我国的地形图比例尺有八种（即八种基本比例尺 ）
作者通过变焦数据模型原理，探讨如何建立多层存贮结构，将数字地图中的大比例尺地图数据库自动导出小比例尺图形。
This paper discusses how to establish multilayer storage structure and automatically reduce small scale graph from large scale mapping database by theory of changed-focuses Datum Model.
对于给定的大比例尺地图多边形图层数据，该算法可以高效率地完成多边形选取与合并的综合操作，输出小比例尺地图图层数据。
The algorithm based on C Tree can complete the polygon selection and combination operation in high efficiency for given large scale polygon map layer and small output scale.
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