3D Geospatial city – Developed Through Digital Photogrammetry & 3D Visualization

Abstract
A fully functional and practical 3D geospatial city is not a dream out of reach. Two cities – Helsinki and Copenhagen, are good exemplarity on how a citywide 3D model can be beneficial and crucial in upgrading the infrastructure of the city. With the increasing demand on accurate and informative data for inter-departmental collaborations, public utilities maintenance, information publishing of various importance, land base information system, visualization consistency of urban development etc, it's not hard to foresee the vital need of a workable and well-rounded 3D model for urban area.

Both cities utilize digital photogrammetry and 3D visualization to serve as the significant input and backbone, to venture into the materialization of 3D geospatial city. For instance, in Helsinki, orthophoto mosaics produced from digital photogrammetry are being used for mapping outdated features and vectorizing buildings and streets in 3D. In Copenhagen, digital photogrammetric mapping serves as a basis for the precise registration of all buildings in the city. On the other hand, 3D visualization techniques are crucial to support the desired analysis, outcomes & services generated from the 3D city models.

As a conclusion, the combination of digital photogrammetry and 3D visualization provides strong platform for a good 3D geospatial city governance.

1 Introduction
It is undeniable that the need of 3D geospatial information is increasing rapidly, especially in 3D urban planning & development, in which brilliant project visualizations & analysis are needed to pave an efficient way for town planning and public administration. Hence, this results the initiation of 3D geospatial city, which integrates various disciplines / technologies of geospatial information system (GIS) to cater the needs of various authorities and industries.

3D geospatial city – developed through digital photogrammetry and 3D visualization, is the ultimate answer on how one can administer a city effectively and transparently in terms of geospatial information management, with a good and interesting emphasis on presentation medium that can connect and increase the involvement of the public. With digital photogrammetry serves as the economical yet reliable input of geospatial data, and 3D visualization as the tool to effectively construct-visualize-manipulate-explore-navigate 3D spatial analysis, such 3D geospatial city deserves to be in the limelight. This paper focuses on two cities – Helsinki and Copenhagen, in their much anticipated efforts on developing their 3D geospatial city, by utilizing digital photogrammetry and 3D visualization. This paper also elaborates the benefits of 3D geospatial city, and provides a glimpse of the challenges encountered.

2 Helsinki – From 2D to Accurate 3D Model
Helsinki – the capital of Finland is the centre of government and the home of the ministries and state administration. With a population of about 560,000, covering a land area of 187 sq. km, Helsinki foresees a significant role of a 3D city model. Hence, some 3D work had begun as early as 1986 for city planning and construction projects.

The Helsinki City Survey Division is leading the migration from 2D to 3D currently, with 142 people being employed, 40 of whom work in its GIS center. The Survey Division has been taking 2D basemaps at a scale of 1:500 and converting the data to an accurate 3D city model. As new data is gathered, the GIS and 3D databases are continuously and automatically updated, ensuring all departments have access to current information.

The City Survey Division is responsible for all city mapping functions, including maintaining a geodetic network, basemap and utility map 1:500, legal maps for city planning, topographic maps, and tourist maps. Additionally, it is responsible for maintaining the legal parcel cadastre and surveys, parcel IDs, building IDs, aerial photography, parcel addresses and updates, and the city's Web GIS services and GIS coordination.

In 2002, a general 3D model of the whole city was completed from the city's building cadastre, orthophotos, and current 2D / 3D basemaps. This model was used in the Helsinki Master Plan 2002 project, which was later approved by the Helsinki City Council. Three scales of 3D models are used in Helsinki – a general 3D city model, an exact 3D model as the new 1:500 basemap, and an exact 3D model with textures included. In these models the Survey Division can provide users a variety of information such as building cadastre information, streets (break lines), trees, and vegetation. It can also add orthophoto coverage on land and facade photos to the buildings.

