Data%20Input%20and%20GIS%20refer%20to%20Chapter%204%20Data%20input,%20verification,%20storage,%20and%20output%20Text%20Book%20Burrough,%20P.%20A.%20and%20R.%20A.%20McDonnell,%201998.%20Principles%20of%20Geographical%20Information%20Systems.%20Oxford%20University%20Press,%20London.

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Data Input and GISrefer to Chapter 4Data
input, verification, storage, and outputText
BookBurrough, P. A. and R. A. McDonnell, 1998.
Principles of Geographical Information Systems.
Oxford University Press, London.

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Overview

• Input of spatial data
• Modes of data input
• Rasterization and vectorization
• Map preparation and the digitizing
• Remote Sensing Special Raster Data Input
• Integrating different data sources
• External Databases
• Exercise

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Input of spatial data

• Need to have tools to transform spatial data of
  various types into digital format
• Data input is a major bottleneck in application
  of GIS technology. Costs of input often consume
  80 or more of project costs
• Many commercial GIS operations generate most of
  their revenue through data input
• Data input is labor intensive, tedious, and
  error-prone
• There is a danger that construction of the
  database may become an end in itself and the
  project may not move on to analysis of the data
  collected

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Input of spatial data-continue

• Essential to find ways to reduce costs and
  maximize accuracy
• Need to automate the input process as much as
  possible, but automated input cab create
  bigger editing problems later
• Source documents (maps) may often have to be
  redrafted to meet rigid quality requirements of
  automated input
• Sharing of digital data is one way around the
  input bottleneck. More and more spatial data is
  becoming available in digital form

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Input of spatial data-continue

• Data input to a GIS involves encoding both the
  locational and attribute data
• The locational data is encoded as coordinates on
  a particular cartesian coordinate system
• Source maps may have different projections and
  scales
• Several stages of data transformation may be
  needed to bring all data to a common coordinate
  system
• Attribute data is often obtained and stored in
  tables (Database Management System)

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Input of spatial data-continue

• There are two methods for spatial data
  acquisition
• Primary methods Surveying, Photogrammetry,
  GPS, and Remote Sensing
• Secondary methods Digitization, Automatic
  line following, and scanning

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The input subsystem

• Designed to transfer data into the GIS from
  external sources (attribute and map data)
• Must allow for encoding in either raster or
  vector(TIN)
• Must provide a means for spatial referencing
  (projections, cartesian coordinate systems, etc)
• Must provide link between storage and editing
  subsystems (ensure input can be saved and any
  errors corrected)

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Modes of data input Input Devices

• Grid overlay
• keyboard
• Digitizer
• Scanner
• Data in digital format (Total station, digital
  photogrammetry, remote sensing, GPS)

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Grid overlay

• Grid on clear material is overlaid on map
• Identity of each cell in the grid is determined
  by what map features are in a particular cell
• Number or code is assigned to each class of map
  features, and used to label cells in grid
• After filling in the grid, numbers or codes are
  typed into the computer to produce a raster layer
  
• Pretty antiquated method, seldom used

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Keyboard

• Keyboard entry (X,Y,Z), (Ø, ?, h), or angle and
  distance
• Input through keyboard is time consuming, but it
  is more accurate
• It is suitable for small areas i.e. when the
  number of points/lines/areas are limited
• Because of its high accuracy, sometimes it is
  used in applications that need high quality e.g.
  cadastral mapping

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Digitizing Digitizing Tablet

• Tablet composed of a flat surface, in which are
  embedded a grid of electronically active wires
  and mouse-like device (puck or stylus) usually
  with cross hairs. When puck is moved over the
  tablet, its location is known because the grid of
  wires senses it location. Puck also has buttons
  which allow communication with the computer
• Grid acts like a cartersian (X,Y) coordinate
  system. To input data, map is taped on digitizing
  tablet. Puck is placed over the feature of
  interest, and message is sent to compute through
  buttons on puck e.g., node is used to mark
  beginning and end of line feature, or point where
  polygon closes on itself

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Digitization

• Digitization is a process of converting existing
  maps to digital form (vector format)
• A digitizer is connected to a computer and map
  features are followed manually
• Digitizers are available at different sizes (A4,
  A3, A2, A0) and different accuracy (0.05 mm)
• Example of digitizers are CalComp 9500 and
  Summagraphic

