Oracle Spatial User's Guide and Reference - InfoLab

SDO_UTIL.SIMPLIFY
Examples
For example, the measure values in a returned LRS geometry will probably not
reflect actual measures in that geometry. In this case, depending on your application
needs, you might have several options after the simplification operation, such as
ignoring the new measure values or redefining the new LRS geometry to reset the
measure values.
This function uses the Douglas-Peucker algorithm, which is explained in several
cartography textbooks and reference documents. (In some explanations, the term
tolerance is used instead of threshold; however, this is different from the Oracle
Spatial meaning of tolerance.)
The returned geometry can be a polygon, line, or point, depending on the geometry
definition and the threshold value. The following considerations apply:
■ A polygon can simplify to a line or a point and a line can simplify to a point, if
the threshold value associated with the geometry is sufficiently large. For
example, a thin rectangle will simplify to a line if the distance between the two
parallel long sides is less then the threshold value, and a line will simplify to a
point if the distance between the start and end points is less than the threshold
value.
■ In a polygon with a hole, if the exterior ring or the interior ring (the hole)
simplifies to a line or a point, the interior ring disappears from (is not included
in) the resulting geometry.
■ Topological characteristics of the input geometry might not be maintained after
simplification. For a collection geometry, individual elements that did not
overlap before simplification might now overlap. If overlapping of elements
occurs in a multipolygon, the geometry is invalid because Open GIS
Consortium rules state that polygon elements may not overlap in a
multipolygon. Under certain conditions, single polygon geometries might also
become invalid.
The following example simplifies the road shown in Figure 7–20 in Section 7.7.
Because the threshold value (6) is fairly large given the input geometry, the
resulting LRS line string has only three points: the start and end points, and (12,
4,12). The measure values in the returned geometry are not meaningful, because this
function considers only two dimensions.
SELECT SDO_UTIL.SIMPLIFY(
SDO_GEOMETRY(
3302, -- line string, 3 dimensions (X,Y,M), 3rd is linear ref. dimension
NULL,
19-32 Oracle Spatial User’s Guide and Reference