The precision of target-based registration is related to the geometry distribution of targets, while the current method of setting the targets mainly depends on experience, and the impact is only evaluated qualitatively by the findings from empirical experiments and through simulations. In this paper, we propose a new quantitative evaluation model, which is comprised of the rotation dilution of precision ( $rDOP$ , assessing the impact of targets’ geometry distribution on the rotation parameters) and the translation dilution of precision ( $tDOP$ , assessing the impact of targets’ geometry distribution on the translation parameters). Here, the definitions and derivation of relevant formulas of the $rDOP$ and $tDOP$ are given, the experience conclusions are theoretically proven by the model of $rDOP$ and $tDOP$ , and an accurate method for determining the optimal placement location of targets and the scanner is proposed by calculating the minimum value of $rDOP$ and $tDOP$ . Furthermore, we can refer to the model ( $rDOP$ and $tDOP$ ) as a unified model of the geometric distribution evaluation model, which includes the $DOP$ model in GPS.