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Van Geems, B, Barbara, Ms
09-28-1995, 06:05 PM
Dear biomch-L'ers

I am new to the biomech network. By training I am a Professional Land
Surveyor and now work as a Photogrammetrist - or using the medical
term - Biostereometrican in Biomedical Engineering. So the comments I
am making on the subject of coordinate axes is from my working point
of view and do not represent any right or wrong answers - especially
as I have only recently begun the learning curve of biomechanics.

The Manual of Photogrammetry published by the American Society of
Photogrammetry states "The photo-coordinate system is defined by the
axes x,y,z ... The z-axis coincides with the optical axis of the
camera and is positive along the direction towards the image plane of
the camera. The x-axis is parallel to the x-axis on the plane of the
photograph and is positive in the direction of flight. The y-axis is
parallel to the y-axis on the photographic plane."

To give an example, when setting up two CCD cameras to monitor human
movement and later using say the DLT to solve for 3D coordinates the
coordinate system is as follows:
(to understand the setup fully one should think of standing behind
the cameras baseline, with the cameras between oneself and the
subject)
the x-axis runs from left to right along the "flight-line" /
baseline, i.e. the axis is horizontal increasing to the right,
the y-axis is vertical, increasing upward,
the z-axis is increasing positively toward one.

To reiterate once again, the above statement pertains to
the photogrammetric setup and obviously has relevance to those using
say photogrammetric methods in studying human movement.

I hope this does not confuse the debate further on setting up
coordinate axes standards.

Barbara



Barbara van Geems
Department of Biomedical Engineering
University of Cape Town Medical School
Observatory 7925
South Africa
Tel: (021) 406-6547
Fax: (021) 448-3291
Pmail: GTSBAR@ANAT.UCT.AC.ZA