3 32 in 3 and is in the preferred embodiment approximately 492 cm 3 31 5 in 3 the total external volume of the scanner including scan head 22 and scanner tower 24 but excluding base 26 preferably does not exceed approximately 1229 cm 3 75 in 3 more preferably does not exceed approximately 1065 cm 3 65 in 3 most preferably does not exceed approximately 967 cm 3 59 in 3 and is in the preferred embodiment approximately 949 cm 3 57 9 in 3 as used herein volume refers to the displacement volume of a solid structure having the same exterior surfaces as scanner 20 still referring to fig3 through 6 scanner tower 24 is in the preferred embodiment a two piece structure the first piece of which includes part of a mounting 40 for scan head 22 referring now to fig7 in the preferred embodiment of scanner 20 mounting 40 includes a splined positioning mechanism 41 which has a head ring 102 on scan head 22 a tower ring 104 on tower 24 and a rotation limiter 106 which is operable in the preferred embodiment both to limit rotation of scan head 22 relative to tower 24 to a rotation of 270 u00b0 in.