Representation and Input
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Transcript of Representation and Input
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Representation and Input
This is lecture 3
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Digital vs Analog
• The only really useful GIS information is digital.
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Analog
• In the case of analog or traditional maps, information is fixed.
• Analog maps literally use analogies (lines for roads, blocks for houses, blobs for towns) to represent the earth.
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Digital 1
• A digital map displays information on the screen but the properties such as scale and projection are not fixed.
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Digital 2
• In its digital form, the road is represented by a series of coordinates.
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Digital 3
• In contrast to the analog model, it is the geographic data that are the basis of the GIS representation, and not the maps displayed on the screen.
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Getting information into the computer
• Need to geocode spatial data.
• Geocoding is the conversion of spatial information into digital form.
• Geocoding involves capturing the map, and sometimes also capturing the attributes.
• The process also involves formatting data so that it can be used by the computer.
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Three traditional methods of geocoding
• Digitizing
• Scanning
• Field data collection
• New methods include: WWW download and GPS
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Important things to remember about data
• data input is a major bottleneck in application of GIS technology
• costs of input can be a major (or the major) expense associated with a project
• data input is labor intensive, tedious, error-prone
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More about data
• sharing of digital data is one way around the input bottleneck
• more and more spatial data is becoming available in digital form
• It is also important to remember that data input to a GIS involves encoding both the locational and attribute data
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Location data
• the locational data are encoded as coordinates on a particular cartesian coordinate system
• source maps or downloaded data may have different projections, scales
• several stages of data transformation may be needed to bring all data to a common coordinate system
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Digitizing 1
• Historically the first method of inputting data was digitizing.
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Main steps in digitizing
• The map, photo, or other document is placed on the flat surface of the digitizing tablet
• the position of an indicator as it is moved over the surface of the digitizing tablet is detected by the computer and interpreted as pairs of x,y coordinates
• the indicator may be a pen-like stylus or a mouse.
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Steps 2
• three or more control points ("reference points", "tics", etc.) are digitized for each map sheet
• these should be easily identified points
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Digitizing modes
• Digitizing the map contents can be done in two different modes:
• In point mode, the operator identifies the points to be captured explicitly by pressing a button
• In stream mode points are captured at set time intervals (typically 10 per second) or on movement of the cursor by a fixed amount
• Most digitizing is done in point mode.
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Problems with digitizing
• most maps were not drafted for the purpose of digitizing
• paper maps are unstable
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Problems 2
• errors occur on these maps, and these errors are entered into the GIS database as well
• the level of error in the GIS database is directly related to the error level of the source maps
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Problems 3
• maps are meant to display information, and do not always accurately record locational information
• discrepancies across map sheet boundaries can cause discrepancies in the total GIS database
• user error causes overshoots, undershoots (gaps) and spikes at intersection of lines
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Digitizing errors
• Error 1 – sliver error (caused by space between separately closed polygons);
• Errors 2 & 3 – line closing errors. Some systems allow you to set a certain tolerance and if points fall within a tolerance zone of each other, they are assumed to be at the same location.Errors 4, 5, & 6 – due to missing segments, wrongly labelled segments of twice-digitized segments
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Errors 2
• Errors 7 & 8 – (weird polygons) due to careless digitizing or poor quality source document
• Error 9 – occurs when two maps have been separately digitized and then need to be combined. This is a process called edge-matching.
• User fatigue and boredom. • Luckily, many digitizers detect some errors
automatically and correct them
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Scanning as a form of data input
• Scanners are a faster, less labour intensive method of data input.
• There are various types of scanners including:- drum scanners (very high quality) and - flat bed scanners.
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Requirements for scanning
• In order to scan a document:
• documents must be clean
• lines should be at least 0.1 mm wide
• complex line work provides greater chance of error in scanning
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Problems with scanning
• text may be accidently scanned as line features. i.e. text streamed along a river becomes a wider part of the river.
• contour lines cannot be broken with text • automatic feature recognition is not easy (two contour
lines vs. road symbols) • special symbols (e.g. marsh symbols) must be recognized
and dealt with • if good source documents are available, scanning can be an
efficient time saving mode of data input • Unfortunately, most maps do not lend themselves to
efficient scanning.
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Vector maps are rasterized in order to scan. Then the polygons are re-created in vector.
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Field data collection
• Traditionally, geographers have conducted their own data collection.
• This usually involves collecting samples or recording events at know geographic positions.
