用4、16、和49点/(英寸)~2的网格在比例尺为1:12,000的航片上进行点数抽样,估测航片面积。这三种密度网格测定面积都会产生无偏估计。当点密度大于27点/(英寸)~2(使每个点的方差减少的最佳密度值)时,用假设的随机抽样和二项式模型估计的方差是有效的。变差系数可由网格密度和测定相应覆被类型的点的百分率预测出来。 Use 4, 16, and 49 dots / (inch) ~ 2 grids to sample points on aerial photos with a scale of 1: 12,000 to estimate the area of ​​aerial photos. All three densities measure the area and produce an unbiased estimate. Variances estimated using the assumed random sample and binomial models are valid when the point density is greater than 27 points / (inch) ~ 2 (the optimal density value that reduces the variance at each point). The coefficient of variation can be predicted from the grid density and the percentage of points at which the corresponding cover types are measured.