包含着很多多边形，但是这些多边形大多永远被排除在视野以外，对于一个大型数据库，就像地形数据库来说是很重要的。

This can be very important for large databases that contain may polygons, most of which will not be in the field-of-view at any one time such as a terrain database.

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不懂拉丁文的人就像是在雾天去到一个不错的国家，他面前的视野非常狭窄，只能看到眼前的一点点东西，超出几步范围以外的东西都变得模糊起来了。

To be entirely ignorant of the Latin language is like being in a fine country on a misty day. The horizon is extremely limited. Nothing can be seen clearly except that which is quite close;

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通过在器具视野以外的触发命令如远程通信、声控或控制总线等控制器具。

NOTE An infra-red control by itself is not considered one used for remote operation. However, it may be incorporated as part of a system such as a telecommunication, sound control or bus system.

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