There are following general restrictions:

§ **The
triangulation points and constraints must be projectable**: a 2d CDT can
intrinsically not triangulate points with identical xy-coordinates or recessing
caves. Therefore, triangulation points with identical xy-coordinates will be
removed during the triangulation process. The point with the highest
z-coordinate is kept.

General rule: the CDT can only “see” the
xy-projection of the triangulation points and constraints in UCS. It has no
height information during the triangulation
process.

Use the CC:POINTS:ELIM2D command to eliminate points with
identical xy-coordinates before triangulating. This keeps your input data
clean.

§ **Constraints
must not overlap and must not be self-intersecting**. Constraints may have
identical start or end points and may be collinear. However, they must not
overlap, be self-intersecting or coincident: since the CDT operates in the UCS xy-plane only,
degenerate constraints define over-determined points along their intersection
(i.e. possibly different z-heights at the same xy-coordinate).

Use the AutoCAD command _overkill to eliminate
overlapping or coincident lines.

§ **Boundaries
must be closed and linear**. The CDT only accepts closed linear polyline objects as
boundaries. The polylines must exclusively consist of line
segments.

Use the AutoCAD command _decurve to linearize a
polyline if it does not exclusively consist of line segments.

§ **Boundaries
must lie inside the convex hull of triangulation points and constraints**. Boundaries allow defining
arbitrary shaped convex and concave shaped holes, islands and bounds in the
__triangulated__ surface. This implies that boundaries can only be defined
where a triangulated surface exists, precisely being the area inside the convex
hull of triangulation points and
constraints.

§ **A Delaunay
triangulation is not unique over an evenly spaced rectangular raster.** As a
consequence, the direction of the diagonal in a raster may alter when
triangulating identical point data twice depending on the insertion order of the
triangulation points.

Figure 36: Two
valid Delaunay triangulations over a rectangular raster