一、什么是SLD？
SLD全称：Styled Layer Descriptor ，翻译：风格化图层描述器；其是一种基于XML的标记语言，在一些软件尤其是geoserver中专门使用sld进行样式设计，因此“sld”基本等同于样式。
二、为什么使用SLD?
地理空间数据没有内在的视觉成分。为了查看数据，必须对其进行样式设置。设置样式指定用于在地图上渲染数据的颜色、厚度和其他可见属性。
三、SLD简介
（一）概念：
In GeoServer styling is most often specified using XML SLD style documents. Style documents are associated with GeoServer layers (featuretypes) to specify how they should be rendered. A style document specifies a single named layer and a user style for it. The layer and style can have metadata elements such as a name identifying them, a title for displaying them, and an abstract describing them in detail. Within the top-level style are one or more feature type styles, which act as “virtual layers” to provide control over rendering order (allowing styling effects such as cased lines for roads). Each feature type style contains one or more rules, which control how sty...