GIS Data Models
Geographic information
 Characteristics of Geographic Information
Location!
volume
Dimensionality
Point
Line
Area
Continuity
Feature
field
Building complex features
Simple geographic features can be used
to build more complex ones.
Areas are made up of lines which are
made up of points represented by their
coordinates.
Areas = {Lines} = {Points}
Properties of Features
size
distribution
pattern
contiguity
neighborhood
shape
scale
orientation.
Basic
properties
of
geographic
features
GIS Analysis
Much of GIS analysis and description
consists of investigating the properties of
geographic features and determining the
relationships between them.
GIS Capability
A GIS package should be able to move
between
map projections,
coordinate systems,
datums, and
ellipsoids.
Draw a map of your
favorite place in Texas.
GIS Data Models
Maps as Numbers
GIS requires that both data and maps be
represented as numbers.
The GIS places data into the computer’s
memory in a physical data structure (i.e. files
and directories).
Files can be written in binary or as ASCII text.
Binary is faster to read and smaller, ASCII can
be read by humans and edited but uses more
space.
The Data Model
A logical data model is how data are
organized for use by the GIS.
GISs have traditionally used either raster
or vector for maps.
Two approaches to handling
spatial data with GIS:
Raster model
Vector model
Points,
lines,
polygons
Features and Maps
A GIS map is a scaled-down digital
representation of point, line, area, and
volume features.
While most GIS systems can handle raster
and vector, only one is used for the
internal organization of spatial data.
Rasters and vectors can be
flat files … if they are simple
Vector-based line
Raster-based line
Flat File
4753456 623412
4753436 623424
4753462 623478
4753432 623482
4753405 623429
4753401 623508
4753462 623555
4753398 623634
Flat File
0000000000000000
0001100000100000
1010100001010000
1100100001010000
0000100010001000
0000100010000100
0001000100000010
0010000100000001
0111001000000001
0000111000000000
0000000000000000
A raster data model uses a
grid.
 One grid cell is one unit or holds one attribute.
 Every cell has a value, even if it is “missing.”
 A cell can hold a number or an index value standing for
an attribute.
 A cell has a resolution, given as the cell size in ground
units.
Raster GIS
Raster Data Model
Rows and Columns of Cells (Array)
Area of Cell equals Spatial Resolution
Value for each cell records type of object or condition
Cells do not correspond to spatial entities in real
world
 A road is a group of cells, not a single entity
Cells are considered Homogeneous Units
Two approaches to handling
spatial data with GIS:
Raster model
Vector model
Points,
lines,
polygons
Generic structure for a
grid
Grid extent
Rows
Grid
cell
Resolution
Columns
Figure 3.1 Generic structure for a grid.
Definitions
Raster - A format for storing, processing, and
displaying graphic data in which graphic images
are stored as values for uniform grid cells or
pixels.
Pixels - Abbreviation for picture element, the
smallest indivisible element that makes up an
image. In raster processing, data is represented
spatially on a matrix of grid cells, called pixels,
which are assigned values for image
characteristics or attributes.
More Definitions
Resolution - A measure of the accuracy or detail
of a graphic display, expressed as dots per inch,
pixels per line, lines per millimeter, etc.
Spatial Resolution - The accuracy associated
with the capture of ground information as
reproduced in a digital format or graphic display.
For example, 10-foot pixels vs. 100-foot pixels.
Definitions
Minimum Mapping Unit - The smallest
element we can uniquely represent in our
data.
Sources of Raster Data
Satellite data
LANDSAT
SPOT
Scanned aerial photography
Digital Orthophotography
Scanned maps and documents
From where do we get
Raster Data?
SCANNED Aerial photographs
photographs are NOT raster images but
SCANNED images ARE
SCANNED maps
Satellite images
From where are the data in
a raster cell taken?
Why does it matter where
the cell data come from?
It’s hard to tell just by looking at the
image!
