Data Source of 3D City Models
The need for 3D city models is growing and expanding rapidly in a variety of fields. In a steady shift from traditional 2D-GIS toward 3D-GIS, a great amount of accurate 3D city models has become necessary to be produced in a short period of time and provided widely on the market.
Compared to traditional modeling techniques for 3D city models, new techniques have resulted in reduction of time in production, efficiency in texture mapping and increased quality and accuracy of 3D city models through automatic generation of programs.
New techniques use various methods of data capture for generation of 3D city models –
The Photogrammetric method is proven and provides exact and definite interpretation results.
Laser Scanning Method
The laser scanner method provides a large quantity of unstructured elements it cannot be used optimally for achieving the interpretation processing.
The above mentioned methods can further be categorized as aerial imagery, terrestrial imagery, satellite imagery and laser scanner data. These are used as raw data for developing 3D city models.
In aerial imagery, the building footprints are considered as roof boundaries because the footprints are usually not visible on the ground level. The 3D city model obtained from serial and ground-based laser scans, and oblique imagery can be combined for texture mapping.
For capturing and creating the 3D point cloud, the stereo pairs of the images are looked-for. Manual feature extraction for base map production from stereo aerial images has been done by photogrammetry operators. Photogrammetric methods are well suited for the economic acquisition of 3D city models as it allows to recover the structure and the dimensions.
If the 3D models developed through airborne data are used in applications like realistic visualizations of urban scenes from terrestrial viewpoints, a refinement using terrestrial data collection is required.
Terrestrial images provide highly detailed images and accurate volume calculation even with a small number of input images. Photo realistic visualizations of building facades require the availability of texture from terrestrial images.
For applications aiming on the highest visual quality, the geometry of the available building model must be improved by terrestrial images. If geometric data capture is aspired, the airborne data is almost equivalent to terrestrial images.
The satellite imagery is based upon the principle of reflected sunlight. Medium and high resolution satellite imagery can be utilized by trained photo geologists to interpret surface lithology's, identify clay, oxides, soil types and identify potential locations of minerals. In case of large areas, recently high resolution satellite imagery is used. The data capturing process is the same as with aerial images, but the accuracy is less, measurement error can be up to 1 meter in height.
Laser Scanner Data
Laser scanning is a technology which uses laser beams to measure and capture environments in 3D with high speed and accuracy. It also allows to run the relief geometric objects in a short time with a high level of detail and completeness, based on the signal emitted by the laser and the corresponding return signal.
The new terrestrial laser scanner devices have increased the possibility of more accurate and complete 3D models of the acquired objects to be obtained. Laser scanning offers direct 3D data capture at high resolutions and permits to extract roof detail.
For the generation of 3D city models from laser scanner data, a density of laser scanner points of more than 2 points/sqm are required. The procedure for the calculation of 3D building models from laser scanner data uses a tangential plane as a first approximation that suits the laser scanner points.
Visualization of 3D Models
In the first attempt of 3D models visualization, small objects like architectural models, etc., were displayed in wire-frame format or using CAD packages, while terrain models were visualized in perspective wire-frame models with draping of orthophotos or orthophoto maps. Today, with the increase in computer memories, shade and texture are added to all the models. To visualize big data sets accurately, much information contained in photogrammetric models is often reduced. The consequences are –
the accuracy of the data is lost because many tools use single precision files,
the geo-referencing because most of the software has their own coordinate systems and
high resolution textures are unusable because of the control on the Level of Detail.
On the other end, low accuracy in the visualization does not attract the end-users and cannot justify the high cost of producing the photogrammetric 3D model. After the creation of a triangular mesh, the results are usually visualized, according to the used package and the requirements, in the following manners:
Wire-frame mode: It is the easiest way of representing a 3D object. It consists of points, lines and curves and describes only the edges in a transparent drawing, without texture or shading information. This technique is mainly used in computer-aided design (CAD) packages.
Shaded mode: It is based on the optical theory which states that the brightness of any small area of a perfectly diffuse undulating surface arises as the cosine of the angle of incident parallel light.
Textured mode: It is used for photo realistic visualization of the 3D models that is, image-based rendering. Texture mapping in its simplest form involves a single texture like image or orthophoto that is being mapped onto the surface composed of one or more polygons.
Creating realistic 3D models like shaded mode or textured mode helps to visualize the final result much better than a wire-frame representation.