====== DATA NOT FOUND ======

**DATA_NOT_FOUND** is a text string that represents data that does not exist.

For example, a TARGET that has a reliability of -1 does not exist, and is set internally to DATA_NOT_FOUND

Under many conditions a signal may have a value that cannot be interpreted mathematically.

One example is a TARGET that is occluded in a frame (e.g. a reliability of -1)

==== NO_DATA ====

NO_DATA is a text string used in Visual3D commands to represent DATA_NOT_FOUND

==== NO DATA ====

Some users may inadvertently omit the underscore, as does the wiki presentation.

==== NaN ====

NaN is a text string used by Matlab to indicate **Not a Number**

**DATA_NOT_FOUND** is not equivalent to **NaN** in Matlab because Visual3D will process a signal that contains DATA_NOT_FOUND entries.