Editing Content :: Admin Scraper

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The manual data input methods depend on whether the database has a vector topology or grid cell (raster) structure. The most common ways of inputting spatial data in to a GIS are through: • Digitisation • Scanning With the entity model, geographical data are in the form of points, lines and/ or polygons (areas)/pixels which are defined using a series of coordinates. These are obtained by referring to the geographical referencing systems of the map or aerial photograph, or by overlaying a graticule or grid onto it. The use of digitisers and the scanners greatly reduce the time and labour involved in writing down coordinates. We shall, briefly, discuss how the spatial data are created in GIS core using a scanner. Scanners Scanners are the devices for converting analogue data into digital grid-based images.Theyareusedinspatialdatacapturetoconvertalinemaptohigh-resolution raster images which may be used directly or further processed to get vector topology. There are two basic types of scanners : • Scanners that record data on a step-for-step basis, and • Those that can scan whole document in one operation. The first type of scanners incorporates a source of illumination on a movable arm (usually light emitting diodes or a stabilised fluorescent lamp) and a digital camera with high-resolution lamp. The camera is usually equipped with special sensors called Charged Coupled Devices (CCDs) arranged in an array. These are semi-conductor devices that translate the photons of light falling on their surface into counts of electrons, which are then recorded as a digital value. The movement of either the scanner or the map builds up a digital 54 two-dimensional image of the map. The map to be scanned can be mounted either on a flat bed, or on a rotating drum. With flatbed scanners, the light source is moved systematically up and down over the surface of the document. For I r largemaps,scannersareusedwhicharemountedonastandandtheillumination P source and camera array are fixed in a position. The map is moved past by a , feedingmechanism.Moderndocumentscannersresemblelaserprintersinreverse p because the scanning surface is manufactured with a given resolution of light r sensitive spots that can be directly addressed by the software. There are no e G moving parts except a movable light source. The resolution is determined by the i geometry of the sensor surface and the amount of memory rather than by a k mechanical arm. o The scanned image is always far from perfect even with the best possible l scanners, as it contains all the smudges and defects of the original map. The c excess data, therefore, in a digital image must be removed to make it usable. c r P Entering the Attribute Data Attribute data define the properties of a spatial entity that need to be handled in the GIS, but which are not spatial. For example, a road may be captured as a set of contiguous pixels or as a line entity and represented in the spatial part of the GIS by a certain colour, symbol or data location. Information describing the type of road may be included in the range of cartographic symbols. The attribute values associated with the road, such as road width, type of surface, estimated number of traffic and specific traffic regulation may also be stored separately Reprint 2026-27