Velocity uncertainties. Left: comparison of the GC velocity estimates obtained with the two different template spectra. For graphic convenience, the error-bars (the two fxcor-uncertainties added in quadrature) are only shown for objects where the estimates differ by more than 75   km   s^-1. The grey area shows the region encompassing 68 per cent of the data points. Middle: velocity uncertainties as determined by fxcor. Left sub-panel: fxcor-uncertainty vs. R-magnitude. Crosses and dots represent blue and red GCs, respectively. The solid and dashed curve show a quadratic fit to the blue and red GC data, respectively. Right sub-panel: fxcor-uncertainty vs. colour. The dashed line at indicates the colour dividing blue from red GCs. The solid line illustrating the increase of the uncertainties towards bluer colours is a 2nd order polynomial fit to the data. The dashed lines at Δv = 65   km   s^-1 show the limit adopted for the sample definition in Sect. 5.2. Right: comparison of the duplicate velocity measurements. Dots and crosses represent GCs and stars, respectively. The dashed lines show the rms scatter of 84 km   s^-1 found for the GCs.