# Django 2.2.9.dev20191202104323 文档

# Measurement Objects

The `django.contrib.gis.measure` module contains objects that allow for convenient representation of distance and area units of measure. [1] Specifically, it implements two objects, `Distance` and `Area` -- both of which may be accessed via the `D` and `A` convenience aliases, respectively.

## 例如

`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 D, Distance
    >>> 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 `Distance` objects multiplied together will yield an `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

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

注解

`Area` attributes are the same as `Distance` attributes, except they are prefixed with `sq_` (area units are square in nature). For example, `Area(sq_m=2)` creates an `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 unit attribute name set with desired value. For example, the following creates a distance object representing 5 miles:

      >>> dist = Distance(mi=5)

  `__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 `Distance` class.

### `Area`

class `Area`(**kwargs)

  To initialize an area object, pass in a keyword corresponding to the desired unit attribute name set with desired value. For example, the following creates an area object representing 5 square miles:

      >>> a = Area(sq_mi=5)

  `__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 `Area` class.

Footnotes

[1] | Robert Coup is the initial author of the measure objects, and was inspired by Brian Beck's work in geopy and Geoff Biggs' PhD work on dimensioned units for robotics.