## 3 Proposed Procedure
### CityGML Transformation and Schema Validation
#### 3.5.2 CityGML Schema Validation  
Following the CityGML transformation, it is essential to validate whether the transformed data comply with the CityGML 3.0 standard schema. In this study, we employed the CityGML Validator tool to automatically verify the consistency of the transformed data.  
It was confirmed that the LoD1 object and texture information generated as shown in Table 13, was accurately converted in accordance with the CityGML 3.0 standard, and each object and attribute was properly defined. Depending on whether the corresponding CityGML file fails/succeeds, the error information of the file can be found in the detailed items.  
The validation process consists of three main components:  
* Since CityGML 3.0 utilizes XML-based data formats, it is crucial to verify that the transformed XML files comply with CityGML 3.0 XSD (schema definition) specifications.
* This study validated the transformed CityGML data against the CityGML 3.0 XSD files to ensure there were no schema violations. Key validation criteria include the following:  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**No.** & **File Name** & **Status** & **Details** \\ \hline
1 & Building.gml & Fail & Missing schema definition \\
2 & Load.gml & Success & Success \\
3 & Transportation.gml & Fail & Geometry validation error \\
4 & AdministrativeDistrict.gml & Success & Success \\ \hline \hline \end{tabular}
\end{table}
Table 13: CityGML Validation Results Compliance Check Summary with Success and Error Details.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**No.** & **CityGML Module** & **Namespace Prefix** \\ \hline
1 & CityGML Core & core \\
2 & Appearance & app \\
3 & Building & build \\
4 & CityObjectGroup & grp \\
5 & Generics & gen \\
6 & Transportation & tran \\
7 & TexturedSurface & tex \\ \hline \hline \end{tabular}
\end{table}
Table 12: List of CityGML mapping modules and their associated namespace prefixes.  
* Verification of attribute definitions (such as gml:id and gml:posList) in accordance with the CityGML schema.
* Examination of correct XML structure implementation for objects including buildings, transportation, and ReliefFeature.
* Validation of coordinate and geometric structure (gml:Polygon, gml:Surface) compliance with schema requirements. To verify the geometric consistency of transformed 3D objects, we employed Topology Checker and OGR Geometry Validator to examine the geometric structure of each object. The primary validation criteria include the following:
* Verification of closed surface conditions for building objects.
* Assessment of correct TIN structure implementation in terrain mesh.
* Inspection for duplicate gml:id or reference errors. Using the CityGML Validator, we examined the proper definition of object attributes and reviewed for the inclusion of undefined properties. Key error validation criteria include the following:
* Verification of proper BuildingPart and BuildingInstallation relationships within building objects.
* Assessment of clear definitions for TrafficArea and AuxiliaryTrafficArea in road objects.
* Validation of elevation value representation (Z-values in gml:posList).  
The verified CityGML file is shown in Figure 8.  
Figure 8: Generated CityGML file of result.