Cartographic and GIS Data Structures Dr. Ahmad BinTouq E-mail: [email protected] URL: abintouq.
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Transcript of Cartographic and GIS Data Structures Dr. Ahmad BinTouq E-mail: [email protected] URL: abintouq.
![Page 1: Cartographic and GIS Data Structures Dr. Ahmad BinTouq E-mail: abintouq@uaeu.ac.ae URL: abintouq.](https://reader031.fdocuments.us/reader031/viewer/2022013011/56649ec65503460f94bd1ed7/html5/thumbnails/1.jpg)
Cartographic and GIS Data Structures
Dr. Ahmad BinTouq
E-mail: [email protected]
URL: http://faculty.uaeu.ac.ae/~abintouq
![Page 2: Cartographic and GIS Data Structures Dr. Ahmad BinTouq E-mail: abintouq@uaeu.ac.ae URL: abintouq.](https://reader031.fdocuments.us/reader031/viewer/2022013011/56649ec65503460f94bd1ed7/html5/thumbnails/2.jpg)
Overview
• Map as an Abstraction of Space
• Methods of representing geographic space
– Raster Model
– Vector Model
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Map as an Abstraction of Space
• Spatial features can be represented as point, lines, areas, or surfaces • Some phenomena or objects are selected for inclusion, others are
not spatial features and there attributes are simplified, aggregated, and classified
• When we want to enter this data into a GIS, certain decisions need to be made based upon how the data can be entered into a computer (geocoding vs. drawing)
• How do you get simple spatial concepts into the computer (e.g., a map which identifies a lake within an island, surrounded by ocean, covered by forest on north side, and a cleared beach on the other side)
• Inside, surrounded, by, north, south
![Page 4: Cartographic and GIS Data Structures Dr. Ahmad BinTouq E-mail: abintouq@uaeu.ac.ae URL: abintouq.](https://reader031.fdocuments.us/reader031/viewer/2022013011/56649ec65503460f94bd1ed7/html5/thumbnails/4.jpg)
Methods of representing geographic space
Raster
Vector
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The diagram below shows how real-world objects can be represented on a computer monitor by x,y coordinates.
The coordinate pairs 1,5 3,5 5,7 8,8 and 11,7 represent a line
(road) The coordinate pairs 6,5 7,4 9,5 11,3 8,2 5,3 and 6,5 represent a
polygon (lake). The first and last coordinates of the polygon are the same; a
polygon always closes.
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Raster Models• Raster - from the Greek word meaning "to rake" • Quantizes or divides space into discrete packets (cells), each representing
a part of the whole • Cells are of equal size square, rectangular, hexagon, triangles • Loose the ability to represent exact locations (e.g., point represented as
single cell) • Zero dimensional object rep. with 2D feature • Lines represented as a series of connected cells • Multiple cells joined at edges or corners, usually with only 1 or 2
neighbors, 1D objects represented in 2D • Areas represented as a series of connected cells • 2D objects represented in 2D, cells distort area and shape - stairs-
stepped appearance
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Raster Models-continue
• Two general ways of associating attribute data with raster entities
• 1. store an attribute for every grid cell problem is redundancy in storage
• 2. link cells to RDBMS
• Permits more than one attribute to be associated for a single cell
• Only have to store attributes once
• Cell value linked to attribute table
• Essentially many to one - "many cells being linked to one record in separate attribute table"
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Generic structure for a grid
Row
s
Columns
Gridcell
Grid extent
Resolution
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• CELLS: a representation of geographic data based on rows and columns (e.g.. continuous surface data such as elevation or temperature, and categorical representations derived from vector data)
• PIXELS: a group of independent points with a color value but no other associated data (e.g.. scanned documents, orthophotography, satellite images)
Geographic Representations
![Page 10: Cartographic and GIS Data Structures Dr. Ahmad BinTouq E-mail: abintouq@uaeu.ac.ae URL: abintouq.](https://reader031.fdocuments.us/reader031/viewer/2022013011/56649ec65503460f94bd1ed7/html5/thumbnails/10.jpg)
• Like the vector data model, the raster data model can represent discrete point, line and area features.
• A point feature is represented as a value in a single cell, a linear feature as a series of connected cells that portray length, and an area feature as a group of connected cells portraying shape.
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• Because the raster data model is a regular grid, spatial relationships are implicit. Therefore, explicitly storing spatial relationships is not required as it is for the vector data model.
