Chapter 18. Units of Measurement 18-1 Chapter 18. Units of Measurement The uniform use of units of measure facilitates broad catalog searches across archive systems. The PDS standard system for units, where applicable, is the Systeme Internationale d’Unites (SI). The default units for data elements in the Planetary Science Data Dictionary(PSDD) are determined as each element is defined and added to the dictionary. Specific unit definitions are also included in the PSDD. In cases where more than one type of …

SI units are all written in mixed case; symbols are also mixed case except for those derived from proper names. No periods are used in any of the symbols in the international system. Derived units — In addition to the base units of the system, a host of derived units, which stem from the base units, are also employed. One class of these is formed by adding a prefix, representing a power of ten, to the base unit. For example, a kilometer is equal to 1,000 meters, and a millisecond is .001 (that is, 1/1,000) second. The prefixes in current use are as follows: SI PREFIXES Factor Prefix Symbol Factor Prefix Symbol 10**18 exa E 10**-1 deci d 10**15 pe ta P 10**-2 cent i c 10**12 tera T 10**-3 mil li m 10**9 giga G 10**-6 micro 10**6 meg a M 10**-9 na no n 10**3 kilo k 10**-12 pic o p 10**2 hecto h 10**-15 fem to f 10**1 de ka da 10**-18 at to a Note that the kilogram (rather than the gram) was selected as the base unit for mass for historical reasons. Notwithstanding, the gram is the basis for creating mass units by addition of prefixes. Another class of derived units consists of powers of base units and of base units in algebraic relationships. Some of the more familiar of these are the following: QUANTITY NAME OF UNIT SYMBOL area square meter m**2 volume cubic meter m**3 density kilogram per cubic meter kg/m**3 velocity meter per second m/s angular velocity radian per second rad/s acceleration meter per second squared m/s**2 angular acceleration radian per second squared rad/s**2….

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