Topic: Data Models

Data Model: A consistent way of defining and representing real world entities or phenomena in a GIS. Two Primary Types of Data Models: –Vector (points, lines and polygons) –Raster (Pixels or grid cells)

Real World Raster Representation Vector Representation line polygon point Which is better?

Parcels  Vector

Elevation  Raster DEM

Layer (Generic Definition): A spatial dataset containing a common feature type. Raster Layers Vector Layers Layers and Themes

Types of vector data Points Polylines Polygons

One pair of coordinates defines the location of a point feature.

Two or more pairs of coordinates that are connected define a line feature.

Multiple pairs of coordinates that are connected and closed* define a polygon feature. * The beginning and ending coordinates for a polygon are the same.

Types of vector data Points –Individual X,Y Polylines (LineStrings) –A series of connected points –Actually: A set of series of connected points Polygons –A series of connected points that loop back to the first point –Actually: Multiple “polygons” Can have internal polygons or “holes”

Definitions Features: Something we can reference spatially Arcata California This building This room The pacific ocean General Sherman (the tree) Your house You! Themes: A type of feature Cities States Buildings Rooms Oceans Sequoias Houses People