一种基于动态投影和扫描线裁剪的快速瓦片缓存生成方法

A Quick Tile Cache Generation Method Based on Dynamic Projection and Scan-Line Cropping

摘要: 瓦片地图缓存技术是在网络GIS发展过程中产生的一种能够实现静态数据快速浏览的服务器缓存技术，同时近年来也被广泛用于三维地形显示中作为底层数据组织方式。在瓦片地图生产过程中，由于原始影像来源广泛，通常数据格式、投影方式、影像分辨率等均不一致，同时在切片过程中会存在根据地域范围或保密要求等因素需要对影像进行裁剪的等其他约束条件，在传统的切图流程中，会首先在遥感影像处理软件中对影像进行重投影、裁剪、拼接等操作，生成统一格式的中间影像，然后再输入到切图系统中，这种方式不仅效率低下，同时占用大量存储空间。本文在传统切图方法基础上，提出了在切图过程中逐瓦片使用一次多项式进行数值重投影实现的动态投影方法，以及基于扫描线填充算法的影像快速裁剪算法。通过实验对比分析，在多约束条件下使用本文算法能够实现一次性自动化切图，切图效率同无约束条件下切图效率相当，能够在基本不影响切图效率的同时完成瓦片金字塔构建，同时省去了影像预处理过程，极大的提升了处理自动化程度，降低了处理时间。

Abstract: Tile map cache is a kind of service cache technique for quick accessing static map. It is developed during the progress of Web GIS. In recent years, it is widely used in 3D terrain display as the funda-mental data organization method. In the progress of tile map production, original image data are usually different in data format, projection, resolution and so on because of various data sources. Meanwhile, there are other restrictions such as image cropping by area of interest because of application localization or secrecy. So the traditional map tile generation begins with image re-projection, cropping and mosaic to produce united semi-finished image, and then puts it into map tiling software. This method is inefficient and takes lots of storage. Based on the method, this article suggests a dynamic projection using linear polynomial numerical projection and image quick cropping algorithm based on scan-line filling. The experimental results show that the method can realize automatic tile generation in one-time processing. The time it used is nearly the same as traditional methods which only deal with united semi-finished image. The algorithm can finish the map tiling without generating semi-finished image. As a result, it can improve the automation and reduce the processing time.