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178 lines
5.8 KiB
Text
178 lines
5.8 KiB
Text
===================
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Measurement Objects
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===================
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.. module:: django.contrib.gis.measure
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:synopsis: GeoDjango's distance and area measurement objects.
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The :mod:`django.contrib.gis.measure` module contains objects that allow
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for convenient representation of distance and area units of measure. [#]_
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Specifically, it implements two objects, :class:`Distance` and
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:class:`Area` -- both of which may be accessed via the
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:class:`D` and :class:`A` convenience aliases, respectively.
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Example
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=======
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:class:`Distance` objects may be instantiated using a keyword argument indicating the
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context of the units. In the example below, two different distance objects are
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instantiated in units of kilometers (``km``) and miles (``mi``)::
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>>> from django.contrib.gis.measure import D, Distance
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>>> d1 = Distance(km=5)
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>>> print(d1)
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5.0 km
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>>> d2 = D(mi=5) # `D` is an alias for `Distance`
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>>> print(d2)
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5.0 mi
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Conversions are easy, just access the preferred unit attribute to get a
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converted distance quantity::
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>>> print(d1.mi) # Converting 5 kilometers to miles
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3.10685596119
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>>> print(d2.km) # Converting 5 miles to kilometers
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8.04672
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Moreover, arithmetic operations may be performed between the distance
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objects::
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>>> print(d1 + d2) # Adding 5 miles to 5 kilometers
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13.04672 km
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>>> print(d2 - d1) # Subtracting 5 kilometers from 5 miles
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1.89314403881 mi
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Two :class:`Distance` objects multiplied together will yield an :class:`Area`
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object, which uses squared units of measure::
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>>> a = d1 * d2 # Returns an Area object.
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>>> print(a)
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40.2336 sq_km
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To determine what the attribute abbreviation of a unit is, the ``unit_attname``
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class method may be used::
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>>> print(Distance.unit_attname('US Survey Foot'))
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survey_ft
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>>> print(Distance.unit_attname('centimeter'))
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cm
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.. _supported_units:
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Supported units
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===============
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================================= ========================================
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Unit Attribute Full name or alias(es)
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================================= ========================================
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``km`` Kilometre, Kilometer
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``mi`` Mile
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``m`` Meter, Metre
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``yd`` Yard
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``ft`` Foot, Foot (International)
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``survey_ft`` U.S. Foot, US survey foot
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``inch`` Inches
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``cm`` Centimeter
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``mm`` Millimetre, Millimeter
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``um`` Micrometer, Micrometre
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``british_ft`` British foot (Sears 1922)
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``british_yd`` British yard (Sears 1922)
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``british_chain_sears`` British chain (Sears 1922)
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``indian_yd`` Indian yard, Yard (Indian)
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``sears_yd`` Yard (Sears)
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``clarke_ft`` Clarke's Foot
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``chain`` Chain
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``chain_benoit`` Chain (Benoit)
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``chain_sears`` Chain (Sears)
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``british_chain_benoit`` British chain (Benoit 1895 B)
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``british_chain_sears_truncated`` British chain (Sears 1922 truncated)
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``gold_coast_ft`` Gold Coast foot
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``link`` Link
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``link_benoit`` Link (Benoit)
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``link_sears`` Link (Sears)
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``clarke_link`` Clarke's link
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``fathom`` Fathom
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``rod`` Rod
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``nm`` Nautical Mile
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``nm_uk`` Nautical Mile (UK)
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``german_m`` German legal metre
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================================= ========================================
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.. note::
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:class:`Area` attributes are the same as :class:`Distance` attributes,
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except they are prefixed with ``sq_`` (area units are square in nature).
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For example, ``Area(sq_m=2)`` creates an :class:`Area` object
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representing two square meters.
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Measurement API
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===============
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``Distance``
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------------
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.. class:: Distance(**kwargs)
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To initialize a distance object, pass in a keyword corresponding to the
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desired :ref:`unit attribute name <supported_units>` set with desired
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value. For example, the following creates a distance object representing 5
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miles::
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>>> dist = Distance(mi=5)
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.. method:: __getattr__(unit_att)
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Returns the distance value in units corresponding to the given unit
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attribute. For example::
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>>> print(dist.km)
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8.04672
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.. classmethod:: unit_attname(unit_name)
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Returns the distance unit attribute name for the given full unit name. For
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example::
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>>> Distance.unit_attname('Mile')
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'mi'
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.. class:: D
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Alias for :class:`Distance` class.
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``Area``
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--------
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.. class:: Area(**kwargs)
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To initialize an area object, pass in a keyword corresponding to the
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desired :ref:`unit attribute name <supported_units>` set with desired
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value. For example, the following creates an area object representing 5
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square miles::
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>>> a = Area(sq_mi=5)
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.. method:: __getattr__(unit_att)
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Returns the area value in units corresponding to the given unit attribute.
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For example::
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>>> print(a.sq_km)
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12.949940551680001
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.. classmethod:: unit_attname(unit_name)
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Returns the area unit attribute name for the given full unit name. For
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example::
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>>> Area.unit_attname('Kilometer')
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'sq_km'
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.. class:: A
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Alias for :class:`Area` class.
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.. rubric:: Footnotes
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.. [#] `Robert Coup <https://koordinates.com/>`_ is the initial author of the measure objects,
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and was inspired by Brian Beck's work in `geopy <https://github.com/geopy/geopy/>`_
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and Geoff Biggs' PhD work on dimensioned units for robotics.
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