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GIS data represents real world objects (roads, landuse, elevation) with digital data. Real world objects can be divided into two abstractions: discrete objects (a house) and continuous fields (rain fall amount or elevation). Traditionally, there are two broad methods used to store data in a GIS for both abstractions: Raster and Vector.
A raster data type is, in essence, any type of digital image represented in grids. Raster data type consists of rows and columns of cells, with each cell storing a single value. Raster data can be images (raster images) with each pixel (or cell) containing a color value. Raster data is stored in various formats, from a standard file-based of TIF, JPEG, etc.
In a GIS, geographical features are often expressed as vectors, by considering those features as geometrical shapes. Different geographical features are expressed by different types of geometry:
a) points - Zero-dimensional point are used for geographical features that can best be expressed by a single pint refrence; in other words, simple location. For example, the locations of wells, peak elevations, features of interest or trailheads. Points convey the least amount of information of these file types. Point can also be used to represent areas when displayed at a small scale. For example, citites on a map of the world would be represented by points rather than polygons. No measurements are possible with point features.
b) lines - One-dimensional lines or polylines used for linear features such as rivers, roads, railroads and topographic lines. Again, as with point features, linear features displayed at a small scale will be represented as linear features rather than as a polygon. Line features can measure distance.
c) polygon - Two-dimensional polygons are used for geographical features that cover a particular area of the earth´s surface. Such features may include lakes, park boundaries, buildings, city boundaries, or land uses. Polygons convey the most amount of information of the file types. Polygon features can measure perimeter and area.
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