# 4.4. Import¶

To load 3D city model content into a 3D City Database instance, the Importer/Exporter supports the import of CityGML and CityJSON files on the Import tab of the operations window.

Fig. 4.32 The import dialog.

Input files and formats

The list of files to be imported must be provided at the top of the import dialog [1]. Files can be selected through clicking on the Browse button. Alternatively, you can drag&drop files from your preferred file explorer onto the Import tab. If the file list already contains entries, the drag&drop operation will replace them by default. If you want to keep the previous entries and only append additional files, keep the CTRL key pressed while dropping (on Windows). The Remove button or DEL key lets you remove selected entries from the input files. Note that adding folders to the list is also supported. Each folder will be recursively scanned for input files to be imported.

The import operation supports the following file formats and extensions:

 Format File extensions CityGML versions 2.0, 1.0, and 0.4 *.gml, *.xml CityJSON version 1.0.x *.json, *.cityjson GZIP compressed files *.gz, *.gzip ZIP archives *.zip

The file formats are mainly detected based on the file extension, so please make sure to use one of the supported file extensions from Table 4.7. ZIP archives are recursively scanned for contained CityGML and CityJSON files. Additional files referenced from the CityGML/CityJSON files such as texture images will also be imported into the database if the references can be correctly resolved during import. This also holds true if the additional files are located inside the same ZIP archive as the CityGML/CityJSON files.

Caution

While even large CityGML files can be read in a streaming fashion (i.e., one top-level feature after the other), large parts of a CityJSON file must be kept in main memory while reading the entire file. To avoid memory issues, make sure the file size of the CityJSON input file is small enough, otherwise the import process will terminate with an exception. You can also increase the available memory for the Importer/Exporter application (see Section 4.1).

Import filters

The import dialog allows for setting thematic and spatial filters to narrow down the set of top-level city objects that are to be imported from the input files [2]. The following filters are offered and discussed in separate sections of this chapter:

To enable a filter, simply select its checkbox. This will automatically make the filter dialog visible. Make sure to provide the mandatory input for the filter to work correctly. If more than one filter is enabled, the filters are combined in a logical AND operation, i.e. all filter criteria must be fulfilled for a city object to be imported. If no checkbox is enabled, no filters are applied and, thus, all features contained in the input files will be imported.

Note

All import filters are only applied to top-level features but not to nested sub-features.

Schema validation

Before importing, the input files can be validated against the official CityGML XML and CityJSON schemas. Simply click the Just Validate button [4] in order to run the validation process. Filter settings are not considered in this process. Note that this operation does not require internet access since the schemas are packaged with the application. The features from the input files are not imported into the database during validation. The validation results are printed to the console window.

Note

It is strongly recommended that only input files having successfully passed the validation are imported into the database. Otherwise, errors in the data may lead to unexpected behavior, error messages or even abnormal termination of the import process.

Note

CityGML ADE schemas are automatically considered in the validation process if the ADE has been correctly registered with the Importer/Exporter (see Section 5.3 for more details). This way, also ADE data can be checked before importing. CityJSON Extension schemas are, however, not supported by the validation process. Please use an external tool like cjio to validate such datasets.

Import preferences

More fine-grained preference settings affecting the import operation are available on the Preferences tab of the operations window [5]. Make sure to check these settings before starting the import process. A full documentation of the import preferences is provided in Section 4.4.6. The following table provides a summary overview.

 Preference name Description General options like behaviour in error situations to be used for imports. Metadata that is stored for every object in the database such as the data lineage, the updating person or the creationDate property. Generates UUIDs where object identifiers are missing on input features or replaces all identifiers with UUIDs. Defines whether appearance information should be imported. Allows for applying an affine transformation to the input geometry. Address (CityGML only) Controls the way in which xAL address fragments are imported into the database. XML validation (CityGML only) Performs XML validation automatically and excludes invalid features from being imported. XSL Transformation (CityGML only) Defines one or more XSLT stylesheets that shall be applied to the city objects in the given order before import. Defines import options for CityJSON input files. Settings for automatically enabling/disabling spatial and normal indexes during imports. Creates a list of all successfully imported CityGML top-level features. Allocation of computer resources used in the import operation.

Starting the import process

Once all import settings are correct, the Import button [3] starts the import process. If a database connection has not been established manually beforehand, the currently selected entry on the Database tab is used to connect to the 3D City Database. The separate steps of the import process as well as all errors and warnings that might occur during the import are reported to the console window, whereas the overall progress is shown in a separate status window. The import process can be aborted at any time by pressing the Cancel button in the status window. The Importer/Exporter will make sure that all pending city objects are completely imported before it terminates the import process.

After having completed the import, a summary of the imported CityGML top-level features is printed to the console window.

Caution

The Importer/Exporter does not check by any means whether a top-level feature from an input file already exists in the database. Thus, if an import is executed twice on the same dataset, all CityGML features contained in the dataset will be imported twice.

One way to avoid duplicate features might be, for instance, to manually set a UNIQUE constraint on the GMLID column of the CITYOBJECT table.

Hint

To improve the speed of the import operation for a very large number of features (bulk imports), the spatial indexes can be disabled before and re-activated after the import. For a smaller number of features though, disabling and enabling the spatial indexes might take longer than the actual import itself. Normal indexes should never be disabled before an import.

Importing into version-enabled tables under Oracle typically takes considerably more time than importing into non-version-enabled tables.

Note

The import operation does not automatically apply a coordinate transformation to the internal reference system of the 3D City Database instance. Thus, if the coordinate reference system of the CityGML input data does not match the coordinate reference system defined for the 3D City Database instance, the user must transform the coordinate values before importing the data (or use an affine transformation during import if this is enough). A possible workaround procedure can be realized as follows:

1. Set up a second (temporary) instance of the 3D City Database with an internal CRS matching the CRS of the CityGML instance document.
2. Import the dataset into this second 3D City Database instance.
3. Export the data from this second instance into the target CRS by applying a coordinate transformation (see CityGML export documentation in Section 4.5).
4. The exported CityGML document now matches the CRS of the target 3D City Database instance and can be imported into that database. The temporary database instance can be dropped.

Alternatively, you can change the reference system in the database to the one used by the imported geometries (see the corresponding database operation in Section 4.3.4).