Scan region in Bruker NanoScope .spm

How a NanoScope .spm file states the scanned area, and how the reader turns it into NeXus under the scan region convention.

Raw keys

Raw key	Meaning	How the reader uses it
\X Offset, \Y Offset	Centre position of the scan	scan_offset_value_x, _y
\Scan Size	Edge length of the frame	scan_range_x; y is divided by \Aspect Ratio
\Aspect Ratio	Ratio of x to y range	Malformed values fall back to 1:1 with a warning
\Samps/line, \Lines	Pixels per line, number of lines	scan_points_x, _y
\Frame direction	Up or Down, the slow direction	Sign of the slow axis
\Rotate Ang.	Rotation of the frame	scan_angle_x, not applied

Trace and Retrace layers are both stored, so the fast axis stays unsigned. \X Position and the coarse stage \Stage X are not used for the scan region: the stage is a different frame of reference.

Open question. No Bruker document states the order in which the rows are stored, so the image orientation rests on how Gwyddion reads the format. A document stating the row order, or an openly licensed Up and Down scan of one area, would settle it.

Evidence

The centre reading is graded vendor manual: the NanoScope 6.13 User Guide defines the parameters on the Scan Controls panel as "X offset, Y offset: Controls the center position of the scan in the X and Y directions, respectively" (p. 60), and adds that "each volt of X or Y offset reduces the maximum scan size by 2V" (p. 85), which only a centred frame does. See how the evidence is graded.