Spatial Data Entry – Specific Theory
introduction The input of data from analogue maps required the conversion of the features into coordinate values. Digitising is the transformation of information from analog format, such as a paper map, to digital format, so that it can be stored and displayed with a computer . Digitising can be manual, semi-automated (automatically recorded while manually following a line), or fully automated (line following). Manual digitising involves an operator using a digitising table (or tablet) (known as heads-down digitising), or with the operator using a computer screen (heads-up digitising). The digitising table has a fine grid of wires embedded in it that acts as a Cartesian coordinate system. The coordinate may be in plane or geographic coordinates. The procedure involves tracing map features in the form of points, lines or polygons with a mouse (puck) which relays the coordinate of each sample point to be stored in the computer (see Figure 1). The tablet and puck acting together with the computer can locate the puck s position relative to reference information provided by the operator. There are two modes of digitising: point-mode and stream-mode (see Figures 2 and 3). The resolution of coordinate data is dependent on mode of digitizing. In point-mode the digitizing operator specifically selects and encodes those points deemed ‘critical’ to represent the geomorphology of the line or significant coordinate pairs. This requires some knowledge about the line representation that will be needed. On-screen digitising is an interactive process in which a map is created using previously digitised of scanned information. This method of geocoding is commonly called ‘heads-up’ digitising because the attention of the user is focused up on the screen, and not on a digitising tablet. This technique may be used to trace features from a scanned map or image to create new layers or themes. On-screen digitising may also be employed in an editing session where there is enough information on the screen to accurately add new features without a reference image or map. Source.