The Cadastre GML does not accept arcs: how to discretize

Summary

The plot format and the Cadastre GML do not accept arcs or circles: curves must be approximated by straight polyline segments. Two problems can therefore appear when working with curved geometry: when a closed polyline containing arcs is converted into a plot, the program warns that the plot "intersects itself"; or, when an arc or circle is exported to GML and that GML is imported again, the arc appears as a straight line. In both cases the solution is to discretize (segment) the curves into straight polyline segments beforehand, before converting to a plot or exporting to GML.

Applies to

• MDT 9, MDT 25 and MDT26 — plot creation and Cadastre GML export functions.

Symptoms

• When trying to create a plot from a closed polyline that contains arcs, the program reports that the plot intersects itself.

• When generating the GML of an arc or circle, the area is calculated correctly, but when that GML is imported again for display the arc becomes a straight line.

Cause

The plot format / Cadastre GML does not accept arcs. The Cadastre requires curves to be approximated by polyline segments, so that:

• An arc exported to GML does not keep its curved geometry and, when reimported, is drawn as a straight line between its end points.

• The process of converting a polyline into a plot may detect a self-intersection when the polyline contains arcs.

Solution

Case: creating a plot from a polyline with arcs ("intersects itself")

1. Discretize the polyline beforehand with the Discretize Polylines utility (in MDT26, Contour Lines/Mapping > Utilities > Discretize Polylines, or the MDT26_cvdispol command on the command line). The intervals are specified separately for Lines and Contours.

2. Then convert the polyline into a plot: this way it can be converted without the self-intersection warning appearing.

You can also have it done automatically: the plots configuration includes the Discretize Arcs option, which discretizes the arcs automatically when importing or converting a plot from a polyline.

Case: exporting an arc or circle to Cadastre GML without it becoming a straight line

1. Before exporting, discretize the curves (arcs or circles) of the plot into straight polyline segments with Discretize Polylines, setting the interval to the accuracy you need.

2. Then export the GML: the geometry will already consist of straight segments accepted by the Cadastre, so it will be preserved when the file is reimported.

If the problem persists

• Adjust the discretizing interval, which is set separately for Lines and Contours: a smaller interval approximates the curve better at the cost of generating more vertices. There is no single recommended value: choose it according to the accuracy the Cadastre requires and the size of the geometry.

• If the self-intersection warning still appears after discretizing the polyline, check that the geometry does not contain real crossings between segments, and contact Aplitop technical support attaching the drawing.

Category: Cadastre and GML · Level: Basic