Visibility analysis is an important aspect in terrain analysis. Terrain visibility calculation is a time-consuming process, and visibility algorithms are dedicated to reduce the algorithm complexity to enhance calculation speed. We use digital elevation model to represent terrain. This equal-distance discrete data structure can be processed by geometric algorithm. The paper judges line of sight (LOS) visibility through geometric vector cross product, then delivers an incremental method to calculate terrain visibility with comparison of elevation difference using increments base on grid size. We analyze the algorithm performance in two aspects: horizontal and vertical comparisons. Through vertical analysis, the run-time of the algorithms has been measured for different terrain configurations and different heights of the viewpoint in terrain region. The method is more simple, lower complexity and easier to realize. We give out many computational results of viewshed with different location, and analyze the their distribution characteristics. We conclude that this method have high efficiency in undulant terrain with long line of sight or deep valleys. Also we compare this method with ARCGIS viewshed function horizontally through many tests and find that incremental method is faster and finer than ARCGIS model.