Rasters are most commonly used to represent continuous data, as they allow for more efficient storage of values than an equivalent vector or point-based lattice system at the point densities generally required. In contrast, pixels could represent the value at the grid’s exact center this is sometimes referred to as a grid (Briese, 2010) or lattice type raster (Wise, 2000). In the most common case, when pixels represent a 2D area, pixels may be thought of as a “bin”, where the pixel’s value is a summary statistic (e.g., mean, median, standard deviation) of all values of the field within the bounds of the pixel. A digital elevation model (DEM) at 30 m (left), 10 m (center), and 3.3 m resolution (right). Figure 2 illustrates the effect of increased resolution on a digital elevation model (DEM).įigure 2. The number of cells in a raster increases quadratically as resolution increases a doubling of the resolution of a raster reduces the linear distance of a cell by one-half (e.g., moving from a 2 m cell to a 1 m cell), but increases the number of cells by a factor of four (doubling the number of cells in two directions). The resolution of the raster is one component that dictates the memory storage requirements for it, with finer resolutions requiring more space in memory. The size or extent of each cell indicates the resolution of the raster, and is given in linear units of distance (e.g., number of feet, meters, kilometers along one side of the cell) or in degrees or fractions of degrees of latitude and longitude (e.g., one arc second, or one-third arc second). Tessellations of triangles, squares, and hexagons that can be used as the basis for cell shape in a raster model. Cells of the raster are most often square, but may be rectangular (with differing resolutions in x and y directions) or other shapes that can be tessellated such as triangles and hexagons (Figure 1 Peuquet, 1984).įigure 1. The raster is sometimes referred to as an image, array, surface, matrix, or lattice (Wise, 2000). Digital rasters most often take the form of a regularly spaced, grid-like pattern of rows and columns, with each element referred to as a cell, pixel, or grid point. Its origin as a description for images comes from the drawing performed by electron beams on cathode ray tube (CRT) screens in the early days of analog television, and the metaphor was subsequently extended to digital images as well. The term raster originated from the German word for screen, implying a series of orthogonally oriented parallel lines. It is typically used to record, analyze and visualize data with a continuous nature such as elevation, temperature, or reflected or emitted electromagnetic radiation. The raster data model, along with the vector data model, is one of the earliest and most widely used data models within geographic information systems (Tomlin, 1990 Goodchild, 1992, Maguire, 1992). In raster sensor arrays, resolution is defined by the dimensions of the individual sensors in terms of ground units (i.e., the width of one pixel in meters on the Earth). Data are stored and rendered at some degree of representation resolution. Resolution: The degree of detail to which a phenomenon is detected or represented. Sometimes referred to as a cell or grid point. Pixel: A portmanteau of “picture element”, the smallest unit of a raster. Mixed pixel: A condition whereby more than one category of object is present within a single grid cell. In cartography and GIS, the extent of a representation is the size of the real space being represented.įile format: The specification for how data is stored a computer file. Important distinctions include those between binary and plaintext approaches, and between proprietary and free and open formats. Advantages and Disadvantages of the Raster Data ModelĢ.5D: A system of recording values on a raster in which each grid cell has one and only one z-value.Ĭontinuous data: Field-like data in which values are present at any point within the spatial extent, such as elevation or temperature.ĭigital elevation model (DEM): A data model used to process, store, analyze, and display elevation data.ĭigital surface model (DSM): A type of DEM that represents a maximum value within a grid cell, thereby recording the tops of buildings, trees, and other objects.ĭigital terrain model (DTM): A type of DEM that aims to represent an idealized land surface where surface objects (buildings, trees, etc.) have been digitally removed.ĭiscrete data: Object-like data, in which the spatial extent or boundaries of the features are definable.Įxtent: The area or distance in real space over which some geographic entity exists.
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