Lines - Curving Line Features

There is often the requirement to display part or all a line feature as curved segments.  Kerbs in towns and cities are seldom straight.

The Curving comma code, normally C, specifies that the line feature generated using a given point will be curved at that point.  A curve can only be generated if one of the curve comma codes has been applied to both points of a line feature segment.  If the line segment prior to the point is straight, a discontinuity in the curve will exist.  The same applies where the following segment is straight.  If both prior and following segments are curved, there is a common tangent direction at the point.

The Curve Discontinuity comma code, normally D, specifies that if both the prior and following line feature segments are curved, there is no continuity of the curve direction through the point.

Straight Feature	Curved Feature	Curved Feature with Discontinuity

The first of the examples above show the starting point of a feature made up of straight line segments.  The second shows how curve fitting is applied to all the segments of the feature.  The third shows how a curve discontinuity can be introduced to one of the points.

The Tangent comma code, normally T, specifies that the line feature segment prior to or following the point is a tangent.  If the prior segment is straight, the direction of this segment is used to constrain the start direction of the following segment.  If the prior segment is curved, the direction of the following segment is used to constrain the end direction of the prior segment.  Some examples are shown below.

Single Tangent	Double Tangent	Quadruple Tangent

The first of the examples above shows how a series of curved segments are followed by a straight segment using a tangent point.  The second shows how a single curved segment can be created between two straight line segments by using two tangent points.  The third shows how a 4-point polygon with four tangents can be used to simulate a pedestrian refuge in a road.