Hall effect based Teslameters/Gaussmeters measure DC and AC magnetic flux densities in the range from a few mu T to about 30 T. For accurate measurement a 3-axis Hall probe is applied with small magnetic field sensitive volume (MFSV) of 100 mu m x 10 mu m x 100 mu m, with vertical and horizontal Hall elements integrated on a single chip. The planar Hall effect, that produces additional measurement errors is suppressed by the spinning current technique. The orthogonality error of the 3-axis Hall probe is reduced to smaller than 0.1 degrees by the described calibration procedure. This paper explains why the above features are crucial for some applications in industry and modern science for accurate measurement of inhomogeneous magnetic fields and how to achieve them. The future technology trends in magnetic metrology are introduced and the newly developed Nanomapper that incorporates a 3-axis Hall probe with a MFSV of smaller than 10 x 10 x 10 micrometer is presented.