| # GRASS Help System

## THE DIGITIZING THRESHOLD

Digitizing using the GRASS v.digit program may be done in either point or stream mode. In point mode, a point is collected at each location specified by the operator. In stream mode, the operator traces a continuous line, and points are collected automatically as the the line is traced. When digitizing in stream mode, the digitizer picks up many points, often more than are needed to adequately depict a line. To limit the total number of points in a data file, a pruning of the original points occurs before the points are saved in a data file. The digitizing threshold is used to determine which points are saved and which are thrown out.

The pruning process occurs during digitizing each time the user accepts a line. It is achieved by setting up a sort of corridor the width of the digitizing threshold. The direction of this corridor is determined by the first two points that were collected by the digitizer. All subsequent points are examined to determine whether they fall within the corridor limits. The first point which lies outside the corridor effectively establishes a new direction for the line. The program saves the original first point of the line segment and the last point that is found to be inside the corridor. The intervening points are discarded, as they are unnecessary to define the line segment. A new corridor is now established between the second saved point and the point collected immediately after it and the process continues.

As a result of this process, the curved portions of a line will contain more points, while a straight portion will have fewer. Theoretically, the user could do what is essentially point mode digitizing in stream mode. In cases where the straight line segments are short and interspersed with curved lines this could be done. It is not reccommended that long straight line segments be done in stream mode, however. They are done more efficiently in point mode. The default digitizing threshold is 0.03 inches, which is equivalent to about .75 mm. This level works quite well when digitizing curved lines of a fairly complex nature. If the user is not satisfied with the default threshold, it may be changed using the v.digit "Customize" menu. A larger threshold (e.g., 0.05 inches) will result in fewer points being saved; a smaller threshold (e.g., 0.02 inches) will increase the number of points saved. The default digitizing threshold is automatically reset at the beginning of each "v.digit" session.

Read "Cartographic Issues in the Development of a Digital GRASS Database" (Ruiz and Messersmith) for further information on map development issues.

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Automatically created on: Sat Oct 15 07:07:26 2005