*errcreate* - Use of error arrays

**It is possible to propagate statistical errors through an analysis
procedure via the use of an error array. Error arrays can be used in
such tasks as imcnts, imcontour
and imsmooth.
An error array contains the
squares of the errors (it is the data file
itself for Gaussian statistics).
**

**With the release of PROS 2.3, library routines and tasks were changed
so as to use Poisson statistics by default, rather than Gaussian
statistics. For normal operation:
**

xs> errcreate ``xdata$snr.qp[bl=2]''

**will make a **
** array with 16**
** pixels from
the Einstein IPC image, SNR.qp. Each array element will contain the
square of the error. The output file will be snr_err.imh.
**

**For Gaussian statistics, one needs to change the value of keyword
POISSERR in the image header. (For more information, see Section 2.6.2
in this Guide; see also help errcreate and help explain_errors.
**

1998-06-10