Intro to GIS | Summer 2012

Spatial Data Models

DATA MODELS

Models & Representations

Why do we need models?

• The world is infinitely complex

• A computer is finite

• A map is finite

We can create digital representations of some

limited part of the earth’s surface

There is a simplified relationship between the digital

model and the feature being represented

Data Models

• Vector

• Raster

• Network

• TIN

VECTOR DATA

Vector

• Objects are defined by points, lines connecting the

points, or polygons bounded by lines

• Each point has x/y values (can also be a z component)

• Each object, or “Feature”, is a single row in the GIS

database

• “Attributes” are information associated with features;

each attribute is a column in the database

• Good way to represent Discrete objects, e.g. buildings

(“discrete” = constituting a separate, distinct entity)

Vector Data Terminology

• points: features defined by a single x/y coordinate

• vertices: points defining a line feature

• nodes: vertices representing the start and end of a line

feature (topologic vector model)

• polyline: feature comprised of a series of straight line

segments or mathematical curves

• polygon: feature represented by a “closed” series of

polylines (a.k.a. “areas”)

Reality

Representation

Vector Data Formats

• shapefiles

• geodatabase (enterprise, file-based, personal)

• coverages

• CAD files (dwg, dgn, dxf)

Types of Vector Data

• Simple

• Topological

• Object

Simple Data Model

Shapefiles

Collections of point,

line, or polygon

features.

They are simple vector

features.

Shapefiles

.shp - feature geometry

.dbf - attribute table

.shx - index file

.prj – projection

.sbx and .sbn – spatial index

.ain and .aih – attribute index

Topological Data

• Have explicit (i.e. unambiguous, defined)

spatial relationships

– Connectivity: Lines that cross or touch are

connected with each other

– Planar: Lines and polygons cannot overlap

– Contiguity: polygons that share a border

are contiguous and adjacent (i.e. don’t

overlap and are next to each other)

Topology

Coverages

Topological vector features

Stored in .e00 file format (aka “Interchange files” or ”Export files)

Shapefiles can be converted into coverages, but steps must be taken to “build”, or define, topology

Object Data Model

• A collection of geographic objects and relationships

between those objects.

• An “object” feature is comprised of geometry, attributes,

and behavior.

• Properties: define the object (e.g. “bike lane”) and its

representation (point/line/polygon), can vary by scale

• Methods: define the behavior of an object (i.e., a bike

lane is 4’ wide and must be within a right-of-way)

Vector Data Model • Advantages

– Geometry independent of data model

– Vector is “correcter” (i.e. more accurate)

– More intuitive

– Analytical capabilities

– Easy to edit

– Better cartography

– Include topology

• Disadvantages

– Complex features

– Time consuming to design

– Software dependent

RASTER DATA

Raster Data Model

• Contains an array of cells (a.k.a., pixels) that are

organized into columns and rows

• Each pixel is a single row in the GIS database

• Attributes are associated with each cell

• Used mostly for Continuous data, e.g. rainfall

• Can also represent discrete data

Examples of Raster Data

• ESRI Grid

• ESRI Geodatabase (file-based, enterprise)

• ERDAS Imagine (.img)

• MrSID (.sid)

• Other image formats (.tif, .jpg)

Raster Data Model

• Advantages

– Simple data structure

– Large # of common, software-independent

formats

– Faster processing times

• Disadvantages

– Large datasets

– Cartographic limitations

– Limited analytical capability

NETWORK DATA

Network Data Model

• A system of points and lines defining how flows move

through a network, e.g. streams or highways

• Linear Referencing: features are stored as distances

along a network (such a feature is aka an event)

TIN DATA

TIN Data Model

• Triangulated Irregular Network

• Created from a series of points with x/y/z coordinates (3D)

• Points are connected by lines to form triangles

• Used to represent continuous surfaces, e.g. elevation

• Best used when the density of data points varies

(i.e. irregular spacing of data points)

DATA SOURCES

Where to find spatial data

• Directly from federal agencies

• Directly from state agencies

• Directly from local governments (sporadic)

• Directly from non-profits

• Clearinghouses (central locations for distributing GIS data)

• Geolibraries

• Commercial sites

• www.christinafriedle.com/data-links.html

Interactive Maps: Portland TriMet

ride.trimet.org

maps.massgis.state.ma.us/map_ol/oliver.php

Interactive Maps: OLIVER – MassGIS’s online mapping tool

www.metro-region.org/index.cfm/go/by.web/id=24869

Static (Paper) Maps: Metro Maps

www.stateofthesalmon.org/resources/maps/

Static (Paper) Maps: State of the Salmon

www.civicapps.org/datasets

GIS Datasets: City of Portland CivicApps

GIS Datasets: State of the Salmon

www.stateofthesalmon.org/resources/sosdb.php

Non-Spatial Data: Nation Master

www.nationmaster.com

Non-Spatial Data: US Census Bureau

METADATA data about data

What’s in your GIS data?

• Nutrition labels are required on

packaged food – they tell you everything

you need to know to make an informed

decision about whether or not you’d like

to consume the food

• Metadata tells you whether the GIS data

is junk food, or a healthy nutritious snack

• Nutrition labels do not tell you how to

prepare the food – Likewise, metadata

does not tell you how to use the GIS data

What can Metadata tell me?

• Name of dataset

• Spatial extent

• Coordinate system

• Attribute descriptions

• How and When the dataset was created

• Who to contact for further information

Metadata – Summary Display

Metadata – Detailed Display

Accessing Metadata

• ArcCatalog: “Description” tab

How to change style of displayed metadata?

Customize > Options > Metadata > Metadata Style

• ArcMap: Open ArcCatalog window, right-click on

layer name, select “Item Description…”

• README.txt file that often arrives with

downloaded GIS data