此种模型绘制类似米字形的网格。由于整个过程递归调用绘图函数，所以可以根据误差判断绘制DEM的精细程度，从而绘制出不同精细程度的DEM，为解决漫游中数据量较大而引起的画面不流畅现象提供了模型基础。 本文并没有对LOD作研究，只是给出了四叉树的建立和遍历绘图的方法。

#include "Pt3d.h" //空间点类(主要记录空间点的x,y,z)

-Rendering such a model similar to rice-shaped grid. Recursive calls as a result of the whole process of mapping function, so it can be judged according to the error map of the fine DEM, thus rendering a different degree of precision DEM, for the settlement of roaming in the amount of data arising from the larger screen does not provide a model of fluid based on the phenomenon. This article does not LOD for research, but given quadtree traversal of the establishment and mapping method.# Include　Pt3d.h // space-point type (mainly recorded space points x, y, z)