Raster and Vector Data

GIS data models Raster model Vector model

Raster types Thematic rasters Image rasters Contain quantities that represent map data such as land use or rainfall May be continuous or discrete Image rasters Contain satellite or air photo data

Thematic raster types Discrete data Continuous data relatively few values Values may change abruptly at boundaries Continuous data Thousands or millions of potential values Values may change from cell to cell

Impact of resolution 90m resolution Storage space increases by the square of the resolution Portraying large areas at high precision is problematic 10m resolution

Vector model Polygon Vertices Line Points Nodes Y X Features are stored as a series of x-y coordinates in a rectangular coordinate system.

Features linked to data Each feature is linked to an entry in a data table containing information about the feature.

Advantages of vectors Precise location of features Storing many attributes Flexible for cartography Compact storage of information Ideally suited for certain types of analysis, especially areas, lengths, connections

Storage of attributes Roads may have other attributes: ownership, speed limit, number of lanes, etc. Would need a new raster for each attribute. Only numeric attributes may be stored Raster contains 1 value indicating a single attribute road type for example

Converting Vector to Raster Data is Easy

Converting Raster Data to Vector is Hard Nearly Impossible

Storing data in ArcGIS

In the vector world everything is a point, a line, or a polygon Lines Points Polygons

Feature Classes A feature class is a collection of similar objects with the same attributes, stored as a single unit. Stored as spatial features with a table of associated attributes for each feature. Because all features must share the same table, all must have the same attributes. Feature classes may contain only one type of geometry (points or lines or polygons). States feature class Rivers feature class 14 Capitals feature class

Data formats Coverages Shapefiles Geodatabases

Creating Shapefiles To create a new shapefile: open ArcCatalog, right-click on a folder, click on New/Shapefile… in the drop down menu, enter the Name for the new shapefile, and then select the Feature type to be inserted. Optionally, you can also select the Spatial reference. To add features to the shapefile, add the shapefile to an ArcMap document.

Shapefiles A shapefile consists of multiple files, and the common ones are *.dbf, *. shp, *.shx, *.prj, *.avl, *.sbn Shapefiles in Windows Explorer Shapefiles in ArcCatalog

Shapefiles have a few parts: "essential": .shp: Shapefile geometry data .dbf: database data .shx: shapefile index (internal). "important": .prj: projection file with coordinate reference system info. "other": .sbn: shapefile (internal) .sbx: shapefile (internal) .avl: ArcView legend file .rtf: data dictionary (rich text document)