Colour‐coded pixel‐based highly interactive Web mapping for georeferenced data exploration

Abstract

This paper describes a pixel‐based technique that enables highly interactive Web choropleth maps for georeferenced data publishing and visual exploration. Instead of delivering geographic knowledge to the client in polygon‐based vector formats, we encode geographic object IDs and shape information into highly compact pixel images (decoding maps). This allows the combination of raster and vector characteristics while avoiding the problems in the currently existing pixel‐based (raster‐image‐based) or vector‐based techniques. Differing from traditional pixel‐based techniques that are static and allow very little user interaction, our pixel‐based technique allows varieties of sub‐second (less than 1 s) interface controls such as dynamic query, dynamic classification, geographic object data identification, user‐setting adjusting, as well as turning on/off layers, panning and zooming, with no or minimum server support. Compared with Web GIS techniques that use vector geographic data, our technique avoids transferring over the network large vector geographic data. It also avoids the non‐trivial client‐side computation to interpret the vector data and render the maps. Our technique features a short initial download time, near‐constant performance scalability for larger numbers of geographic objects, and download‐map‐segment‐only‐when‐necessary which potentially reduces the overall data transfer over the network. As a result, it accommodates general public users with slow modem network connections and low‐end machines, as well as users with fast T‐1 connections and fast machines. The client‐side (browser) is implemented as lightweight Java applets. YMap, an easy‐to‐use, user‐task‐oriented highly interactive mapping tool for visual georeferenced data exploration is implemented using this technique. The performance comparison of YMap to some other vector‐based Web GIS demonstrates the feasibility and benefits of this technique.

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