Download Faculty of Transportation Sciences

Czech Technical University
in Prague
Faculty of Transportation
Sciences
Department of Transport Telematics
Geographical Information Systems
Doc. Ing. Pavel Hrubeš, Ph.D.
Actuality
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Presentations for download
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http://www.lss.fd.cvut.cz/education/gis
Czech Technical University in Prague - Faculty of Transportation Sciences
Department of Transport Telematics
Rehearsal
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GIS
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Users, experts
Hardware
Software, analytical
methods
Databases
Visualization
Modeling
Capturing
Storage
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Modeling of real
world
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Selection of objects or
phenomena
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Digital version of real
and virtual objects
Identification of objects
Definition of way of
representation
Transformation rules
GIS data model
Recording of data into
database
Czech Technical University in Prague - Faculty of Transportation Sciences
Department of Transport Telematics
Physical representation
Czech Technical University in Prague - Faculty of Transportation Sciences
Department of Transport Telematics
Recording of spatial information
 Raster Images
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“pixels”
satellite images
aerial photos
Czech Technical University in Prague - Faculty of Transportation Sciences
Department of Transport Telematics
Raster data
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Raster is a method for the storage, processing
and display of spatial data. Each area is divided
into rows and columns, which form a regular grid
structure.
The spatial location of each cell is implicitly
contained within the ordering of the matrix, unlike
a vector structure which stores topology explicitly.
Areas containing the same attribute value are
recognized as such, however, raster structures
cannot identify the boundaries of such areas as
polygons.
Czech Technical University in Prague - Faculty of Transportation Sciences
Department of Transport Telematics
Raster data
Czech Technical University in Prague - Faculty of Transportation Sciences
Department of Transport Telematics
Raster data - grid size and resolution
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In raster GIS the pixel equivalent is usually
referred to as a cell element or grid cell. Pixel/cell
refers to the smallest unit of information available
in an image or raster map. This is the smallest
element of a display device that can be
independently assigned attributes such as color.
Pixel size and number of rows and columns:
"The size of the pixel must be half of the smallest
distance to be represented" Star and Estes
(1990)
Czech Technical University in Prague - Faculty of Transportation Sciences
Department of Transport Telematics
Raster data - raster data structures
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exhaustive enumeration
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In this data structure every pixel is given a single
value, hence there is no compression when many like
values are encountered.
Czech Technical University in Prague - Faculty of Transportation Sciences
Department of Transport Telematics
Raster data - raster data structures
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run-length encoding
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This is a raster image compression technique. If a
raster contains groups of cells with identical values,
run length encoding can compress storage. Instead of
storing each cell, each component stores a value and
a count of cells with that value. If there is only one cell
the storage doubles, but for three or more cells there is
a reduction.
Czech Technical University in Prague - Faculty of Transportation Sciences
Department of Transport Telematics
Recording of spatial information
 Vector data model
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features: points, lines & polygons
attributes: size, type, length, etc.
Czech Technical University in Prague - Faculty of Transportation Sciences
Department of Transport Telematics
Vector data
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Vector is a data structure, used to store spatial
data. Vector data is comprised of lines or arcs,
defined by beginning and end points, which meet
at nodes.
The locations of these nodes and the topological
structure are usually stored explicitly.
Features are defined by their boundaries only and
curved lines are represented as a series of
connecting arcs.
Czech Technical University in Prague - Faculty of Transportation Sciences
Department of Transport Telematics
Vector data
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co-ordinate
Pairs of numbers expressing horizontal distances along orthogonal axes,
or triplets of numbers measuring horizontal and vertical distances, or nnumbers along n-axes expressing a precise location in n-dimensional
space. Co-ordinates generally represent locations on the earth's surface
relative to other locations.
point
A zero-dimensional abstraction of an object represented by a single X,Y
co-ordinate. A point normally represents a geographic feature too small to
be displayed as a line or area; for example, the location of a building
location on a small-scale map, or the location of a service cover on a
medium scale map.
line
A set of ordered co-ordinates that represent the shape of geographic
features too narrow to be displayed as an area at the given scale
(contours, street centrelines, or streams), or linear features with no area
(county boundary lines). A lines is synonymous with an arc.
polygon
A feature used to represent areas. A polygon is defined by the lines that
make up its boundary and a point inside its boundary for identification.
Polygons have attributes that describe the geographic feature they
represent.
Czech Technical University in Prague - Faculty of Transportation Sciences
Department of Transport Telematics
Vector data
points
lines
areas
Node
Vertex
y
x
Czech Technical University in Prague - Faculty of Transportation Sciences
Department of Transport Telematics
Rehearsal
 Vector Data
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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.
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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Raster Data
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Advantages:
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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.
As well, 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.
Spatial analysis and filtering within polygons is
impossible
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The geographic location of each cell is implied by its
position in the cell matrix. Accordingly, other than an
origin point, e.g. bottom left corner, no geographic
coordinates are stored.
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 rasterbased output devices, e.g. electrostatic plotters.
Disadvantages:
The cell size determines the resolution at which the
data is represented.;
It is especially difficult to adequately represent linear
features depending on the cell resolution. Accordingly,
network linkages are difficult to establish.
Processing of associated attribute data may be
cumbersome if large amounts of data exists. Raster
maps inherently reflect only one attribute or
characteristic for an area.
Since most input data is in vector form, data must
undergo vector-to-raster conversion. Besides
increased processing requirements this may introduce
data integrity concerns due to generalization and
choice of inappropriate cell size.
Most output maps from grid-cell systems do not
conform to high-quality cartographic needs.
Czech Technical University in Prague - Faculty of Transportation Sciences
Department of Control and Telematics
Advantages/disadvantages of raster and
vector data models
precision in graphics
traditional cartography
data volume
topology
computation
update
continuous space
integration
discontinuous
raster
vector
x
x
x
x
v
v
v
v
x
v
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v
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v
Czech Technical University in Prague - Faculty of Transportation Sciences
Department of Transport Telematics
Recording of attribute information
 Record in graphical part
(raster format)
 Spatial and attribute part
divided to two records
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Common in files (ESRI
shapefiles)
 Necessary to use unique identifiers
 Spatial and attribute part
together in one record
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Databases
Czech Technical University in Prague - Faculty of Transportation Sciences
Department of Transport Telematics