Raster Data Model
The raster data model is used in GIS to represent continuous data over space.Raster data are divided into rows and columns, which form a regular grid structure. This grid structure has individual elements commonly called cells or pixels. The most common example of raster data is a digital photograph which store an array of color intensity values. In general, raster data in a GIS can hold any attribute values. Continuous numeric values, such as elevation, and continuous categories, such as vegetation types, are represented using the raster model.
Storage size is of chief concern with the use of raster data. Because raster data store large arrays of values, they take more digital storage space than vector data. This factor has resulted in the invention of many compression algorithms and file types. The most common file types for GIS raster data are JPEG, GIF, MrSID, and TIFF (see http://en.wikipedia.org/wiki/GIS_file_formats for others). Each file type is compressed, or made to take up less digital storage space in different ways. It is up to the user of the data to determine which file type/compression algorithm is best for their project.
Cell size, or resolution, is the other main concern when working with raster data. The cell size determines the smallest unit or object that can be identified. The process of choosing a size and value for each cell is called sampling. For example, when working with raster data of an urban scene that has a cell size of 30 m x 30 m, individual cars on streets would not be distinguishable. The general rule for raster resolution is: use cell sizes of half of the smallest dimension of the smallest object that you want to identify.
- Longley, Paul A. et al. Geographic Information Systems and Science
|Authors||Brian Bunker, Dantley Frehner|