This document discusses different methods of data representation in GIS, including data collection, input, and output devices. It covers three main types of data input: sample ground data, topographic maps, and satellite digital data. Common input devices include digitizers, scanners, keyboards, and disk drives, while common output devices are plotters, printers, visual display units, and tape drives. The document then focuses on different data input methods like keyboard entry, digitizing, and scanning, outlining their processes, advantages, and limitations.

1. TYPES OF DATA
REPRESENTATION
Unit -5
Prepared by
Sampath Kathroju
HOD Civil Dept,
Kshatriya College of Engineering

2. Data collection:
3 types
 Sample ground data
 Topo maps of SOI on 1:25000 scale used to prepare base map
 Satellite digital data
Input Overview :
 It covers and transforms all the data captured in the form of maps
 Sensors such as aerial photos etc
 Field observations
Input Overview
Field Observations Sensors Maps
Text Files Scanners
Magnetic
media
Digitizers
Terminal Display
Unit

3. Data Input Devices :
 Digitizer (Raster to vector)
 Scanner (Data to raster /digital on paper)
 Keyboard
 Disk Drive
Data Output Devices
 Plotter (plot d graphical information on paper)
 Printer
 Visual Display Unit (VDU)
 Tape

5. Data Input
Data input involves both locational (map) and attribute data.
The data can originate from various sources:
- paper maps
- attribute data on paper
- other digital formats- databases (dBase), spreadsheets
(excel), or ASCII files

6. The goal is to use the mode of data input that:
- best translates the source data to electronic form
- is fast
- is easy to use
- reduces cost
- maximizes the accuracy required for the GIS project

7. Keyboard entry:
(the source data only exists on paper)
- mainly attribute data
- type data using keyboard
- data can be entered directly
into a GIS table, spreadsheet,
or database table.

8. 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

9. Why digitize?
• New maps
• Map features are wrong
• Missing features
9
GIS TUTORIAL 1 - Basic
Workbook

10. Digitizer
• Digitizing table
10” x 10” to 80” x 60”
1/100th inch accuracy
• Stylus or
puck with control buttons

11. Digitizing:
Digitizing:
A process that uses a device to extract
spatial features from paper maps or
photos to electronic GIS format
Digitizing hardware:
- table or tablet
- grid of wires embedded in surface
- grid generates a magnetic field
that can be detected by a cursor
- cursor is a small flat device that
contains a wire cross hair
- cursor keypad allows special
GIS functions to allow faster
data capture
Digitizers are available at different
sizes (A4, A3, A2, A0) and different
accuracy (0.05 mm)

12. Digitizer with puck

13. Heads Down Digitizing
• Digitizing table or tablet

14. Numonics Accugrid A90 digitizer

15. The Digitizing Procedure
• Affixing the map to the digitizer
• Registering the map

16. Tablet Digitizing
• Tablet digitizing requires a person to enter
coordinate information through the use of a
digitizing tablet and digitizing puck
– A digitizing tablet is a hardened surface with a
fine electrical wire grid under the surface.
– A digitizing puck is an electrical device with cross
hairs and multiple buttons to perform data entry
operations
– An operator then enters the information using
the puck.

17. • Digitizing tablets
– Transform wire
intersections into
coordinates of the
tablet’s coordinate
system
17

18. Steps for digitizing
• Tape map to the digitizer
• Register control points on
the map (Tics)
18
Control
points
Map
Puck
Tape

19. Tablet Digitizing – how it works
• When the user places the digitizing
puck over a location on the tablet, and
presses one of the buttons, the wire
mesh beneath the tablet records the
location of the puck
• Digitizing tablets are very accurate,
with more expensive tablets able to
measure objects to within 0.006 mm.
This means that if you were to press the
entry button on the puck continuously
at one spot, the coordinate value
received from the tablet would only
vary by 0.006 mm.
• The coordinate, as referenced by the
tablet is then stored in the computer.
x,y

20. Georeferencing
• at least 3 control points
reference points or tics
• easily identifiable on the map
• exact coordinates need to be known
East of Greenwich
72°71° 73°
72°71° 73°
11°
12°
11°
12°
South
Tic Points
Origin:
X = 4 in.
Y = 5 in.
Digitizing Table Coordinates
Entered:
Tic 1: 11° 15' N
30° 30' E
Tic 2: 11° 15' N
73° 30' E

21. Digitizing Modes
• Point mode
– most common
– selective choice of points digitized
– requires judgment
– for man-made features
• Stream mode
– large number of (redundant) points
– requires concentration
– For natural (irregular) features

22. Problems With Digitizing
• Paper instability
– Humidity-induced shrinking of 2%-3%
• Cartographic distortion, aka displacement
• Overshoots, gaps, and spikes
• Curve sampling

23. Problems or Errors using Digitizing:
- Paper maps can stretch/ shrink. Lose accuracy.
- Paper maps meant to display information, not record locational
(x,y) information.
- Discrepancies across map sheets to digitize. (ie roads do not
match when 2 maps digitized.)
- User error such as overshoots, undershoots, or spikes
Overshoot Undershoot Spike

24. Errors From Digitizing
• Fatigue
• Map complexity
– ½ hour to 3 days for a single map sheet
• Sliver polygons
• Wrongly placed labels
5 86 7

25. 25
Advantages of Digitizing
• Low initial capital cost
• Flexible and adapts to different types of data
• Easily mastered skill
• Digitizing devices are reliable
• Generally the quality of data is high

26. 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)

27. Automated scanning of paper maps can cause problems:
-complex line work provides a greater chance of error
-contour lines cannot be broken with text
-feature recognition not easy (road versus 2 contour lines)

28. Types of scanners:
1. Flat Bed Scanner
2. Rotating drum Scanner

29. Drum scanners
• Drum scanners 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

31. powerful scanner called Drum Scanner, they scan very fast in very high quality.

32. Man putting comic book cover into a drum scanner

34. FLATBED SCANNERS

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Scanning Problems
• Higher resolutions aren’t always the answer to better
data; often the additional “noise” and resulting clean
up of data can cause higher resolution to not be the
best solution, a balance between detail and additional
manual clean-up must be struck.
• Paper maps are not “dimensionally stable” and a great
deal of variation occurs as the maps age.
• Documents must be clean (no smudges or extra marks
or lines).

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Scanning Problems #2
• Text may accidentally be scanned as line
features in automatic feature recognition.
• Specialized symbols (for example marsh or
asphalt) may not be detected as such.

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Digitizer vs Scanner
• Scanners
– Speed and ease
– Raster data without
intelligence; manual or
automatic vectorisation
possible.
– Usually produces large files
that need compression
– Hardware is expensive
• Digitizers
– Labor intensive
– Requires skilled operator
– Vector (intelligent) data
– Labor intensive
– Hardware less expensive