To establish an authoritative 3D city model, each procedure and process in the production has to be closely monitored. Descriptive rules for 3D map, the structures of the map database, the new 3D objects and features, as well as the working orders for all steps taken in map updating process have all been created. Despite the continuous updating of the new 3D map database, the conversion of the current 2D map database (to 3D) is still on-going. To date, all updates that have been done since year 2000 have all been converted to 3D, in terms of file format and content elements.

3 Copenhagen – New Perspectives on Urban Development
Being the capital of Denmark, Copenhagen has been long known as one of the most popular international city for tourists in Europe. With a municipal area of 88 sq. km and population of 503,699, Copenhagen is one of the world's most attractive international congress cities. Thus, a reliable 3D city model with realistic visualizations is deemed imperative to allow better decisions and public input.

A number of leading international consulting companies specialized in mapping and GIS have involved in developing & supporting 3D city model of Copenhagen, helmed by the City Surveyor. One of the most impressive projects is a model of Copenhagen consisting of more than 130,000 buildings, all of which are three-dimensionally correct. With the unique level of detail (LOD) of each building, the model will not just be a snapshot of Copenhagen, but will be continuously updated to provide a 3D record of the urban environment.

The urban area is characterized by its buildings, roof shapes, bays, towers, chimneys, and so on. Curved and double-curved shapes, such as those of church towers and castles, require numerous vectors to describe their forms correctly. The city model must include all of these individual features and be easily recognizable. Besides, all essential trees and dense growths of trees are registered in their exact positions and actual heights, which are important for various visualizations. It is critical during the production process to be extremely accurate in object registration. A comprehensive specification containing detailed instructions must be developed prior to full-scale production. Detailed technical specifications and work procedures are essential for the production of complete, accurate, high-quality 3D city models. This includes:

• the accuracy of buildings better than 10cm to 15cm (plan and height)
• the accuracy of DEM better than 50cm
• details in building outlines including 30cm in the plan, and 45cm in the height
• roof details bigger than 3 meters are included, like dormers and chimneys
• roof bends larger than 45cm
• all supplementary new buildings and extensions
• trees taller than 4 meters

So far, more than 175,000 trees and essential group of trees have been registered in the 3D city model, and more than 30,000 man hours have been spent on the production. The yearly updates of the 3D city model are in parallel to the maintenance of the city's basemaps.

4 Digital Photogrammetry & 3D Visualization
Digital photogrammetry is one of the most reliable technologies for digital mapping, orthophoto mosaic & DEM generation. It is also one of the most important input sources of geospatial data, and has been greatly utilized for object interpretation and object measurement. As the hardware & software technology grows stronger, digital photogrammetry technology is also advancing. Both cities – Helsinki and Copenhagen use aerial digital photogrammetry to capture the essential 3D geospatial information. For instance, in Helsinki, collaboration between aerial digital photogrammetry and laser scanning is being utilized to gather 3D information on as-built and terrain objects. Using the digital photogrammetry to produce orthophoto mosaics, these orthophotos, along with the laser point clouds, have subsequently been used in mapping outdated features and in vectorizing buildings and streets in 3D.

In Copenhagen, orthophotos are the basis for precise registration of all buildings, using computers with stereoscopic viewing capabilities for 3D visualization of all features including roofs and building details. Basically, the 3D city model is an advanced DEM, which precisely describes the urban topography. Each building in the model is registered with its exact location in both plan and height positions.

Since digital photogrammetry produces orthophotos, which serve as a great source of geospatial information presented through digital aerial photos in various resolutions, it is not hard to correlate it to 3D visualization. With the advancement of computer technology, 3D visualization has also shown encouraging evolutions over the years. Technology breakthroughs in computer graphics have made visual media one of the protagonists not to be missed, in terms of interfacing, interaction and communication between the 3D models and the users. Many stand-alone applications and plug-ins have been developed to quickly visualize and navigate through 3D models for a variety of applications.

