TYPES OF DATA REPRESENTATION

Unit -5

Prepared by Sampath Kathroju

HOD Civil Dept,Kshatriya College of Engineering

Data collection:3 types Sample ground data Topo maps of SOI on 1:25000 scale used to prepare base map Satellite digital dataInput 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

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

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

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

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.

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

Why digitize?

• New maps• Map features are wrong• Missing features

9GIS TUTORIAL 1 - Basic Workbook

Digitizer

• Digitizing table10” x 10” to 80” x 60”

1/100th inch accuracy

• Stylus or puck with control buttons

Digitizing:

Digitizing:

A process that uses a device to extractspatial 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 captureDigitizers are available at different sizes (A4, A3, A2, A0) and different accuracy (0.05 mm)

Digitizer with puck

Heads Down Digitizing• Digitizing table or tablet

Numonics Accugrid A90 digitizer

The Digitizing Procedure

• Affixing the map to the digitizer

• Registering the map

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.

• Digitizing tablets– Transform wire

intersections into coordinates of the tablet’s coordinate system

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Steps for digitizing

• Tape map to the digitizer• Register control points on

the map (Tics)

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Control points

Map

Puck

Tape

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

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

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

Problems With Digitizing

• Paper instability– Humidity-induced shrinking of 2%-3%

• Cartographic distortion, aka displacement• Overshoots, gaps, and spikes

• Curve sampling

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

Errors From Digitizing• Fatigue• Map complexity– ½ hour to 3 days for a single map sheet

• Sliver polygons

• Wrongly placed labels

5 86 7

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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

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)

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)

Types of scanners: 1. Flat Bed Scanner

2. Rotating drum Scanner

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

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

Man putting comic book cover into a drum scanner

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