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Digitizing-continue

• Digitizing the map contents can be done in two
  different modes point or stream
• Point mode the operator identifies the points to
  be captured explicitly by pressing a button
• Stream mode points are captured at set time
  intervals (typically 10 per second) or on
  movement of the cursor by a fixed amount
• In point mode the operator selects points
  subjectively, two point mode operators will not
  code a line in the same way
• Stream mode generates large numbers of points,
  many of which may be redundant

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Digitizing- Problems

• Paper maps are unstable each time the map is
  removed from the digitizing table, the reference
  points must be re-entered when the map is affixed
  to the table again
• If the map has stretched or shrunk in the
  interim, the newly digitized points will be
  slightly off in their location when compared to
  previously digitized points
• 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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Digitizing- Problems-continue

• Maps are meant to display information, and do not
  always accurately record locational information,
  for example, when a railroad, stream and road all
  go through a narrow mountain pass, the pass may
  actually be depicted wider than its actual size
  to allow for the three symbols to be drafted in
  the pass
• Discrepancies across map sheet boundaries can
  cause discrepancies in the total GIS database
  e.g. roads or streams that do not meet
  exactly when two map sheets are placed next to
  each other
• User error causes overshoots, undershoots

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Scanners

• Types of scanners Line following and drum
• Line following placed on a line and follow line
  using a guiding device such as a laser
• Two short comings
• 1. sample lines at regular time or distance
  intervals (more complex parts of the line should
  have more samples, less complex need less
  samples)
• 2. lines that converge then diverge (e.g.,
  contours along a cliff, road intersections, etc),
  device doesnt know which line
  is which also broken lines (dashes, interrupted
  by label etc.)
• Line following technology can be reproduced in a
  software environment (line tracing software)

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Drum scanners

• Drum scanners (Fig 5.2, p. 129) as the drum
  rotates about its axis, a scanner head containing
  a light source and photo-detector reads the
  reflectivity of the target graphic, and
  digitizing this signal, creates a single row of
  pixels from the graphic. The scanner head moves
  along the axis of the drum to create the next
  column of pixels, and so on through the entire
  scan
• Systems may have a scan spot size of as little as
  25 micrometers, and be able to scan graphics of
  the order of 1 meter on a side an
  alternative mechanism involves an array of
  photo-detectors which extract data from several
  rows of the raster simultaneously. The detector
  moves across the document in a swath when all
  the columns have been scanned, the detector moves
  to a new swath of rows initially, scanning
  produces a raster image, which can be converted
  to vector using on screen digitizing or
  automated line tracing software

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Scanning

• Scanning is a process of converting existing maps
  to digital form (raster format)
• A scanner is connected to a computer and map
  features are scanned automatically
• Scanners are available at different sizes (A4,
  A3, A2, A0) and different accuracy (300 dpi,
  600dpi, 1000 dpi)
• Example of Scanners are UMAX-S12, HP

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Scanners-Problems

• Scanners are generally very expensive
• Editing can take nearly as long as manual
  digitizing would have taken
• Scanners should be thought of as time-saving
  devices only when maps are clear, show good
  contrast, and contain a relatively simple amount
  of content

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Rasterization and vectorization

• Regardless of input device, it is necessary to
  determine if the final product will be raster or
  vector
• Most GIS programs allow conversion between the
  two, but problems are involved
• If vector to raster, cell size is also important,
  but the results are satisfactory
• If raster to vector, lines become blocky and
  step-like, cant reverse the procedure to produce
  original content of vector line also, resolution
  (or cell size) has a direct effect on the spatial
  integrity of the object
• Spline algorithms - apply a smoothing function to
  vector lines
• Examples of software convert from raster to
  vector or vice versa are R2V and ArcScan under
  Arc/Info

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Map Preparation and the Digitizing

• Identification of features to be digitized.
  Sometimes marked directly on the map or on clear
  overlay. Sometimes, identification of nodes
  vs. vertices
• The digitizing process usually starts with
  telling the computer about the coordinate system
  that the map is in. Digitizer operates in its own
  cartesian coordinate system, need to establish
  relationship between digitizer
  coordinates and map coordinates (Transformation)
• Registration points or tick marks identified.
  Allows you to remove map from tablet to allow
  others to access it, then put it back on and
  register the input system using tic marks.
  It is essential to locate these precisely
  because they provide the reference for all other
  spatial data entered

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Reference Frameworks and Transformations

• Digitizer coordinate must be transformed to map
  coordinates using a minimum number of four
  registration marks to cater for Translation
  Object movement Rotation Reorient the
  object Scale change adjust the object
  size (Figure 5.4, p. 133, DeMers)

