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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.

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Presentation on theme: "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."— Presentation transcript:

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 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 Sliver Real river Digitizer #1 Digitizer #2

15 Unsnapped node

16 Over shoots and under shoots

17 Scanning Flat bed Drum DPI File size

18 Scanning example 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? 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

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 Y 0, 0 X Y

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

22 Reality X Y 0, x, 34 y

23 Define control points, image to map X Y 0, x, 34 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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