gis data models.ppt
TRANSCRIPT
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GIS Data Models
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Geographic information
Characteristics of Geographic Information Location! volume Dimensionality
PointLineArea
ContinuityFeaturefield
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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}
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Properties of Features
size distributionpatterncontiguityneighborhoodshapescaleorientation.
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Basic properties of geographic features
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GIS Analysis
Much of GIS analysis and description consists of investigating the properties of geographic features and determining the relationships between them.
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GIS Capability
A GIS package should be able to move between map projections, coordinate systems, datums, and ellipsoids.
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Draw a map of your favorite place in Texas.
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GIS Data Models
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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.
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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.
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Two approaches to handling spatial data with GIS:
Raster model Vector model
Points, lines, polygons
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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.
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Rasters and vectors can be flat files … if they are simple
Vector-based line
Raster-based line
4753456 6234124753436 6234244753462 6234784753432 6234824753405 6234294753401 6235084753462 6235554753398 623634
00000000000000000001100000100000101010000101000011001000010100000000100010001000000010001000010000010001000000100010000100000001011100100000000100001110000000000000000000000000
Flat File
Flat File
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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.
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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
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Two approaches to handling spatial data with GIS:
Raster model Vector model
Points, lines, polygons
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Generic structure for a grid
Figure 3.1 Generic structure for a grid.
Row
s
Columns
Gridcell
Grid extent
Resolution
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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.
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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.
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Definitions
Minimum Mapping Unit - The smallest element we can uniquely represent in our data.
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Sources of Raster Data
Satellite data LANDSAT SPOT
Scanned aerial photography Digital Orthophotography Scanned maps and documents
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From where do we get Raster Data?
SCANNED Aerial photographs photographs are NOT raster images but
SCANNED images ARESCANNED mapsSatellite images
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From where are the data in a raster cell taken?
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Why does it matter where the cell data come from?
It’s hard to tell just by looking at the image!
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The mixed pixel problem
W GW
W W G
W W G
W GG
W W G
W G G
W GE
W E G
E E G
Water dominates Winner takes all Edges separate
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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 thiscase, the zeros in the ocean off of New York and New Jersey.
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Why use Raster?Overlay Analysis/Overlay OperationsArithmetic Operations
Addition Subtraction Division Multiplication
Logical (Boolean) Operations Where conditions occur or do not occur
together AND, OR, NOT, GT, LT, etc.
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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
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Raster Applications
Utility Corridor Siting Environmental Mapping Natural Communities Mapping Forest resource planning Spatial data variability decisions Forest inventory Wildlife habitat analysis
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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
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Any Technology has Pro’s & Con’s
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Raster Limitations
Aesthetics Data storage requirements Overlay operations performed on
every cell Sparse data sets require as much
processing as dense ones
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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.
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GIS Data Models
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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.
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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
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Areas are lines are points are coordinates
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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.
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Raster/Vector Comparison
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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.
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Arc/node map data structure with files
Figure 3.4 Arc/Node Map Data Structure with Files.
1 1,2,3,4,5,6,7
Arcs File
POLYGON “A”
A: 1,2, Area, Attributes
File of Arcs by Polygon
1
23
4
5
6
7
8
9
10
1112
13 1 x y2 x y3 x y4 x y5 x y6 x y7 x y8 x y9 x y10 x y11 x y12 x y13 x y
Po
ints
Fil
e
1
2
2 1,8,9,10,11,12,13,7
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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).
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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.
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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
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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
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Basic arc topology
n1
n2
123 A
B
Arc From To PL PR n1x n1y n2x n2y1 n1 n2 A B x y x y
Topological Arcs File
Figure 3.5 A topological structure for the arcs.
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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.
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Slivers
Sliver
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Unsnapped node
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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.
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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.
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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.
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Sources of Vector Data
RASTER-VECTOR conversions from scanned images
Pre-existing digital data from disks or internet
DIGITIZING
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Vector to raster to vector conversion
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Comparison: Raster and Vector
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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.
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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.
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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.
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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.
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Vector to raster exchange errors
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Transfer Standards
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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.
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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.