Geoscience Reference
In-Depth Information
2 Automatic Validation
2.1 Validation Rules
A validated a data set is expected to be clean, correct and useful for a given
application. This implies that different sets of validation rules exist, depending on
the intended application.
We separate the validation process in two general steps: first a schema valida-
tion for CityGML data to assure schema conformant input to the second step, geo-
metric and semantic validation of the data set. Only the second step is discussed
here because XML schema validation is a standard procedure for which sophisti-
cated tools are available.
For the basics of geodata validation we refer to the explanations in Wagner
et al. (
2013
).
The Special Interest Group 3D has developed guidelines for modeling of 3D
city models. The goal is to clearly specify valid alternatives and recommend
one of them for general usage. This should lead to city models with known
specifications in contrast to the situation today where only the modeler knows
how certain features are reproduced. These recommendations are the base for
geometric validation rules which have been developed and implemented as part
of the research project
CityDoctor
at
University of Applied Sciences Stuttgart
,
Germany. The geometric model as described by Gröger and Plümer (
2011
)
is used.
In addition, some geometric-semantic rules resulting from CityGML require-
ments are included as plausibility checks for consistency of the data set.
A short listing of the checks is given in the following, more detailed explana-
tions are given in Wagner et al. (
2013
). The algorithms and the underlying data
structure are suitable for CityGML LODs 1 and 2.
2.1.1 Polygon Checks
1. A linear ring must consist of a minimum of 4 points.
2. First and last point of a linear ring are identical.
3. All points of a linear ring
R
are different, with exception of first and last
point.
4. Two edges can intersect only in one start-/end point. Other points of intersec-
tion or touching are not allowed (to account for rounding errors or polygons
which are not perfectly planar, a small tolerance is allowed).
5. All points of the polygon must be located in a plane (a small tolerance is
allowed).
NB: since we consider only outer rings, polygons with holes are not processed
currently (they occur only rarely in LOD 1 or 2).
