This technical note describes a comparison of relative errors in one-camera 2D underwater motion analysis when using two different control structure methods for calibrating video images. Eleven male and six female participants performed underwater kicking swim trials. A reference structure of known length and each swimmer's actual (land measured) thigh length were each used to calibrate the movement space from which digitised thigh lengths were calculated from each trial. For each trial, mean thigh lengths calculated using the two methods were then compared to actual thigh length for each swimmer and the differences recorded. Mean differences between actual and computed thigh lengths for the two methods were shown to be similar. However, the spread of mean differences was much less when the actual thigh length was used for calibration. This result strongly supports the use of thigh length as a more accurate method of calibration for one-camera 2D underwater motion analysis.