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Lecture 6

Lecture 6. Data entry. Getting the Map into the Computer. Get data in finished form Analog-to-Digital maps Digitizing Data Entry Editing and validation Scanning Georeferencing data Field and image data Attribute data. GIS maps are digital not analog.

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Lecture 6

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  1. Lecture 6 Data entry

  2. Getting the Map into the Computer • Get data in finished form • Analog-to-Digital maps • Digitizing • Data Entry • Editing and validation • Scanning • Georeferencing data • Field and image data • Attribute data

  3. GIS maps are digital not analog • Maps have a communications function but... • A map has a storage function for spatial data • Somehow, the visually “stored” data must get digital • Real and Virtual maps

  4. Data Conversion • Traditionally most of the cost of a GIS project • One time cost • Depends on reuse • Requires maintenance

  5. Finding Existing Map Data • Map libraries • Reference books • State and local agencies • Federal agencies • Commercial data suppliers e.g. EOSAT, ETAK

  6. GIS data can be • Purchased • Found from existing sources in digital form • Captured from analog maps by GEOCODING

  7. Turning analog (paper) maps to digital • Digitizing (vector) • Scanning (raster) • Field Data Collection • GPS • Survey

  8. Digitizing • Captures map data by tracing lines from a map by hand • Uses a cursor and an electronically-sensitive tablet • Result is a string of points with (x, y) values Heads-up digitizing example

  9. map Pulse is picked up by nearest grid wires under tablet surface. Result is sent to computer after conversion to x and y units. Digitizer cursor transmits a pulse from an electomagnetic coil under the view lens. The Digitizing Tablet\

  10. Digitizing • Stable base map, Medium? • Fix to tablet • Determine coordinate transformation • Trace features • Proof plot • Edit • Clean and build

  11. Digitizing • Cursor data entry • Secondary tablet (menu/template) • Voice command entry • Point select • Stream mode

  12. Selecting points to digitize

  13. Some common digitizing errors • Slivers • Duplicate lines • Duplicate nodes • Unended lines • Gaps

  14. Slivers Real river Digitizer #1 Digitizer #2 Sliver

  15. Unsnapped node

  16. Over shoots and under shoots

  17. Flat bed Drum DPI File size Scanning

  18. Scanning example 15 x 15 cm (3.6 x 3.6 km) grid is 0.25 mm ground equivalent is 6 m 600 x 600 pixels one byte per color (0-255) 1.08 MB This section of map was scanned, resulting in a file in TIF format that was bytes in size. This was a file of color intensities between 0 and 255 for red, green, and blue in each of three layers spaced on a grid 0.25 millimeter apart. How much data would be necessary to capture the features on your map as vectors? Would it be more or less than the grid (raster) file?

  19. Scanning • Places a map on a glass plate, and passes a light beam over it • Measures the reflected light intensity • Result is a grid of pixels • Image size and resolution are important • Features can “drop out”

  20. Image’s coordinates X X 0, 0 0, 0 Y Y

  21. Image’s coordinates and data’s coordinates 117 W, 34 N X 0, 0 Y

  22. Reality -117 x, 34 y X 0, 0 Y

  23. Define control points, image to map -117 x, 34 y X 0, 0 Y

  24. Image moves

  25. Attribute data • Table with rows and columns • Attributes by records • Entries are called values

  26. Attribute data entry errors • Spelling • Alesandro • Allessandro • Alessandro • Allessandro • Locate/reduce spelling errors • Enter the column twice and compare • Use frequency statistics

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