Thanks Tom. That is a very thorough responce, and very enlightening. I guess my last question is then how to figure out how Chimera computed the new origin. It appears after closer inspection that my file is an older MRC file in fact. According to the Volume Viewer the origin in Chimera of this map is (-224,-224,-224). Could you briefly explain how that was computed from the data below? I can't seem to figure out from the map header info how that origin would have been computed (other than header info I can't see, and then maybe you can suggest how I could dump ALL the header data). Other than reading it directly from the file it appears that mrc_format.py may also apply a "scale_and_skew" operation (from griddata.py) to the origin data. Im not sure what that is doing, but it might explain how the origin Chimera computed was derived from the data below ... Thanks again. Albion Grid = 96 96 96 Origin = -48 -48 -48 Extent = 96 96 96 Cell = 213.423 213.423 213.423 90.000 90.000 90.000 UVW = 1 2 3 Spcgrp = 1 Nr of points = 884736 Density min = -6.915E-01 Max = 2.397E+00 Average = 7.247E-11 Sigma = 2.290E-01 Variance = 5.243E-02 Mappage Prod= 8.257E+01 Plus = 57 Dyn. range = -6.903E-01 To = 2.398E+00 Cell vol. = 9.721E+06 Voxel vol. = 1.099E+01 Grid vol. = 9.721E+06 %Cell vol. = 100.00 Origin frac = -0.50000 -0.50000 -0.50000 Origin Cart = -106.712 -106.712 -106.712 Top frac = 0.48958 0.48958 0.48958 Top Cart = 104.488 104.488 104.488 Spacing = 2.223 2.223 2.223 Top = 47 47 47 Changes = F