First of all, this paper points out that the significance of studying the Linear measurement of sample's interval, and then applying different sampling programs on the same plane for collecting, and separately by the least square method, the smallest tolerance zones flatness error evaluation. The result indicated along with the sampling interval increasing or the sampling points reduction, the flatness error always tendency is reduction The result of two methods evaluate along indicated along with the sampling interval increasing or the sampling points reducing, the influence of evaluating method to the evaluate be also smaller.