3D Building Models
TerraScan provides a set of tools for building vectorization based on airborne point cloud (ALS or photogrammetric) data. The 3D vector models are created fully-automatically but for higher accuracy, they can be modified manually with dedicated tools. These tools ensure that the topology of a building model is preserved and allow fast and easy editing. In addition, TerraScan may be used for creating or updating building models stored in 3D City Databases.
According to the common way for describing building models, the models of TerraScan are at level-of-detail (LOD) 2. In LOD 2, roof shapes and the overall structure of roofs are represented but walls are just plain vertical polygons.
The automatic vectorization is based on classified points of the ground and on building roofs. Building footprints can be used in the vectorization process for placing walls or roof edges. Image data loaded in TerraPhoto supports the automatic vectorization of buildings. For manual editing, images in camera views improve the result essentially, because edges of roofs, roof structures, and smaller details may not be detectable accurately in the point cloud data.
The automatic building vectorization runs on loaded laser points using the Vectorize Buildings tool or for a TerraScan project using the Vectorize buildings macro action.
The workflow for automatic building vectorization can be outlined as follows:
1. Classify ground points using the Ground classification routine.
2. Classify high points which may be hits on building roofs using the By distance classification routine. This classification also includes points from high vegetation and other high objects.
3. Classify points on building roofs using the Buildings classification routine.
4. (Optional) If images are available, load a mission and an image list into TerraPhoto. The camera parameters of the mission and the image list should be adjusted in order to provide accurately positioned images.
6. Create vector models of buildings using the Vectorize Buildings tool for loaded points or run a macro including the Vectorize buildings macro action on a TerraScan project.
7. Review and improve building models with the help of the Check Building Models tool and tools in the Building Patches toolbox and the Building Edges toolbox.
The quality of the automatic building vectorization depends on the quality of the laser data processing that is done in preparation of the vectorization, but also on the point density of the data. A higher point density results in more accurate models. The following number may serve as a guideline for estimating the possible results of the automatic vectorization:
•Low density < 2 points / m² - good models of large buildings, more problems with small buildings, loss of small details and roof structures
•Medium density 2-10 points / m² - good models
•High density > 10 points / m² - accurate models with details and roof structures
As alternative to point cloud data, TerraScan can also utilize line elements for the creation of 3D building models. The line elements must represent different types of roof edges, such as outer edges, internal edges along elevation jumps, and intersection lines, and they must form a closed line work for each building. From the line network, the Construct Roof Polygons tool tries to create closed polygons which represent roof planes. Finally, the Create Buildings from Polygons can be used to create the 3D vector models from the roof polygons.
The major advantage of this building vectorization approach is the automatic production of 3D building models for large areas in a comparatively short time. The process can also model complex roofs that are non-planar and contain a lot of detailed roof patches. The tools for improving the result of the automatic process are versatile and make the manual work fast and simple.
A disadvantage of the vectorization process is that it fully relies on the quality of the source data, which is either laser data or a line work for building roofs. If, for example, laser data is missing on parts of a building roof, there is no way to create at least an approximate building model based on the represented roof parts.
TerraScan also provides tools for writing, reading, updating and deleting building models stored in 3D City Databases. The 3D City Database is a free geo database to store, represent, and manage virtual 3D city models on top of a standard spatial relational database. The database schema implements the CityGML standard for storing geometry and semantic information of urban objects. More information, instructions for 3DCityDB setup and examples are available on the official internet pages of 3D City Database.
You may update building models stored in a 3DCityDB or write new building models in a database. A typical update workflow can be outlined as follows:
1. Read building models stored in a 3DCityDB.
2. Review and improve building models with the help of the Check Building Models tool and tools in the Building Patches toolbox and the Building Edges toolbox.
3. If necessary, delete models of demolished buildings from the database.
4. If necessary, create models for new buildings using the Vectorize Buildings tool for loaded points or run a macro including the Vectorize buildings macro action on a TerraScan project.
5. Write modified models to the database using the method Replace by Id or Replace by centroid.
6. Write new models to the database using the method Add to database.
In this scenario, the CAD file acts only as temporary working environment. The building models are only stored in the database.
