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