• Traditionally, the “known geographic location” specification was restricting.
• The introduction of GPS has eliminated this constraint.
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Buying or downloading data
• First choice of many small users.
• Need to convert between formats (FME)
• Sources include:
• USGS (United States Geological Survey) http://www.usgs.gov/
• Census data. www.census.gov/
• GeoWeb. www.ggrweb.com
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Global positioning systems (GPS)
• GPS is becoming one of the most important technology that intersects with GIS
• a new tool for determining accurate positions on the surface of the earth
• computes positions from signals received from a series of satellites (NAVSTAR)
• depends on precise information about the orbits of the satellites
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GPS use and accuracy
• particularly valuable for establishing accurate positional control in remote areas
• accuracy will continue to improve as more satellites are placed in orbit and experts fine tune the software and hardware
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About Navstar satellites
Name: NAVSTAR • Manufacturer: Rockwell International • Altitude: 10,900 nautical miles • Weight: 1900 lbs (in orbit)• Size:17 ft with solar panels extended • Orbital Period: 12 hours • Orbital Plane: 55 degrees to equitorial
plane • Planned Lifespan: 7.5 years
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How GPS works
• GPS uses satellites (at least 3) to record positions on the earth’s surface. The calculations are done through a process of triangulation.
• Radio signals are sent by 24 satellites to GPS receivers.
• The system works by measuring how long it takes a radio signal to get from the satellite to the earth.
• There is error in the calculations and ground stations on earth are used to correct some of that error.
• More precise locational data is possible since May, 2000
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Source: Longley, P.A., M.F. Goodchild, D. J. Maguire, and D.W. Rhind, eds. 2001. Geographical Information Systems and Science. New York: John Wiley & Sons, Inc, p. 212.
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Mobile GPS
• Portability of GPS has led to proliferation of location-based services.
• Allows users to update data in the field in real time.
• Some systems allow two-way data flow.• Allows tracking (digital angels).• Studies of movement (time-space geography)
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Differential GPS
• The quest for greater and greater accuracy has spawned an assortment of variations on basic GPS technology.
• One technique, called "Differential GPS," involves the use of two ground-based receivers.
• One monitors variations in the GPS signal and communicates those variations to the other receiver.
• The second receiver can then correct its calculations for better accuracy.
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Carrier-phase GPS
• Another technique called "Carrier-phase GPS" takes advantage of the GPS signal's carrier signal to improve accuracy.
• The carrier frequency is much higher than the GPS signal.
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Augmented GPS
• The aviation industry is developing a type of GPS called "Augmented GPS" which involves the use of a geostationary satellite as a relay station for the transmission of differential corrections and GPS satellite status information.
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Attributes of spatial data
• Location and shape files have attributes which must also be entered and then linked up to the shape or point.
• Each shape will be associated with a list of variables (Var1…Varn).
• They might represent such things as area in square miles, population, highest altitude within the polygon etc.
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Source: Longley, P.A., M.F. Goodchild, D. J. Maguire, and D.W. Rhind, eds. 2001. Geographical Information Systems and Science. New York: John Wiley & Sons, Inc, p. 212.
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Characteristics of attribute data
• attributes usually obtained through a combination of field collection and interpretation
• categories may be subjective attributes such as these may not be easy to check in the field
• for social data, a major source of inaccuracy is undercounting
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Spatial data infrastructure (base maps)
• Spatial data forms the infrastructure for businesses and government agencies to plan.
• Infrastructure data is based on framework data
• Street centerline databases form the backbone of GIS and computer mapping applications.
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Street network data
• Street centerline spatial data is encoded with attributes such as street names, address ranges, Postal Codes, and census boundaries.
• These are then combined with non-street features such as bodies of water, railroads or landmarks to create a spatial data infrastructure that is used in a variety of business and government applications.
• Street data is important because it expresses the fundamental relationships between street addresses (the most common spatial reference point for most of us) and coordinates or other locational links.
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The role of the Census
• In Canada, the Canada Census street network file is compiled with each new enumeration.
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Maintaining base data
• To be useful, spatial data must be constantly maintained.
• New streets are added every day, and Postal Codes, and letter carrier routes change frequently.
• Successful private database companies, such as DMTI have concentrated on street and address currentness and completeness.
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Data policies in Canada vs US
• US has different spatial data policies than other developed nations.
• Spatial data infrastructure is growing more slowly in Canada, UK, New Zealand and Australia