The mixed pixel problem
W ate r d o m in ate s
W in n er ta ke s a ll
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Grids and missing data
Figure 3.8 GIS data layer as a grid with a large section of “missing data,” in this
case, the zeros in the ocean off of New York and New Jersey.
Why use Raster?
Overlay Analysis/Overlay Operations
Arithmetic Operations
Addition
Subtraction
Division
Multiplication
Logical (Boolean) Operations
Where conditions occur or do not occur together
AND, OR, NOT, GT, LT, etc.
Raster GIS Applications
Integrate images to georeferenced data
i.e., parcel deed image linked to parcel
centroid
Document Imaging
Natural Resource applications where:
Positional accuracy relaxed
Imagery-oriented
Raster Applications
Utility Corridor Siting
Environmental Mapping
Natural Communities Mapping
Forest resource planning
Spatial data variability decisions
Forest inventory
Wildlife habitat analysis
More Raster Applications
Wetlands Vegetation Inventory & Analysis
Agricultural analysis
Planetary analysis (including lunar)
Vector Updating
Digital Terrain Modeling
Flood Control & Emergency Preparedness
Communication System Engineering
Any Technology has Pro’s & Con’s
Raster Limitations
Aesthetics
Data storage requirements
Overlay operations performed on every
cell
Sparse data sets require as much
processing as dense ones
RASTER -- summary
 A grid or raster maps directly onto a programming
computer memory structure called an array.
 Grids are poor at representing points, lines and areas,
but good at surfaces.
 Grids are good only at very localized topology, and weak
otherwise.
 Grids are a natural for scanned or remotely sensed data.
 Grids suffer from the mixed pixel problem.
 Grids must often include redundant or missing data.
 Grid compression techniques used in GIS are run-length
encoding and quad trees.
GIS Data Models
Rasters are faster, but...
 Points and lines in raster format have to move to a cell
center.
 Lines can become fat. Areas may need separately coded
edges.
 Each cell can be owned by only one feature.
 As data, all cells must be able to hold any cell value.
 It is very difficult to precisely position features in
space.
Vector GIS Data Model
Precisely position features in space
Points, Nodes, vertex, single X,Y coordinate
pair
Lines, Arcs, series of X,Y coordinate pairs
Area, Polygons, area as a closed loop of X,Y
coordinate pairs
Areas are lines are points are
coordinates
The Vector Model
 A vector data model uses points stored by their real (earth)
coordinates and so requires a precise coordinate system.
Geographic Coordinate System
• Latitude/Longitude
Cartesian Coordinate Systems
• X,Y Coordinate system
• State Plane
• UTM (Universal Transverse Mercator)
 Lines and areas are built from sequences of points in order.
 Lines have a direction to the ordering of the points.
 Polygons can be built from points or lines.
 Vectors can store information about topology.
Raster/Vector Comparison
VECTOR
 At first, GISs used vector data and cartographic
spaghetti structures.
Collection of coordinate strings with no structure
Cartesian coordinates stored in data structure
No spatial relationships stored
Inefficient data storage technique
 Vector data evolved the arc/node model in the 1960s.
 In the arc/node model, an area consist of lines and a
line consists of points.
 Points, lines, and areas can each be stored in their own
files, with links between them.
13
11
2
12
10
7
POLYGON “A” 5
9
4
2
1
6
3
8
1
1xy
2xy
3xy
4xy
5xy
6xy
7xy
8xy
9xy
10 x y
11 x y
12 x y
13 x y
Points File
Arc/node map data structure with
files
File of Arcs by Polygon
A: 1,2, Area, Attributes
1 1,2,3,4,5,6,7
2 1,8,9,10,11,12,13,7
Arcs File
Figure 3.4 Arc/Node Map Data Structure with Files.
 The topological vector model uses the line (arc) as a
basic unit. Areas (polygons) are built up from arcs.
 The endpoint of a line (arc) is called a node. Arc
junctions are only at nodes.
 Stored with the arc is the topology (i.e. the connecting
arcs and left and right polygons).
