Computer-assisted surgical techniques which seek to avoid relying on CT or MRI scans often require intraoperative location of anatomical features. The conventional optoelectronic probe measures a cloud of points on the feature surface, but the resulting location estimate is subject to bias and variation due both to local deformations in the surface and to measurement noise. We compare three probe designs — the conventional point probe, a flat probe and a V-probe — and show that all exhibit strong directional variability when estimating the centre of a quarter arc. We also show that the V-probe design is superior in tests on a 2D image, reducing the variability in localizing a femoral condyle by 50%.