In the case of Helsinki, three scales of 3D models have been developed to cater different needs. Each type represents different level of details (LOD), hence different level of 3D visualization outcomes and users will certainly be presented with a different amount of information provided. The first scale of the 3D model is a so called 'Rough 3D City Model', which is a general 3D model. This general 3D model covers the whole city successfully in summer 2002, and is good to be used for the building cadastre information (for building heights, number of floors etc), which doesn't need much of 3D visualization outcomes.

The second scale of Helsinki's 3D city model is called 'Exact 3D City Model'. It has a better LOD for 3D buildings, DEMs, map features, and 3D streets (break lines), among others. The map features come with a definitive characteristic for heights, and they can be 'draped' on the DEM. The third scale of the 3D model – 'Exact 3D City Model with Textures' is the most comprehensive and informative model, that comes with the most interesting 3D visualization. On top of a high quality DEM (draped with high quality orthophoto mosaics), all 3D buildings are attached with the relevant façade photos / images to represent the actual appearances in real world. Apart from that, the 3D model also consists of trees, vegetation and street furniture, which are all 3D elements that can be manipulated and visualized freely in 3D mode.

For Copenhagen, the 3D city model consists of two basic construction methods. Firstly, it is the wireframe mode, which serves as the basis of boundary representation. In wireframe mode, each building’s wireframe model will be generated through the construction of roofs and calculation of vertical lines connecting the roof to the bottom of building (or so called “foot print”). Secondly, it is the solid geometry mode, which develops the actual 3D objects that represent the buildings, three-dimensionally. Since these 3D solids can accommodate different types of surfaces, it is possible to calculate the shapes of each surface, which is very critical especially for the roofs and facades of the buildings (see Figure 1) Wireframe Mode Solid Geometry Mode The Detailed Model.
 

The shapes in the 3D city model can be further manipulated by adding colors, textures, or digital photos to the individual elements. For example, a 3D solid object which represents a church can be attached with digital photos taken on its original facades, assigned a proper texture image on its roof and then applied with adjusted color to make it more realistic. The bumpy appearance and image rendering setup can be further fine-tuned on the object surfaces to achieve a photorealistic result. With more than 130,000 buildings photogrammetrically produced, the 3D city model consists of approximately 3 millions vectors, and has generated approximately 6 millions shapes, to date. The combination of both wireframe mode and solid mode objects results a fantastic, resourceful and multi-functional 3D city model, in which further improvements are always possible.

5 The Benefits of 3D Geospatial City
Undoubtedly, the benefits of a 3D geospatial city are countless. In Helsinki, the 3D maps and models are currently used in city and street planning (see Figure 2), civil engineering projects (above and underground), soil surveys, building permit processes, noise modeling, traffic simulation, and military defense applications. In Copenhagen, the 3D city model is being utilized to generate shadow diagrams (see Figure 3), rendered images (still pictures of part of the model), video clips, internet publishing and 3D PDF documentations. One of the major purposes is to increase the effectiveness on information distribution to the public, through visualization dissemination.
 

 

A 3D geospatial city with digital photogrammetry and 3D visualization as backbone makes it possible to present complex problems in a reasonable and more convincing way, and are therefore suitable for public discussions / debate on the positive and negative consequences of a proposed urban development project. This is particularly important as 3D geospatial city provides the possibility of evaluating future works (building, civil, environmental or infrastructural projects) which are already in the design or drafting stage, to ensure that other existing buildings, facilities or services, as well as public sphere or the society norm of the city are taken into consideration. Members of the public can now deeply and actively involve in the urban development debate, because they can clearly see the visualized proposed changes.

