General Notion of Geographic Information

2. The Essential Model for Features

2.1. General Notion of Geographic Information

Geospatial information is anything that you can learn by looking at maps -- not just traditional maps, but new, creative, digital maps and earth visualization systems. A map, after all, is simply a metaphor for the Earth itself. We therefore accept raster Earth imagery as a kind of map, and even less structured collections of samples of Earth phenomena with any kind of instrumentation as acceptable maps. For the purposes of this document, we use the term “map” in a very broad sense – encompassing all earth metaphors from traditional paper maps to 3d earth visualization systems. We can learn about phenomena that vary with time by looking at special maps designed to reveal temporal differences and events. However, the study of the temporal aspect of geospatial information in 4-space is postponed for a later version of this document. For now, we will assume that phenomena do not change, or that temporal aspects of geospatial information can be held as attributes of features. The fundamental unit of geospatial information is called a feature. Features may be defined recursively, so there can be considerable variation in feature granularity. For example, depending on the application or interests of the information gatherer, any of the items in Figure 2-1 could be a feature. Potential Features A segment of a road between consecutive intersections A numbered highway consisting of many road segments A georeferenced satellite image A single pixel from the image mentioned at the left A temperature overlay on a weather map A triangulated irregular network A dynamically segmented road A drainage network A set of siesmic event magnitude contours Figure 2-1: Example Features The collection and use of geospatial information has one purpose: to communicate knowledge about phenomena that have location. For example, the knowledge imparted by the map answers two kinds of questions: where and what. Maps can tell us where things are, both in relation to other nearby things. Maps also can tell us what things are, either through symbology e.g., by use of color or line pattern whose meaning is explained in a legend or through text or tabular annotations or multi-media links. The same goes for attributes that modify or extend our knowledge of things. Digital geospatial information is geospatial information that has been encoded into a digital form. The encoding is done so that computer resources can be applied to automate the business of geospatial information processing: storage, transmission, analysis, visualization and so forth. There are many different ways to create digital representations of geospatial information. This richness of alternatives is more a curse than a blessing since it has created the confusing and apparently chaotic variety of Geographic Information System GIS data structures and formats now confronting GIS users. The OpenGIS ® Abstract Specification Page 3 Volume 5: Topic 5: Features This Abstract Specification exists to help bring order to this chaos and to provide the foundation model upon which the OGC standards are based.

2.2. Introduction to the Notion of a Information Community