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DATA INPUT BY DIGITIZATION SCANNING
The data to be input for GIS are typically acquired in a diverse variety of forms.Some data come in graphic and tabular forms. These would include maps and photographs, records from site visits by specialists, related to non-spatial information from both printed and digital files (including descriptive information about the spatial data, such as date of compilation, and observational criteria). Other data come in digital form. These would include digital spatial data such as computer records of demographic or land ownership data, magnetic tapes containing information about topography and remotely sensed imagery. The data to be input for GIS are of different forms. These include key board entry or key coding, digitising, scanning and digital data. The process of data encoding and editing is often called as data stream.
1 DATA INPUT METHODS
Before explaining the input methods, it is necessary to make a distinction between analogue (non-digital) and digital sources of spatial data. Analogue data are normally in paper form and include paper maps, tables of statistics and hardcopy aerial photographs. All these forms of data need to be converted to digital form before use in a GIS. Digital data like remote sensing data are already in compute-readable formats and are supplied on diskette, magnetic tape or CD-ROM or across a computer network. All data in analogue form need to be converted to digital form before they can be input into GIS. There are four methods of data input which are widely used: keyboard entry, manual digitising, automatic digitisation, and scanning. Digital data must be downloaded from their source media and may require reformatting to convert them to an appropriate format for the GIS being used. Reformatting or conversionmay also be required after analogue data have been converted to digital form. For example, after scanning a paper map, the file produced by the scanning equipment may not be compatible with the GIS, so it needs reformatting. For both the analogue anrJ digital data, keyboard entry method, manual digitising and automatic digitizing and scanning methods are very important as detailed below.
2 MANUAL DIGITISING
Manual digitising is the most common method of encoding spatial features frompaper maps. It is a process of converting the spatial features on a map into a digital format. Point, line, and area features that form a map, are converted into (x, y) coordinates. A point is represented by a single coordinate, a line by a string of coordinates, and, when one or more lines are combined with a label point inside an outline, then an area (polygon) is identified. Thus digitising is the process of capturing a series of points and lines. Points are used for two different purposes: to represent point features or to identify the presence of a polygon. Manual digitising requires a table digitiser that is linked to a computer work station (Fig 10.2). To achieve good results, the following steps are necessary. Before, discussing these steps, the description of digitisers is provided for the beginners in this field of technology. Digitisers are the most common device for extracting spatial information from maps and photographs. The position of an indicator as it is moved over the surface of the digitizing tablet is detected by the computer and interpreted as pairs of x, y coordinates. The indicator may be a pen-like stylus or a cursor. Frequently, there are control buttons on the cursor which permit control of the system without having to turn attention from the digitising tablet to a computer terminal. The current most popular digitiser is contemporary tablets using a gird of wires embedded in the tablet to a generate magnetic field which is detected by the cursor. The accuracy of such tables are typically better than 0.1 mm which is better than the accuracy with which the average operator can position the cursor. Sometimes the functions for transforming coordinates are built into the tablet and used to process data before it is sent to the host.
3 THE DIGITIZING OPERATION
The map is affixed to a digitizing table. Three or more control points are to be identified and digitized for each map sheet. These points should be those that can be easily identified like intersections of major streets and prominent landmarks. The points are called reference points or tics or control points.
The coordinates of these control points will be known in the coordinate system to be used in the final data, such as, latitude and longitude. The control points are used by the system to calculate the necessary mathematical transformations to convert all coordinates to the final system. The more the control points, the better the accuracy of digitisation. Digitising the map contents can be done in two different modes: point mode and stream mode. Point mode is the mode in which the operator identifies the points to be captured explicitly by pressing a button, and stream mode is the mode in which points are captured at set time intervals, typically 10 per second, or on movement of the cursor by filed distance. Most digitizing is currently done in point mode.
Problems with Digitising Maps
The problems that come during the process of converting the maps into digital mode through the process of digitisation vary from one CAD operator to another. It depends upon the experience and skill of the operator and density of points, lines and polygons of the map. The accuracy of the output of the digitisation also depends upon the selection and distribution of the control points. Some of the commonly occurred problems during the digitisation of any paper map are as follows:
(i) Paper maps are unstable; each time the map is removed from the digitising table, the reference points must be re-entered when the map is affixed to the table again.
(ii) If the map has stretched or shrunk in the interim, the newly digitised points will be slightly off in their location when compared to previously digitised points.
(iii) Errors occur on these maps, and these errors are entered into the GIS data base as well.
(iv) The level of error in the GIS database is directly related to the error level of the source maps.
(v)Maps are meant to display information, and do not always accurately record vocational information.
A digital image of the map is produced by moving an electronic detector acrossthe map surface. The size of the map area viewed by the detector and scanning should be processed or edited to improve the quality and convert the raster to vector after online digitisation. The accuracy of the scanned output data depends on the quality of the scanner, the quality of the software used to process the scanned data, and the quality of the source document. A very important feature that a GIS user should observe after scanning the paper map is the occurrence of splines, which is black appearance on the scanned output. This can be removed by using a process called thinning.
The resolution of the scanner used affects the quality and quantity of outputdata. The cheaper flat-bed scanners have resolutions of 200-500 mm whereas the more expensive drum scanners use resolutions of 10-50 mm. The higher the resolution, the larger the volume of the data produced.
Scanning and Automatic Digitising
Scanning is the most commonly used method of automatic digitising. 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 as a background raster dataset for the over-plotting of vector infrastructure data, such as, pipelines and cables.
A scanner is a piece of hardware Fig for converting an analogue source document to a digital raster format (Jackson Woodsford, 1997). There are two types of scanners, (i) Flatbed scanner and (ii) rotating drum scanners. The cheapest scanners are small flatbed scanners, and high quality and large format scanners are rotating drum scanners in which the sensor moves along the axis of rotation.
Fig 5.4 Scanned output map
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