GUS: 0265 Applications in GIS Lecture Presentation 1: Vector Data Model and Operations Jeremy Mennis Department of Geography and Urban Studies Temple University

Vector Data Models Geometric Primitives: Points, lines, polygons Two Primary Types: Spaghetti Topologic (and TIN)

Spaghetti Vector Data Model Each point, line, or polygon is stored as a record in a file that consists of that entity’s ID and a list of coordinates that define geometry. IDCoordinates 13,4 25,5 1 2 For Points:

Spaghetti Vector Data Model Each point, line, or polygon is stored as a record in a file that consists of that entity’s ID and a list of coordinates that define geometry. IDCoordinates 1(0,1), (3,4), (5,6) 2(3,1), (5,2), (4,3) 1 2 For Lines:

Spaghetti Vector Data Model Each point, line, or polygon is stored as a record in a file that consists of that entity’s ID and a list of coordinates that define geometry. IDCoordinates 1(2,4), (4,3), (3,6), (2,4), 2(3,1), (5,2), (4,3), (3,2), (3,1) 1 2 For Polygons:

ESRI Shapefile Designed by ESRI for ArcView Implementation of the spaghetti vector model An individual layer stores a single type of geometry (i.e. point, line, polygon) No topology (but it can be calculated on the fly...) Draws relatively fast ‘Open’ file format

ESRI Shapefile Three primary files in a shapefile:.shp,.shx, and.dbf All files must share the same prefix for one shapefile, e.g. road.shp, road.shx, and road.dbf.shp : stores the feature geometry (binary).shx : index for.shp file.dbf : attribute data stored in dBASE format

Spaghetti Vector Data Model Advantages simple efficient for display and plotting Disadvantages inefficient for most types of spatial analysis

Vector Topologic Data Model Composed of points, lines, and polygons Node: a point at the intersection of three or more lines In addition to coordinate locations, the topologic relationships among geometric features are explicitly recorded

A B C a1a2 a3 a4 n1 n2 ID Arcs A a1, a2 B a2, a4 C a3, a4 Polygon Topology NodeArcs n1a4, a2, a1, a3 n2a2, a4, a3, a1 Node Topology Arc Start End Left Right a1 n1 n2 A a2 n1 n2 A B a3 n1 n2 C a4 n2 n1 C B Arc Topology ArcStartXYIntermediateXYEndXY a14,5(4,8), (8,8), (8,1), (4,1)4,3 a24,5(6,7), (6,3)4,3 a34,5(1,3)4,3 a44,34,5 Arc Coordinate Data Vector Topologic Data Model

Planar Enforcement: No two individual features can overlap. There are no ‘holes’ or ‘íslands’ that are not themselves features. Every feature is represented as a record in the attribute table.

Vector Topologic vs. Spaghetti Spaghetti: can encode as 2 or 3 polygons (and have 2 or 3 records in the attribute table) Topologic: must be encoded as 3 polygons (and have 3 records in the attribute table)

ESRI Coverage Designed by ESRI for ArcInfo Implementation of the vector topologic data model ‘Closed’ file format Each coverage is a directory, with numerous files that store feature geometry, projection, registration, etc. Attribute data is stored in a separate INFO directory, which stores all attribute data for all coverages in its parent directory.

Triangulated Irregular Network (TIN)

Vector Operations Classification Buffer Overlay

Classification Line Dissolve (Map Dissolve) 1 grain crops 2 orchards 3 residential 4 commercial 1 agricultural 2 non-agricultural

Buffer Definition of what is within/without a given proximity Point buffer Line buffer Polygon buffer

Overlay An overlay operation takes two or more data layers as input and results in an output data layer Three types of overlay: Point in polygon Line in polygon Polygon (polygon on polygon)

Point in Polygon Overlay A B C ID Tree A Elm B Maple C Elm Point Table ID Tree Cover A Elm Rural B Maple Rural C Elm Urban Point Table ID Cover 1 Rural 2 Urban Poly Table 12 + A B C= Land CoverTreesNewTrees

Line in Polygon Overlay A B C ID Street A Race B Race C Arch Line Table ID Street Cover A Race Rural B Race Urban C Arch Urban D Race Urban Line Table ID Cover 1 Rural 2 Urban Poly Table 12 += Land CoverStreetsNewStreets A B C D

Polygon Overlay Intersection (and)Union (or) Identity

Polygon Overlay: Intersection Agriculture A B A Land Cover ID Owner A Brown B Smith ID Cover A commercial B industrial B Area of intersection New node

Polygon Overlay: Union Agriculture A B A ID Owner A Brown B Smith ID Cover A commercial B industrial B Area of union New node Land Cover

Polygon Overlay: Identity Agriculture (input layer) A B A Land Cover (identity layer) ID Owner A Brown B Smith ID Cover A commercial B industrial B Area of identity New node