Abstract: A probability algorithm based on the envelop points of the equivalent probability density error model(marked as Tepdem) is put forward,then the probability algorithm based on the normal plane of the random line segment is compared with the former.A case study is conducted and the results are visualized and analyzed.It is discovered that Tepdem and its corresponding boundary envelop as well as the probability calculation value are same under both of the probability algorithms,and the former is suitable for the random linear segment while the latter is more general for the probability calculation of the linear entity in vector GIS.