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Setting up digitizing environment to handle errors

• Fuzzy tolerance - attempts to account for errors
  caused by the "shaky hand. Based on the idea
  that you will not be able to place the cursor
  exactly the same location twice. Essentially
  defines a distance for maximum separation . If
  two nodes are within the limits of fuzzy
  tolerance, the are snapped together. Same idea
  for line features. Can be done before digitizing
  starts or can be implemented in post-digitizing
  editing process
• Other variables Material of map shrink/swell
  with changes in humidity and temperature and
  stable medium such as plastic (Mylar) is
  preferred

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What to Input

• Define your purpose before hand and make sure the
  data you are using are suitable for the goals of
  your project and pre-plan carefully
• Use the most accurate data, but not data that is
  too accurate for your purpose
• Check to see if data are already available
• Keep coverages simple and use the same map to
  extract different coverages when possible
• How Much to Input
• Scale dependent
• General rule - more complex features at larger
  scales require more detail (more vertices,
  smaller cell size)
• Sample more for more information

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Methods of Vector Input

• Manual digitizing, Registration marks
• Location of nodes, lines not become nodes and
  nodes dont become just points
• Building of topology
• Correcting of digitizing errors
• Transformation and projection
• Adding attribute data
• Checking the accuracy of attribute data

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Methods of Raster Input

• Presence/absence method If object occurs in a
  cell (anywhere) it is recorded as present (
  simplicity ) best method for coding
  points and lines (Fig. 5.6 p. 143)
• Centroid of cell method Presence only recorded
  if object is at the center of the cell . Disadv.
  - less simple, requires calculation of centroid,
  location of object relative to centroid.
  Generally restricted to raster encoding of
  polygons
• Dominant type method Commonly used for
  encoding polygons into raster format . Identified
  as present if it occupies more than 50 of the
  cell
• Percent occurrence Not only encodes
  presence/absence, but occurrence (Urban/Rural)
• Generally, each attribute is recorded as a
  separate coverage e.g., one grid of percent
  urban, one of percent rural, percent water,
  percent forest, etc.

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Remote Sensing Special Raster Data Input

• Remote Sensing data is considered as special
  raster data (in digital form). Image processing
  software can be used to extract/classify remote
  sensing imagery (cover later in the semester)
• Attention should be paid to geometric and
  radiometric corrections and method of
  classification (supervised/unsupervised),
  different radiometric, geometric, and temporal
  resolutions
• Institutional problems related to remote sensing
  data include availability of data (limited
  coverage, cloud cover), cost, education and
  training, and organizational infrastructure

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External Databases

• An efficient method of building a GIS database is
  to limit the amount of time and cost necessary to
  develop database
• A plenty of data already available in different
  digital format an in different media 9-inch tape,
  8 mm tape, CD-ROM, etc.
• Need to evaluate data for its utility/quality for
  projects and ability to import
• Meta-data or data dictionary should be prepared
  for the GIS database (information about the
  content)

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Exercise 2 Digitization

• 1. Load any image of the campus into ArcView.
• 2. Create three new themes (point, line,
  polygon) (hint View/new theme)
• a. Create a point theme to show your classroom
  buildings, where you park, and where you have
  lunch.
• b. Create a line theme of the paths that you walk
  between those points
• c. Create a polygon theme of each building that
  has a point.
• 3. Give a unique id number to each point, line
  and polygon in each of the new themes tables.
  (hint Table/start editing)
• 4. Change the legend of each theme to show the
  different id numbers.
• 5. Give the points a new symbol that represent
  what is happening there.
• 6. Give the lines arrows to show what direction
  you are walking.
• 7. Create a layout that has a title, north arrow,
  your name, date, and a custom legend.

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Digitization-continue

• To create a custom legend
• a. First load the legend into a layout.
• b. Second select the legend and right click
  simplify. (This will separate the legend)
• c. Now edit the legend to give it a unique look
  and take out the polygon theme part of the
  legend.
• d. Once you are finish with the legend select
  everything in the legend. When everything is
  selected go to graphics/group. This will group
  the graphic back together.
• 9. In the layout, select File and Export the
  layout to JPEG format, but before that make sure
  the JPEG (JFIF) image support Extension is loaded
• What to hand in.
• 1. Jpeg layout
• 2. Layout contains
• a. Campus image with three themes
• b. Custom legend
• c. Title, scale, north arrow and your name