GIS data displays use symbols to represent real-life objects or geographical features. There are two basic categories into which all real-life objects represented on a map fall: discrete objects, such as a building or river, and continuous fields, which are features that are neither static nor locked to a specific place. Rainfall, elevation, and weather are good examples of features that fall under the category of continuous fields. GISs store data and layer it atop digital geographical visual displays via two methods: raster and vector.

Raster
Raster data is any kind of digital image displayed in grids. Raster data is not concerned with producing an exact representation of reality, but instead reflects an abstraction of reality. Aerial photos are an example of a commonly employed form of raster data. The purpose of an aerial photo in geographic information systems is to display a detailed map or to provide an image to be digitized. Raster datasets contain elevation information, or a digital elevation model (DEM) as well as discrete values and continuous values information.

Raster data can be stored in several formats, including standard file-based forms like TIFF or JPEG, and also as binary large object (or BLOB) data that’s kept in a relational database management system. When properly organized and categorized, raster data can be searchable and easy to find.

Vector Mapping
Vector mapping is a fairly straightforward method of displaying data on a map. It uses points for small discrete values, lines for rivers and other tangible but large discrete values, and polygonal shapes to represent aspects of a terrain that occupy a whole area. Features like lakes and certain aspects of the weather are given polygonal representations on vector maps. In essence, vector mapping divides the types of places in a map and codes them using geometric shapes.

• Points – Points are zero-dimensional. They are used to show geographical data best expressed as a static location, which could serve as a point of reference for those looking to orient themselves. Houses, wells, and places of interest on a map are usually represented by points. Other than its location, you cannot measure any other feature of the object using a point as its visual representation.
• Lines – Lines are one-dimensional and they can be used to represent roads or moving bodies of water in mapping. Railroads, trains, and anything having to do with a system of land transportation will be represented by vector lines. Lines measure an object’s distance or length.
• Polygons – Polygons are two-dimensional graphics used to represent area. A large lake, a state boundary, or the region affected by a flu epidemic, for example, can be represented, and its area and perimeter measured, by polygons.

These geometries work in conjunction with other datasets that describe more detailed features of the objects being displayed. For example, polygon areas on a map that represent lakes could be color coded, with each color assigned to a different level of depth, pollution, salinity, or any other piece of information to be conveyed on a GIS display.

GISs can take all of this information and categorize it in different ways to help people find information. For example, a GIS can help a user find the location of a point of interest, such as a restaurant, in relation to another point of interest, such as the user’s house.

Tags: gis map data, gis mapping, gis maps, raster, vector mapping

This entry was posted on Saturday, March 7th, 2009 at 4:03 pm and is filed under 3D Maps, GIS. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.