Getting data into ArcGIS

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Getting data into ArcGIS

• J426 Class 13
• February 24, 2009

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Overview

• Digitizing
• Scanning
• Coordinate data for point locations

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Getting data into GIS

• Major part of establishing working gis
• Can consume 80 percent or more of project costs
• Data input is labor intensive
• Data input introduces errors

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Digitizing with tablet

• Has been most common method for entering
  information from maps and air photos
• Digitizing tablets
• Tablet senses position of cursor (puck) on tablet
• Uses grid of wires to produce magnetic field
  sensed by cursor
• Outputs x- and y-coordinates
• Accuracy typically better than 0.1 mm

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Digitizing tablets
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Digitizing process

• Affix map to tablet
• Identify 3 or more control points (reference
  points) with known coordinates
• Control points used for transformation from
  arbitrary tablet coordinates to geographic
  coordinates
• Process requires knowledge of and consistency
  with map projection

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Digitizing modes

• Point mode
• Operator identifies points (vertices) that define
  feature
• Relies on operator judgment
• Stream mode
• Points captured at regular intervals or upon
  movement of cursor by set amount
• Generates large number of points, many redundant

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Digitizing problems

• Instability of paper maps
• Maps meant to display information and do not
  always accurately record positional information
• Discrepancies across map sheet boundaries
• User error
• Overshoots, undershoots
• Inaccuracies due to fatigue, boredom

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Digitizing issues

• Data will require editing after digitizing
• Some of the editing can be automated
• Other editing will require user intervention
• Quality control
• Need to establish standards, procedures
• E.g., redigitizing samples of features as test
• Digitizing costs
• Rule of thumb one boundary per minute

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Heads-up digitizing

• Digitizing from image displayed on screen
• Has become more common alternative to digitizing
  with tablet
• Sources of images
• Aerial photographs
• Satellite images
• Scanned maps

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Images for heads-up digitizing

• Aerial photos and satellite images must be
  rectified
• All images must be georeferenced, with known
  geographic coordinates being specified
• Digital orthophotographs becoming widely available

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Digital orthophoto for downtown Indianapolis
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Scanning

• Automated alternative to manual digitizing
• Types of scanners
• Video scanners
• Used to scan all types of images
• Fast and inexpensive
• Optical distortion, limited accuracy
• Electromechanical scanners
• Expensive and slow, map on drum
• More accurate, less distortion

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Map scanners
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Scanning issues

• Scanned data in raster format
• Must be converted for use in vector gis
• Problem of identifying geographic features
• Text may be interpreted as features
• Scanned data requires extensive post-processing,
  editing
• Much of this must be done manually

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Requirements for scanning

• Documents must be clean
• Smudges, folds scanned as features
• Simpler maps work better
• Complex maps with many features, labels, create
  problems
• Sometimes maps are redrawn for scanning

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Scanning potential

• For very complex maps, difficulty of interpreting
  and editing scanned data may make scanning
  impractical
• For very simple maps with relatively small
  numbers of line features, digitizing may be
  easier and faster
• Only certain maps that fall in the middle are
  currently candidates for scanning

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Coordinate data for point locations

• Data for point locations including coordinates of
  points
• Coordinates could be either latitude and
  longitude or
• in projected coordinate system
• Will include additional attribute data on points,
  e.g., type of crime, date, etc.

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Possible sources for point location coordinate
data

• GPS (Global Positioning System) receiver capture
• Geocoding using address information or other
  geographic identifier

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Using coordinate data for point locations

• Data must be in suitable data file format
• Simply add table to map, map the coordinate data
  creating point layer
• Data frame must use coordinate system in which
  point data are reported