What does "sigma" mean during a map calculation?

SWEET at BNLARM.BNL.GOV SWEET at BNLARM.BNL.GOV
Thu Oct 26 09:20:08 EST 1995


Can someone explain, please.

I calculate a map that ends up looking about right, but I don't understand
the final scaling of the map.  The text below appears in the output:

	.
	.
	.
 ASSFIL: file obsmap_0.3.map opened.
 XMAP: using grid [ 96( 97), 96( 97),108]
 XRTEST: number of selected reflections   12238
 XMAP: ave. elect. density in unit cell= -0.60E-19 sigma= 0.226E-01 e/A^3
 XMAP: density map has been scaled such that one sigma = 1.0
 XMAP: extend NA=(  96  37 110) NB=(  96 -30  49) NC=( 108   4  73) 
 XMAP: a= 64.03, b= 63.70, c= 69.50, alpha= 90.00, beta= 90.00, gamma= 90.00
 XMAP: written in ZYX mode (z-sections)
 XMAP: section #   0 ave. dens.= 0.246E-01 sigma=  3.28     e/A^3
 XMAP: section #   1 ave. dens.=-0.130E-01 sigma=  3.18     e/A^3
 XMAP: section #   2 ave. dens.=-0.556E-01 sigma=  3.10     e/A^3
 XMAP: section #   3 ave. dens.=-0.190E-01 sigma=  3.04     e/A^3
	.
	.
	.

Line 5 says that the map has been rescaled so that the standard deviation
over the whole map is 1.0.  However, the lines that appear with the map
itself suggest that the average value is actually 3-something. 

What is the truth, please, and what do these numbers mean?

Thank you,
Bob Sweet

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	Robert M. Sweet			E-Dress: SWEET at BNL.GOV 
	Biology Dept.			Phones:
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