Scan region in Nanonis .sxm

How a Nanonis .sxm 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
:SCAN_OFFSET:	Centre of the scan frame, x and y	scan_offset_value_x, _y
:SCAN_RANGE:	Width and height	scan_range_x, _y
:SCAN_PIXELS:	Pixels per line, number of lines	scan_points_x, _y
:SCAN_DIR:	up or down, the slow direction	Sign of the slow axis; orients the image
:SCAN_ANGLE:	Rotation of the frame	scan_angle_x, _y, not applied

Both passes of the fast axis are stored (DATA_INFO Direction is both), so the fast axis stays unsigned. The header repeats the same geometry in :Scan>Scanfield: as centre_x;centre_y;width;height;angle; the reader does not read that key.

Open question. That SCAN_OFFSET is the centre follows from what Nanonis writes into the file and from how other readers treat it, not from a statement by SPECS. The Nanonis TCP Protocol Document, which defines the scan frame directly, needs a MySPECS account and has not been checked.

Evidence

The centre reading is graded vendor file content and third-party readers: it follows from the :Scan>Scanfield: key that Nanonis itself writes and from Gwyddion's nanonis.c, which places the frame origin at SCAN_OFFSET - 0.5 * SCAN_RANGE. See how the evidence is graded.