Hi Hazel, We are glad you like Chimera! There is a "Volume Mean, SD, RMS" tool (under Tools in the Volume Viewer dialog, among other places) that reports statistics for a density map and allows calculation of what contour level corresponds to a particular value of sigma. There are a few more details in this previous message: http://www.cgl.ucsf.edu/pipermail/chimera-users/2008-May/002651.html At least when this message was written, we were not sure whether the official definition of sigma is standard deviations from the mean or standard deviations from zero. Perhaps it depends on the prior processing of the map. Either way, however, the conversion can be performed using the values reported by "Volume Mean, SD, RMS." I hope this helps, Elaine ----- Elaine C. Meng, Ph.D. meng@cgl.ucsf.edu UCSF Computer Graphics Lab and Babbitt Lab Department of Pharmaceutical Chemistry University of California, San Francisco http://www.cgl.ucsf.edu/home/meng/index.html On Jul 2, 2008, at 8:01 AM, Levy, Hazel C. wrote:

Hi, I am a postdoctoral researcher and an enthusiastic Chimera user. We are trying to finish up a manuscript to submit tomorrow to JSB and we have been frantically trying to figure out how to convert from Chimera's "contour level" to the more conventional "sigma level" for density map viewing at different noise levels.

We understand that the higher the contour level the smaller the map volume displayed (less noisy), but we are not quit sure how to report this as a sigma level.

Each map that we are comparing has a different contour range assigned, I am wondering if sigma level corresponds to some proportion of the maximum volume.

Please let us know how to do this conversion or please point us in the direction toward the person that can help us.

Thanks you so much...this information will settle a long-standing mystery for us. Hazel Levy