[phenixbb] Average maps (*not* NCS averaging)

Vellieux Frederic Frederic.Vellieux at ibs.fr
Thu Mar 10 23:22:05 PST 2011


I just want to point out that for averaging maps (i.e. averaging between 
crystals instead of averaging within a single crystal as is usually 
done), the 2 maps to be averaged do not necessarily have the same 
distribution of density, the same density histogram, the same dynamic 
range if you prefer. Averaging 2 maps which do not have the same 
histograms without "real space scaling" by histogram matching (or 
mapping) can lead to map deterioration instead of map improvement.

See the example given in Methods in Enzymology 277, pp18-53, the figure 
showing this is on pages 36-37.


Stephen Graham wrote:
> Hi there,
> I'd like to average 2Fo-Fc maps from 2 (approximately) isomorphous
> data sets (one with ligand, one without) to see whether I get better
> density for some of the noisier bits of the structure.  I don't want
> to do any kind of solvent flattening, histogram matching, cyclical
> averaging, etc; I just want to sum the equivalent pixels and divide by
> 2.
> Is it possible to do this easily using any of the phenix tools?  It
> seems like all the required plumbing is there with get_cc_mtz_mtz or
> superpose_maps, but I can't find a phenix utility for averaging two
> superposed maps.
> Thanks,
> Stephen

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