Unfortunately, the results of overlay in these early systems was often prone to error. It used the Point in polygon algorithm to find intersections quickly. Another early vector GIS, the Polygon Information Overlay System (PIOS), developed by ESRI for San Diego County, California in 1971, also supported polygon overlay. The first true GIS, the Canada Geographic Information System (CGIS), developed during the 1960s and completed in 1971, was based on a rudimentary vector data model, and one of the earliest functions was polygon overlay. Ian McHarg was perhaps most responsible for widely publicizing this approach to planning in Design with Nature (1969), in which he gave several examples of projects on which he had consulted, such as transportation planning and land conservation. Jacqueline Tyrwhitt published instructions for the technique in an English textbook in 1950, including: Īs far as possible maps should be drawn on transparent paper, so that when completed the maps to the same scale can be ‘sieved’-i.e., placed one on top of another in turn so that correlations or their absence can be noted. Warren Manning appears to have used this approach to compare aspects of Billerica, Massachusetts, although his published accounts only reproduce the maps without explaining the technique. The technique was largely developed by landscape architects. Prior to the advent of GIS, the overlay principle had developed as a method of literally superimposing different thematic maps (typically an isarithmic map or a chorochromatic map) drawn on transparent film (e.g., cellulose acetate) to see the interactions and find locations with specific combinations of characteristics. A range of specific operators allows for different types of input, and different choices in what to include in the output. The basic approach of a vector overlay operation is to take in two or more layers composed of vector shapes, and output a layer consisting of new shapes created from the topological relationships discovered between the input shapes. It is also based on the mathematics of set theory and point-set topology. Overlay is based on the fundamental principle of geography known as areal integration, in which different topics (say, climate, topography, and agriculture) can be directly compared based on a common location. Some overlay operations, especially Intersect and Union, are implemented in all GIS software and are used in a wide variety of analytical applications, while others are less common. Overlay has been one of the core elements of spatial analysis in GIS since its early development. Terms such as polygon overlay, map overlay, and topological overlay are often used synonymously, although they are not identical in the range of operations they include. Vector overlay is an operation (or class of operations) in a geographic information system (GIS) for integrating two or more vector spatial data sets.
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