It is believed that one of the major advantages of 3D geospatial city is the ability of navigating and manipulating the 3D model ‘thoroughly’. This means that the 3D city model takes care of all three levels of 3D dimension – above, on and underground. Actually this is one of the major focuses of both cities in venturing and exercising 3D geospatial city. For example, the Metro System of Helsinki (see Figure 4), where 3D modeling combining the above ground level (buildings, transportations, traffic and infrastructure), on the ground level (2D land parcels with image draping, demarcations on the DEMs), and underground level (subway or metro, underground train systems), provides a complete simulation, analysis and visualization for the best decision-making and potential-seeking.
 

The integration of 3D geospatial city model into the urban development process allows a higher degree of realism in the visualization of proposed projects. For example, more consistent visualizations can be produced in architectural tender projects, since the basic 3D geometry – the areas surrounding the project site and the digital terrain – is the same for all bidding companies. Thus, the decision process and public involvement can be improved.

3D geospatial city provides an unbiased, realistic and economical platform for project presentation. Whether the projects are initiated by the government or the private sectors, the focus will be on the actual architecture and workability, rather than the key-players involved or excessive elaboration of the planned projects. Ironically, at the same time, this platform also provides an indirectly great publicity for all parties involved in the projects. The materialization of the 3D geospatial city itself is enough to show the commitment of the government and the private sectors involved, to fabricate a more decisive and firm, yet captivating and impressive approach as part of the effort to continuously bridge the communication gap between them and their fellow urban citizens.

6 Challenges Encountered
Rome wasn’t built in a day, so has 3D geospatial city. It is certainly not a smooth-sailing ‘3D city journey’ for Helsinki and Copenhagen to be at where they are today. Even though both cities have achieved a certain milestone in 3D city modeling, there are still some challenges that are currently being attended to.

One of the common challenges encountered is in the process of updating and synchronizing the old 2D basemap features. The 3D development team always finds numerous digitizing errors from time to time, from the old 2D basemaps. Since all detected digitizing errors will need to be traced back and rectified, the corrections procedures will take sometime and hence the 3D development schedule will need to be revised. Compare to 2D works, 3D updating (modeling) needs more skills and time, especially in 3D visualization setup (photo-true texturing). Hence having a full-time dedicated development team is one criteria of the utmost importance. For instance, a 3D object needs further touch-ups after being built-up. Each of its surface texture (roofs and facades) must be matched exactly and accurately, by re-scaling the texture images and re-adjusting its resolution (pixel dimensions), fine-tuning its brightness, toning the overall appearances, testing the final outcome by rendering partly or the whole model. A few necessary repetitive attempts are needed and commons for 3D visualization works, in order to achieve the best and acceptable results. Besides, the consistency of the 3D basemaps has to be monitored from time to time, to produce a truly reliable 3D city model. Therefore, each member of the development team needs to be persevering, consistent and sensitive to changes, on top of the basic skill set in 3D modeling. Since 3D geospatial city is multi-functionally useful and its potential is growing, there’s a need for better systems coordination in the cities’ administration, and this lies in the hand of the authority who governs the 3D city model. One of the workable solutions will be the development of a common documentation and standards repository by updating the city’s current GIS metadata through internet. This will involve the collection of all data modeling documentations, object and feature catalogs used in various departments and divisions that utilize the 3D city model to offer services to the public.

7 Conclusions
The computer technology will only get better and better, hence the combination of digital photogrammetry and 3D visualization as backbone for 3D geospatial city governance will only get stronger. With software platforms that forge interoperability, it is not an unattainable task to maintain and further improve a 3D geospatial city model. The current achievement of Helsinki and Copenhagen in realizing 3D geospatial city modeling is believed to further convince the skeptics and change the way people think of its practicalities.

Acknowledgements
I would like to express my sincere gratitude to Oscar Custers and Gijsbert Noordam from Bentley Systems Europe, as well as Todd Rothermel from Bentley Systems U.S. for their invaluable guidance and advices for this paper. I would also like to thank Alex Chew of Bentley SEA Regional Office who suggested the idea of writing such a paper and left it to me as ‘assignment’. Many thanks are also due to my Bentley SA team members for their accommodating supports and collaborations.

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