Producing a 3D Digital City Model of a dense urban area of Hong Kong called for spatially correct and photorealistic models, deployed in a versatile and functional 3D GIS.
Hong Kong experiences extreme pressure on its urban space, with high population density, tightly knit high-rise buildings, complex architecture and a rapidly changing cityscape. This physical environment co-exists with a growing public awareness of community consultation, public planning involvement and complex planning legislation. Hong Kong’s authorities required a 3D digital city model with spatial accuracy to support legal scrutiny, photorealism to engage public consultation and the functionality to analyse and present complex planning scenarios.
With a defined 27-week schedule, AAM, which was commissioned to complete this project, utilised a number of resources to maximise the functionality of the 3D city model. A regional perspective was provided with publically available satellite imagery and SRTM terrain data. Around the project site, existing government orthophotos and terrain data increased the resolution to improve the realism of the underlying terrain and landuse. First captured oblique imagery was captured over the project area to define the built environment. This process involves capturing five simultaneous images from a fixedwing survey aircraft, from cameras pointing forward, left, behind, right and under the aircraft, thus capturing every façade of each building. Nadir imagery from this process was used to create a higher-resolution orthophoto of the project area to better reflect the land-use at the time of capture. The oblique and nadir imageries were used to construct a spatially correct building geometry of all structures within the project area. The aerial perspective allowed definition of every building, with roof-top elements captured to further increase the realism and precision of the city model. High resolution images were then used to texture every building façade to make them look photorealistic. Aerial oblique images provide a high degree of realism as they reflect what the human eye sees of the city, including sun/shadows, window reflections, advertising hoardings and all of the other visual components which make up a complex cityscape. However, a limitation of aerial imagery is that the geometry and photorealism of the lower portions of tall buildings are often affected by shadows, building awnings or street vegetation. This was overcome on key thoroughfares of this project by capturing and applying street-level imagery and adding photorealism to streetscapes.
The diagram illustrates how street-level capture can also be limited by verandas, fences, steep observation angles and vegetation.
Aerial versus terrestrial cityscape capture
Aerial imagery provides the most efficient means of defining the geometry and photo-realistic facades of a cityscape since it can define all sides of buildings and rooflines. This provides a distinct advantage over street-level capture as the latter approach is limited to the capture of faces accessible by vehicle or foot. The aerial perspective also supports multiple capture of buildings from many positions, maximising the opportunities for clear sight of the facades.
A cityscape is therefore almost always more realistic and complete when captured and textured with an aerial approach (in terms of both geometry and photo-realism), and supplemented with street-level imagery as required. This approach is also supported by commercial considerations of the project; it is more efficient to define the entire city in one aerial survey (which generally has greater startup but lower incremental costs), and then supplement with street-level capture where required (as this generally has low startup times and times).
3D visualisation and analysis
The layers collected for the project were combined to construct a 3D city model and deployed using AAM’s K2Vi software to provide 3D analysis, presentation and linkages to the client’s data layers.
The benefit of creating a virtual 3D city model is the flexibility it provides to present scenarios or proposals for wider consideration. Existing buildings can be removed from the scene, and proposed developments presented to stakeholders for their input. Variations to proposed developments can be modelled as buildings are shortened, enlarged, moved, rotated or altered. A rigorous city model with such software allows proposals to be evaluated in the knowledge that the spatial rigour and photorealism will allow the development to be evaluated on its merits.
In addition to the hard infrastructure of buildings and terrain, further realism is provided as K2Vi models and visualises the natural environment of water (including wind, waves and reflections) and atmospherics (including wind, precipitation and cloud type and height).
Hong Kong now has a realistic 3D city model of the project area which can be analysed to present development scenarios. The model has the spatial rigour to support shadow analysis, viewshed and interaction with planning zones, plus the realism to stimulate community consultation. AAM conducted user sessions to provide training on the software while stakeholder workshops reviewed the various survey methods available to roll out a structured city model over HKSAR.
The cityscape model and software provides quicker ways to find responses to various planning options. Hong Kong now has a tool which avoids past experiences where a plan proposal is often undermined during the consultation process by poor presentation. A video showing all the combined elements of the Hong Kong 3D citymodel is available by viewing Hong Kong 3D City Model by AAM under the Video tab at https://tinyurl.com/qhkeweh.