#### Description:

This function returns a Region which represents the uncertainty associated with positions within the supplied Region. See astSetUnc for more information about Region uncertainties and their use.

#### Synopsis

AstRegion ∗astGetUnc( AstRegion ∗this, int def )

#### Parameters:

##### this

Pointer to the Region.

##### def

Controls what is returned if no uncertainty information has been associated explicitly with the supplied Region. If a non-zero value is supplied, then the default uncertainty Region used internally within AST is returned (see `"` Applicability`"` below). If zero is supplied, then NULL will be returned (without error).

#### Applicability

##### CmpRegion

The default uncertainty for a CmpRegion is taken from one of the two component Regions. If the first component Region has a non-default uncertainty, then it is used as the default uncertainty for the parent CmpRegion. Otherwise, if the second component Region has a non-default uncertainty, then it is used as the default uncertainty for the parent CmpRegion. If neither of the component Regions has non-default uncertainty, then the default uncertainty for the CmpRegion is 1.0E-6 of the bounding box of the CmpRegion.

##### Prism

The default uncertainty for a Prism is formed by combining the uncertainties from the two component Regions. If a component Region does not have a non-default uncertainty, then its default uncertainty will be used to form the default uncertainty of the parent Prism.

##### Region

For other classes of Region, the default uncertainty is 1.0E-6 of the bounding box of the Region. If the bounding box has zero width on any axis, then the uncertainty will be 1.0E-6 of the axis value.

#### Returned Value

##### astGetUnc()

A pointer to a Region describing the uncertainty in the supplied Region.

#### Notes:

- If uncertainty information is associated with a Region, and the coordinate system described by the Region is subsequently changed (e.g. by changing the value of its System attribute, or using the astMapRegion function), then the uncertainty information returned by this function will be modified so that it refers to the coordinate system currently described by the supplied Region.

- A null Object pointer (NULL) will be returned if this function is invoked with the AST error status set, or if it should fail for any reason.

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