The values of an axis variance array may also be accessed directly
as standard deviation values by specifying the special component name
`Error' in a call to NDF_AMAP.
This facility operates in exactly the same way as the equivalent operation
on the main variance component of an NDF (see §) and
causes the square root of the mapped values to be taken before they are
returned.
If a mapping mode of `UPDATE' or `WRITE' is specified, then the new or
modified values are also squared before being written back to the
axis array when it is unmapped.
Note that the component name `Error' may only be used with mapping routines. The name `Variance' should be used when later unmapping values accessed in this way.