Tuesday, September 22, 2015

SCANNERS (AUTOMATIC DIGITIZING)

Manual digitizing is a time-consuming and tedious process. In order to digitize several complex maps, two expensive alternatives currently in use are:
-Scanning and
-Automatic line following

Scanning is the most common method used for automatic digitization. This method is mainly used when raster data are required since it is the automatic output format from most scanning software. Scanned image of a topographic map can be used as a background raster data set for plotting a vector infrastructure data (such as pipelines or cables). In such cases, the raster background image is very useful as a context for the data.
A scanner is a piece of hardware for converting an analog source document into digital raster format. All scanners work by sampling the source document using transmitted or reflected light. 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. The accuracy of scanned output data depends on:
-the quality of the scanner
-the quality of the image-processing software used to process the scanned data and
-the quality and complexity of the source document

The resolution of the scanner used affects the quality and quantity of output data.
High resolution scanners produce large volumes of data. This in-turn places heavy burden on:
-computing power and
-storage space
thereby increasing time spent editing the scanned data to make it usable by a GIS

Current scanning technology can produce data of a higher quality than manual digitizing. Hence, manual digitizers are becoming obsolete.

Another form of automatic digitizer is the automatic line follower. This method is used when digital versions of clear, distinctive lines on a map are required. This method is similar to manual digitizing with the exception that it uses a laser and light-sensitive device to follow lines on a map. Scanners produce raster output while the automatic line follower produces a vector output as (x,y) coordinate strings. The data produced by this method is suitable for vector GIS. Automatic line followers are not as common as scanners due to their complexity. Automatic line followers face difficulties when digitizing features such as dashed or contour lines.