Accurate estimation of object edge lines is the key for accurate dimension estimations. However, due to the limited spatial resolution of terrestrial laser scanning and the existence of mixed pixels along object edges, edge line estimation is always imperfect. Most of the existing edge line estimation algorithms simply remove mixed pixels before edge line estimation without utilizing mixed pixel data. This study developed high-accuracy edge line estimation algorithms for flat surfaces using 3D terrestrial laser scanning. In this study, four different edge line estimation algorithms are proposed and compared, and two of them leverage information from mixed pixels to improve the edge line estimation accuracy. The effects of scan data parameters on the performance of the edge line estimation algorithms are investigated using numerical simulations and validation experiments. Based on the simulation results, recommendations of edge line estimation algorithm and scanning settings are provided, which are further validated through experiments.