The data type for that cell value can be either integer or floating-point (Section 5.1). 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. Geological Survey, Earth Resources Observation and Science (EROS) Center, Sioux Falls, SD.īecause of the reliance on a uniform series of square pixels, the raster data model is referred to as a grid-based system. Figure 4.3 Common Raster Graphics Used in GIS Applications: Aerial Photograph (left) and USGS DEM (right) 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). Accordingly, the vast majority of available raster GIS data are built on the square pixel (Figure 4.3). 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 (Figure 2.6 illustrates how a land parcel can be converted to a raster representation). 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 neoimpressionist 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 (Figure 4.2). Notably, the foundation of this technology predates computers and digital cameras by nearly a century. Figure 4.1 Digital Picture with Zoomed Inset Showing Pixilation of Raster Imageįurthermore, 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. The ubiquitous JPEG, BMP, and TIFF file formats (among others) are based on the raster data model (see Figure 4.1). 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 (GISs). The objective of this section is to understand how raster data models are implemented in GIS applications.
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