Spatial Analysis – vector data analysis Topic 2 Starting 2/1/2007 Recap: three data models Vector Raster Geodatabase (object-oriented data model) • Vector data and table • feature class is the basis • Raster data The real world is complex. Either discrete (object) or continuous (field) can not efficiently represent some of real world situations. We need combined model: dual, hybrid, or object-oriented approach Spatial Analysis tools in ArcToolBox Shapefile & Feature class/table Coverage Raster Details Shapefile and feature class/table Coverage Raster Spatial Analyst extension 1. Extract To create a new subset from the input (shapefile, features and attributes in a feature class or table) based on spatial intersection or an attribute query. • Clip • Select • Split • Table select only Clip ff Select Split 2. Overlay Joining two existing sets of features into a single set of features to identify spatial relationships between the input features. • Erase • Identify • Intersect • Symmetrical difference • Union • Updata 3. Proximity Identify features that are closest to one another, calculate the distances around them, and calculate distances between them. • Buffer • Multiple ring buffer • Near • Point distance Not dissolved Dissolved Example Some specials for coverage How to form Thiessen polygons Also known as 'Voronoi networks' and 'Delaunay triangulations', Thiessen polygons were independently discovered in several fields of study, including climatology and geography. They are named after a climatologist who used them to perform a transformation from point climate stations to watersheds. Thiessen polygons can be used to describe the area of influence of a point in a set of points. If you take a set of points and connect each point to its nearest neighbor, you have what's called a triangulated irregular network (TIN). If you bisect each connecting line segment perpendicularly and create closed polygons with the perpendicular bisectors, the result will be a set of Thiessen polygons. The area contained in each polygon is closer to the point on which the polygon is based than to any other point in the dataset.