7/26/2023 0 Comments Define raster approachSecond, a cell can hold any alphanumeric index that represents an attribute. Often, an arbitrary, readily identifiable value (e.g., −9999) will be assigned to pixels for which there is no data value. Furthermore, if no data are present for a given pixel, a data value placeholder must be assigned to this grid cell. First, each pixel must hold at least one value, even if that data value is zero. Imagery employing the raster data model must exhibit several properties. Instead, the pole would be assumed to be located somewhere within that cell (typically at the center). For example, if the location of telephone poles was mapped on a coarse raster graphic, it would be clear that the pole would not fill the entire cell. In the case of raster graphics with coarse spatial resolution, the data values associated with specific locations are not necessarily explicit in the raster data model. An effective pixel resolution will take both the map scale and the minimum mapping unit of the other GIS data into consideration. Care must be taken when determining the resolution of a raster because using an overly coarse pixel resolution will cause a loss of information, whereas using overly fine pixel resolution will result in significant increases in file size and computer processing requirements during display and/or analysis. A raster model with pixels measuring 1×1 kilometer (1 square kilometer) in the real world would be said to have a spatial resolution of 1 kilometer. A raster model with pixels representing 10×10 meters (or 100 square meters) in the real world has a spatial resolution of 10 meters. Specifically, resolution is determined by measuring one side of the square pixel. The area covered by each pixel determines the spatial resolution of the raster model from which it is derived. Therefore, the more area covered per pixel, the less accurate the associated data values. The raster model will average all values within a given pixel to yield a single value. The advance of computer technology has made this second methodology increasingly feasible as computer storage issues no longer constrain large datasets as they were previously. Alternatively, the raster graphic can reference a database management system wherein open-ended attribute tables can be used to associate multiple data values to each pixel. The data type for that cell value can be either integer or floating-point. Each cell in a raster carries a single value, which represents the characteristic of the spatial phenomenon at a location denoted by its row and column. Typically, a single data value will be assigned to each grid locale. These squares are typically reformed into rectangles of various dimensions if the data model is transformed from one projection to another (e.g., from State Plane coordinates to UTM coordinates).īecause of the reliance on a uniform series of square pixels, the raster data model is referred to as a grid-based system. Accordingly, the vast majority of available raster GIS data are built on the square pixel. Although pixels may be triangles, hexagons, or even octagons, square pixels represent the simplest geometric form with which to work. These pixels are used as building blocks for creating points, lines, areas, networks, and surfaces. The raster data model consists of rows and columns of equally sized pixels interconnected to form a planar surface. If you are as generous as the author, you may indeed think of your raster dataset creations as sublime works of art. The neo-impressionist artist, Georges Seurat, developed a painting technique referred to as “pointillism” in the 1880s, which similarly relies on the amassing of small, monochromatic “dots” of ink that combine to form a larger image. Notably, the foundation of this technology predates computers and digital cameras by nearly a century. Each of these uniquely colored pixels, when viewed as a whole, combines to form a coherent image.įurthermore, all liquid crystal display (LCD) computer monitors are based on raster technology as they are composed of a set number of rows and columns of pixels. If you zoom deeply into the image, you will notice that it is composed of an array of tiny square pixels (or picture elements). Take a moment to view your favorite digital image. The ubiquitous JPEG, BMP, and TIFF file formats (among others) are based on the raster data model. Most likely, you are already very familiar with this data model if you have any experience with digital photographs. The raster data model is widely used in applications ranging far beyond geographic information systems (GIS).
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