Unit – II
Data Input and Editing
GIS data almost always need to be corrected and manipulated to ensure that they
can be structured according to the required data model.
Problems that may have to be addressed at this stage of a GIS project include:
1. the re-projection of data from different map sources to a common projection;
2. the generalization of complex data to provide a simpler data sets;
3. the matching and joining of adjacent map sheets once the data are in digital
form.
Methods of data input
Data in analogue or digital form need to be encoded to be compatible with the
GIS being used.
Possible Encoding Methods for Different Data Sources
S.no Data Source Possible Encoding Methods
Analogue
1 Keyboard entry
Tabular Data
Text Scanning
2 Manual Digitizing
Map data Automatic Digitizing
Scanning
3 Manual Digitizing
Aerial Photographs Automatic Digitizing
Scanning
Digital
1 Tabular Data Digital File Transfer (with reformatting if necessary)
2 Map data Digital File Transfer (with reformatting if necessary)
3 Aerial Photographs Digital File Transfer (with reformatting if necessary)
4 Digital File Transfer
Satellite Imagery
Image Processing and Reformatting
,Methods of data input/ encoding are
1. Keyboard entry
2. Scanning
3. Digitizing
4. Electronic Data Transfer
Keyboard Entry
Keyboard entry, often referred to as keycoding, is the entry of data into a file at a
computer terminal. This technique is used for attribute data that are only available on
paper.
Text scanners and optical character recognition (OCR) software can be used to
read in data automatically. Attribute data, once in a digital format, are linked to the
relevant map features in the spatial database using identification codes. These are unique
codes that are allocated to each point, line and area feature in the data sets.
The co-ordinates of spatial entities can be encoded by keyboard entry, although
this method is used only when co-ordinates are known and there are not too many of
them.
Scanning
Scanning is the most commonly used method of automatic digitizing. Scanning is
an appropriate method of data encoding when raster data are required, since this is the
automatic output format from most scanning software. Thus, scanning may be used to
input a complete topographic map that over-plotting of vector infrastructure data.
A scanner is a piece of hardware for converting an analogue source document into
digital raster format. (Jackson and Woodsford, 1991)
The cheapest scanners are small flat-bed scanners. High-quality and large format
scanners require the source document to be placed on a rotating drum, and a sensor
moves along the axis of rotation.
i) Flat-bed Scanner
The map is placed on the flat bed and the detectors of the scanner moves across
the analog map. This type of scanners available in A4 to A3.
, ii) Rotating Drum Scanner
In this type of scanner, when a map is placed over a drum the detectors of scanner
a part of the map; then the drum rotates along with the map. This process continues, till
the entire map is scanned. The scanned part of the map will appear on the monitor; line
by line scanning can be seen. If the scanned images is not clear; the scanning of map can
be repeated all over again.
iii) Large-format Scanner
Maps/satellite imageries of small scale purchased from Survey of India, National
Remote Sensing Centre are of large size. These data can be scanned using large format
scanners or to size scanners. This is similar to a rotating drum scanner. To scan a map,
start and end value i.e the width of the map has to be specified.
Data Input and Editing
GIS data almost always need to be corrected and manipulated to ensure that they
can be structured according to the required data model.
Problems that may have to be addressed at this stage of a GIS project include:
1. the re-projection of data from different map sources to a common projection;
2. the generalization of complex data to provide a simpler data sets;
3. the matching and joining of adjacent map sheets once the data are in digital
form.
Methods of data input
Data in analogue or digital form need to be encoded to be compatible with the
GIS being used.
Possible Encoding Methods for Different Data Sources
S.no Data Source Possible Encoding Methods
Analogue
1 Keyboard entry
Tabular Data
Text Scanning
2 Manual Digitizing
Map data Automatic Digitizing
Scanning
3 Manual Digitizing
Aerial Photographs Automatic Digitizing
Scanning
Digital
1 Tabular Data Digital File Transfer (with reformatting if necessary)
2 Map data Digital File Transfer (with reformatting if necessary)
3 Aerial Photographs Digital File Transfer (with reformatting if necessary)
4 Digital File Transfer
Satellite Imagery
Image Processing and Reformatting
,Methods of data input/ encoding are
1. Keyboard entry
2. Scanning
3. Digitizing
4. Electronic Data Transfer
Keyboard Entry
Keyboard entry, often referred to as keycoding, is the entry of data into a file at a
computer terminal. This technique is used for attribute data that are only available on
paper.
Text scanners and optical character recognition (OCR) software can be used to
read in data automatically. Attribute data, once in a digital format, are linked to the
relevant map features in the spatial database using identification codes. These are unique
codes that are allocated to each point, line and area feature in the data sets.
The co-ordinates of spatial entities can be encoded by keyboard entry, although
this method is used only when co-ordinates are known and there are not too many of
them.
Scanning
Scanning is the most commonly used method of automatic digitizing. Scanning is
an appropriate method of data encoding when raster data are required, since this is the
automatic output format from most scanning software. Thus, scanning may be used to
input a complete topographic map that over-plotting of vector infrastructure data.
A scanner is a piece of hardware for converting an analogue source document into
digital raster format. (Jackson and Woodsford, 1991)
The cheapest scanners are small flat-bed scanners. High-quality and large format
scanners require the source document to be placed on a rotating drum, and a sensor
moves along the axis of rotation.
i) Flat-bed Scanner
The map is placed on the flat bed and the detectors of the scanner moves across
the analog map. This type of scanners available in A4 to A3.
, ii) Rotating Drum Scanner
In this type of scanner, when a map is placed over a drum the detectors of scanner
a part of the map; then the drum rotates along with the map. This process continues, till
the entire map is scanned. The scanned part of the map will appear on the monitor; line
by line scanning can be seen. If the scanned images is not clear; the scanning of map can
be repeated all over again.
iii) Large-format Scanner
Maps/satellite imageries of small scale purchased from Survey of India, National
Remote Sensing Centre are of large size. These data can be scanned using large format
scanners or to size scanners. This is similar to a rotating drum scanner. To scan a map,
start and end value i.e the width of the map has to be specified.
