On 05/30/2013 07:10 PM, Nobody wrote: > This is why technical drawings which include regularly-spaced features > will normally specify the positions of features relative to their > neighbours instead of (or as well as) relative to some origin.
If I am planting trees, putting in fence posts, or drilling lots of little holes in steel, I am actually more likely to measure from the origin (or one arbitrary position). I trust that the errors accumulating as the tape measure marks were printed on the tape is less than the error I'd accumulate by digging a hole, and measuring from there to the next hole. And definitely when drilling a series of holes I'll never measure hole to hole to mark. If I measure from the origin than any error for the hole is limited to itself as much as possible rather than passing on the error to subsequent hole positions. If I was making a server rack, for example, having the holes consistently near their desired position is necessary. Tolerances are such that my hole can be off by as much as a 1/16" of inch of my desired position and it would still be fine, but not if each hole was off by an additional 1/16". I guess what I've described is accuracy vs precision. In the case of the server rack accuracy is important, and precision can be more coarse depending on the screw size and the mount type (threaded hole vs square hole with snap-in nut). -- http://mail.python.org/mailman/listinfo/python-list
