine what the attribute abbreviation of a unit is, the ``unit_attname``
class method may be used::

    >>> print Distance.unit_attname('US Survey Foot')
    survey_ft
    >>> print Distance.unit_attname('centimeter')
    cm

.. _supported_units:

Supported units
===============

=================================  ========================================
Unit Attribute                     Full name or alias(es)
=================================  ========================================
``km``                             Kilometre, Kilometer   
``mi``                             Mile
``m``                              Meter, Metre
``yd``                             Yard
``ft``                             Foot, Foot (International)
``survey_ft``                      U.S. Foot, US survey foot
``inch``                           Inches
``cm``                             Centimeter
``mm``                             Millimetre, Millimeter
``um``                             Micrometer, Micrometre
``british_ft``                     British foot (Sears 1922)
``british_yd``                     British yard (Sears 1922)
``british_chain_sears``            British chain (Sears 1922)
``indian_yd``                      Indian yard, Yard (Indian)
``sears_yd``                       Yard (Sears)
``clarke_ft``                      Clarke's Foot
``chain``                          Chain
``chain_benoit``                   Chain (Benoit)
``chain_sears``                    Chain (Sears)
``british_chain_benoit``           British chain (Benoit 1895 B)
``british_chain_sears_truncated``  British chain (Sears 1922 truncated)
``gold_coast_ft``                  Gold Coast foot
``link``                           Link
``link_benoit``                    Link (Benoit)
``link_sears``                     Link (Sears)
``clarke_link``                    Clarke's link
``fathom``                         Fathom
``rod``                            Rod
``nm``                             Nautical Mile
``nm_uk``                          Nautical Mile (UK)
``german_m``                       German legal metre
=================================  ========================================

.. note::

    :class:`Area` attributes are the same as :class:`Distance` attributes,
    except they are prefixed with ``sq_`` (area units are square in nature).
    For example, ``Area(sq_m=2)`` creates an :class:`Area` object
    representing two square meters.

Measurement API
===============

``Distance``
------------

.. class:: Distance(**kwargs)

   To initialize a distance object, pass in a keyword corresponding to
   the desired :ref:`unit attribute name <supported_units>` set with
   desired value.  For example, the following creates a distance
   object representing 5 miles::

       >>> dist = Distance(mi=5)

   .. method:: __getattr__(unit_att)

   Returns the distance value in units corresponding to the given unit
   attribute.  For example::

       >>> print dist.km
       8.04672

   .. classmethod:: unit_attname(unit_name)

   Returns the distance unit attribute name for the given full unit name.
   For example::

       >>> Distance.unit_attname('Mile')
       'mi'

.. class:: D

   Alias for :class:`Distance` class.

``Area``
--------

.. class:: Area(**kwargs)

   To initialize a distance object, pass in a keyword corresponding to
   the desired :ref:`unit attribute name <supported_units>` set with
   desired value.  For example, the following creates a distance
   object representing 5 square miles::

       >>> a = Area(sq_mi=5)

   .. method:: __getattr__(unit_att)

   Returns the area value in units corresponding to the given unit
   attribute.  For example::

       >>> print a.sq_km
       12.949940551680001

   .. classmethod:: unit_attname(unit_name)
   
   Returns the area unit attribute name for the given full unit name.
   For example::

        >>> Area.unit_attname('Kilometer')
        'sq_km'

.. class:: A

   Alias for :class:`Area` class.

.. rubric:: Footnotes
.. [#] `Robert Coup <http://koordinates.com/>`_ is the initial author of the measure objects, 
       and was inspired by Brian Beck's work in `geopy <http://code.google.com/p/geopy/>`_ 
       and Geoff Biggs' PhD work on dimensioned units for robotics.