Vectors
 TIN must be used to represent volumes.
 Vector can represent point, line, and area features very
accurately.
 Vectors work well with pen and light-plotting devices
and tablet digitizers.
 Vectors are not good at continuous coverages or plotters
that fill areas.
Topological Model
Topology: mathematical method to define
spatial relationships
Arc-node data model
Arc: a series of points that start and end at a
node
Node: an intersection point where two or
more arcs meet
Topological Data Spatial
Operations
Contiguity: spatial relationship of
adjacency
i.e., stand of coniferous trees adjacent to
deciduous trees
Connectivity: interconnected pathways or
networks
i.e., street and trail networks, stream
networks
Basic arc topology
3
n2
2
A
1
n1
B
Topological Arcs File
Arc
1
From To PL PR n1x n1y n2x n2y
n1 n2 A B x y
x y
Figure 3.5 A topological structure for the arcs.
TOPOLOGY
 Topological data structures dominate GIS software.
 Topology allows automated error detection and
elimination.
 Rarely are maps topologically clean when digitized or
imported.
 A GIS has to be able to build topology from
unconnected arcs.
 Nodes that are close together are snapped.
 Slivers due to double digitizing and overlay are
eliminated.
Slivers
Sliver
Unsnapped node
Topology Matters
The tolerances controlling snapping,
elimination, and merging must be
considered carefully, because they can
move features.
Complete topology makes map overlay
feasible.
Topology allows many GIS operations to
be done without accessing the point files.
Vectors and 3D
Volumes (surfaces) are structured with the
Triangulated Irregular Network model, including
edge or triangle topology.
TINs use an optimal Delaunay triangulation of a
set of irregularly distributed points.
 TINs are popular in CAD and surveying
packages.
TIN: Triangulated Irregular
Network
Way to handle field
data with the vector
data structure.
Common in some
GISs and most
AM/FM packages.
More efficient than a
grid.
Sources of Vector Data
RASTER-VECTOR conversions from
scanned images
Pre-existing digital data from disks or
internet
DIGITIZING
Vector to raster to vector
conversion
Comparison: Raster and
Vector
FORMATS
Most GIS systems can import different
data formats, or use utility programs to
convert them.
Data formats can be industry standard,
commonly accepted or standard.
Vector Data Formats
Vector formats are either page definition
languages or preserve ground
coordinates.
Page languages are HPGL, PostScript, and
Autocad DXF.
True vector GIS data formats are DLG and
TIGER, which has topology.
Raster Data Formats
Most raster formats are digital image
formats.
Most GISs accept TIF, GIF, JPEG or
encapsulated PostScript, which are not
georeferenced.
DEMs are true raster data formats.
EXCHANGE
 Most GISs use many formats and one data structure.
 If a GIS supports many data structures, changing structures
becomes the user’s responsibility.
 Changing vector to raster is easy; raster to vector is hard.
 Data also are often exchanged or transferred between different GIS
packages and computer systems.
 The history of GIS data exchange is chaotic and has been wasteful.
Vector to raster exchange
errors
Transfer Standards
GIS Data Exchange
 Data exchange by translation (export and import) can lead to significant
errors in attributes and in geometry.
 In the United States, the SDTS was evolved to facilitate data transfer.
 SDTS became a federal standard (FIPS 173) in 1992.
 SDTS contains a terminology, a set of references, a list of features, a
transfer mechanism, and an accuracy standard.
 Both DLG and TIGER data are available in SDTS format.
 Other standards efforts are DIGEST, DX-90, the Tri-Service Spatial Data
Standards, and many other international standards.
 Efficient data exchange is important for the future of GIS.
Attribute data
 Attribute data are stored logically in flat files.
 A flat file is a matrix of numbers and values stored in
rows and columns, like a spreadsheet.
 Both logical and physical data models have evolved over
time.
 DBMSs use many different methods to store and
manage flat files in physical files.
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GIS Data Models - University of Texas at San Antonio