The AutoCAD unit definition file, acad.unt, allows you to define the factors to convert data one set of units to another set of units.
The acad.unt file is a plain ASCII text format file and is used by the unit-conversion function cvunit. You can add new and modify the unit definitions available by using a text editor. A definition consists of two lines in the file—the unit name and the unit definition. The first line must have an asterisk (*) in the first column, followed by the name of the unit. A unit name can have several abbreviations or alternate spellings, separated by commas. If a unit name has singular and plural forms, you can specify these using the following format:
*[ [common] [ ( [singular.] plural) ] ]...
You can specify multiple expressions (singular and plural). They do not have to be located at the end of the word, and a plural form is not required. The following are examples of valid unit name definitions:
*inch(es) *milleni(um.a) *f(oot.eet) or (foot.feet)
The line following the *unit name line defines the unit as either fundamental or derived.
Fundamental Units
A fundamental unit is an expression in constants. If the line following the *unit name line begins with something other than an equal sign (=), it defines fundamental units. Fundamental units consist of five integers and two real numbers in the following form:
c, e, h, k, m, r1, r2
The five integers correspond to the exponents of these five constants:
c Velocity of light in a vacuum
e Electron charge
h Planck's constant
k Boltzman's constant
m Electron rest mass
As a group, these exponents define the dimensionality of the unit: length, mass, time, volume, and so on.
The first real number (r1) is a multiplier, and the second (r2) is an additive offset (used only for temperature conversions). The fundamental unit definition allows for different spellings of the unit (for example, meter and metre); the case of the unit is ignored. An example of a fundamental unit definition is as follows:
*meter(s),metre(s),m -1,0,1,0,-1,4.1214856408e11,0
In this example, the constants that make one meter are as follows:
Derived Units
A derived unit is defined in terms of other units. If the line following the *unit name line begins with an equal sign ( = ), it defines derived units. Valid operators in these definitions are * (multiplication), / (division), + (addition), - (subtraction), and ^ (exponentiation).
You can specify a predefined unit by naming it, and you can use abbreviations (if provided). The items in a formula are multiplied together unless some other arithmetic operator is specified. For example, the units database defines the dimensionless multiple and submultiple names, so you can specify a unit such as micro-inches by entering micro inch.
The following are examples of derived unit definitions.
; Units of area *township(s) =93239571.456 meter^2
The definition of a township is given as 93,239,571.456 square meters.
; Electromagnetic units *volt(s),v =watt/ampere
In this example, a volt is defined as a watt divided by an ampere. In the acad.unt, both watts and amperes are defined in terms of fundamental units.
User Comments
Comments can be added to the file by placing a semicolon at the beginning of a line. The comment continues to the end of the line.
; This entire line is a comment.