Tuesday, September 22, 2015

MANUAL DIGITIZING

The most common method of encoding spatial features from paper maps is manual digitizing. It is useful when a selection of features are required from paper maps. Manual digitizing is also used for map encoding where it is important to reflect the topology of features, since information about the direction of line features can be included. It is also used for digitizing features of interest from hard-copy aerial photographs.
Manual digitizing requires a table digitizer that is linked to a computer workstation. The table digitizer is a large flat tablet, whose surface is laid over a very fine mesh of wires. Attached to the digitizer via a  cable is a cursor that can be moved freely over the surface of the table. Buttons on the cursor allow the user to send instructions to the computer. The position of the cursor on the table is registered by reference to its position above the wire mesh.
Most manual digitizers are used either in the point mode or stream mode. In the point mode, each line segment is digitized beginning with a start node. Change in direction of the line is recorded with a digitized point and the line segment ends with an end node. Thus, complex lines require a greater number of points between start and end nodes. In this context, smooth curves are difficult to digitize as they require an infinite number of points to record their true shape. The minimum resolution is governed by the distance between the wires in the digitizing table (typically 0.1mm). Some digitizing packages allow the user to record smooth curves as mathematically defined splines or bezier curves.
In the stream mode, the digitizer records points according to a stated time interval or on a distance basis. An end node is required to stop the digitizer recording further points. The speed at which cursor is moved above the line determines number of points recorded. Choice between point mode and stream mode is a matter of personal preference.
Stream mode digitizing requires more skill than point mode digitizing. Stream mode usually generates more points and hence larger files than point mode.