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Vector Models• Features represented in basically the same way as an analog map,
permits more precise representation than raster model, permits "empty space”, variations of the vector model
• Spaghetti models • Simplest of vector data structures • Does not explicitly store spatial relationships (topology), essentially
X,Y coordinates, and which should be connected by lines • Doesn’t really "know" if points and connected lines form a line entity
or poly entity • Topological models • Recognizes the concept of an entity • Stores spatial relationship information explicitly associated with each
entity, most common in GIS
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Feature Geometry
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To keep track of many features, each is assigned a unique identification number or tag.
Then, the list of coordinates for each feature is associated with the feature’s tag. The objects you see in a vector theme are
actually saved in the theme table
Linking Geographic data with attribute
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Vector Data: Advantages
• Data can be represented at its original resolution and form without generalization.
• Graphic output is usually more aesthetically pleasing (traditional cartographic representation)
• Since most data, e.g. hard copy maps, is in vector form no data conversion is required.
• Accurate geographic location of data is maintained.
• Because it recognizes entities, model allows for efficient encoding of topology, and as a result more efficient operations that require topological information, e.g. proximity, network analysis.
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Vector Data: Disadvantages
• The location of each vertex needs to be stored explicitly
• For effective analysis, vector data must be converted into a topological structure. This is often processing intensive and usually requires extensive data cleaning.
• Topology is static, and any updating or editing of the vector data requires re-building of the topology
• Algorithms for manipulative and analysis functions are complex and may be processing intensive
• Often, this inherently limits the functionality for large data sets, e.g.a large number of features.
• Continuous data, such as elevation data, is not effectively represented in vector form. Usually substantial data generalization or interpolation is required for these data layers
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Raster Data: Advantages
• Due to the nature of the data storage technique data analysis is usually easy to program and quick to perform.
• The inherent nature of raster maps, e.g. one attribute maps, is ideally suited for mathematical modeling and quantitative analysis.
• Discrete data, e.g. forestry stands, is accommodated equally well as continuous data, e.g. elevation data, and facilitates the integrating of the two data types.
• Grid-cell systems are very compatible with raster-based output devices, e.g. electrostatic plotters, graphic terminals.
• Also compatible with digital satellite imagery.
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Raster Data: Disadvantages
• The cell size determines the resolution at which the data is represented.
• Processing of associated attribute data may be cumbersome if large amounts of data exists.
• Raster maps normally reflect only one attribute or characteristic for an area.
• Since most input data is in vector form, data must undergo vector-to-raster conversion.
• Most output maps from grid-cell systems do not conform to high-quality cartographic needs.
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Vector RepresentationVector Representation
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Vector to RasterVector to Raster
![Page 21: Cartographic and GIS Data Structures Dr. Ahmad BinTouq E-mail: abintouq@uaeu.ac.ae URL: abintouq.](https://reader031.fdocuments.us/reader031/viewer/2022013011/56649ec65503460f94bd1ed7/html5/thumbnails/21.jpg)
Raster RepresentationRaster Representation
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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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Grid data (Cells)
![Page 24: Cartographic and GIS Data Structures Dr. Ahmad BinTouq E-mail: abintouq@uaeu.ac.ae URL: abintouq.](https://reader031.fdocuments.us/reader031/viewer/2022013011/56649ec65503460f94bd1ed7/html5/thumbnails/24.jpg)
Satellite image (Pixels)
![Page 25: Cartographic and GIS Data Structures Dr. Ahmad BinTouq E-mail: abintouq@uaeu.ac.ae URL: abintouq.](https://reader031.fdocuments.us/reader031/viewer/2022013011/56649ec65503460f94bd1ed7/html5/thumbnails/25.jpg)
Vector Vs. RasterVector Raster
Compact datastructure - littlestorage space
greater storage needed
Topology more easilymaintained (lines havedirection)
Topology difficult
Arcs moreaesthetically pleasing
Grids not veryaesthetic
Data structure morecomplex
Data structure moresimple
Better geographicspecificity
Limited geographicspecificity (due toresolution)
![Page 26: Cartographic and GIS Data Structures Dr. Ahmad BinTouq E-mail: abintouq@uaeu.ac.ae URL: abintouq.](https://reader031.fdocuments.us/reader031/viewer/2022013011/56649ec65503460f94bd1ed7/html5/thumbnails/26.jpg)
Exercise
• Compare between Raster and Vector Model for representing geographic features; illustrate by figures