转自android官方文档，无法提供链接，因为android官网的链接被和谐了。我在日文版google android中的cache里面翻到的如下内容。赶紧保存了下来。

　　1. You can create common dimensions to use for various screen elements by defining dimension values in XML. A dimension resource is a number followed by a unit of measurement . For example: 10px, 2in, 5sp. Here are the units of measurement supported by Android :

　　(1) px --> Pixels - corresponds to actual pixels on the screen .

　　(2) in --> Inches - based on the physical size of the screen .

　　(3) mm --> Millimeters - based on the physical size of the screen .

　　(4) pt --> Points - 1/72 of an inch based on the physical size of the screen .

　　(5) dp --> Density-independent Pixels - an abstract unit that is based on the physical density of the screen. These units are relative to a 160 dpi screen, so one dp is one pixel on a 160 dpi screen. The ratio of dp-to-pixel will change with the screen density, but not necessarily in direct proportion .Note: The compiler accepts both "dip" and "dp", though "dp" is more consistent with "sp".

　　(6) sp --> Scale-independent Pixels - this is like the dp unit, but it is also scaled by the user's font size preference. It is recommend you use this unit when specifying font sizes, so they will be adjusted for both the screen density and user's preference .

　　2. Dimension values are not normally used as raw resources, but rather as attribute values in XML files. You can, however, create plain resources containing this data type.

　　Source file format: XML file requiring a declaration, and a root element containing one or more tags.

　　Resource source file location : res/values/dimens.xml (File name is arbitrary; standard practice is to put all dimensions in one file devoted to dimensions.)

　　Compiled resource datatype: Resource pointer to a dimension .

Resource reference name:

　　Java: R.dimen.some_name

　　XML: @[package :]dimen/some_name (where some_name is the name of a specific element)

　　Syntax

　　Xml代码

　　 <dimenname=dimen_name>dimen_value</dimen>

　　<dimen> A valid dimension value. name - The name used in referring to this dimension .

　　Example XML Declaration

　　The following code declares several dimension values.

　　Example Java code:

　　Java代码

　　float dimen = Resources.getDimen(R.dimen.one_pixel);

　　Example XML code:

　　Xml代码