Other grid-based distances have been developed based on the random walk or drunkard's walk, in which route-finding is a stochastic process. Since least-cost distance is affected by the environment, grid-based calculations are necessary. The least-cost distance is implemented in most GIS software and mimics route finding 'as the wolf runs', taking into account obstacles and the local `friction' of the landscape. Other distance measures also imply a route across geographic space. In common language, this is referred to as distance `as the crow flies'. The great-circle distance could be conceived of as the distance measured along a route of a very efficient traveller who knows where to go and has no obstacles to deal with. Implicit in this distance measure is a route. This distance represents the shortest line between two points, taking into account the curvature of the earth. For example, take the great-circle distance, the most commonly used geographic distance measure. Distance is fundamental to geospatial analysis It is closely related to the concept of route. This vignette describes gdistance, a package written for use in the R environment It provides functionality to calculate various distance measures and routes in heterogeneous geographic spaces represented as grids. Introduction: the crow, the wolf, and the drunkard “R Package gdistance: Distances and Routes on Geographical Grids.” Journal of Statistical Software 76 (13): 1–21. This vignette was published as an article in the Journal of Statistical Software: The novel distances implemented in the package are used in geographical genetics (applying circuit theory), but may also have applications in other fields of geospatial analysis. These classes make it possible to manipulate the values of cell-to-cell movement directly, which offers flexibility and the possibility to use asymmetric values. The package implements classes to store the data about the probability or cost of transitioning from one cell to another on a grid in a memory-efficient sparse format. Also the corresponding routes or probabilities of passing each cell can be determined. Least-cost distances as well as more complex distances based on (constrained) random walks can be calculated. The R package gdistance provides classes and functions to calculate various distance measures and routes in heterogeneous geographic spaces represented as grids.
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