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A sensor model describes the properties and characteristics associated with the camera or sensor used to capture an image at the time of capture. The internal (focal length, fiducial marks etc. for aerial photographs) and external characteristics associated with sensor geometry, the image and the ground are frequently known or can easily be determined. Increasingly, airborne Global Positioning System (GPS) and inertial navigation system (INS) information also is obtained during image acquisition. If these data are available, the external sensor model information can be directly input for use in subsequent photogrammetric processing. The user can then automatically compute Tie Points and proceed to calculate Orthophotos or 3D feature collection. Otherwise, most photogrammetric systems can determine the exact position and orientation of each image in a project using the block bundle adjustment approach. Unlike traditional geo-rectification techniques, GCPs in digital photogrammetry have three coordinates: X, Y and Z. GCPs can be collected from existing vector files (i.e., a road intersection), orthorectified images, DTMs, and scanned topographic and cartographic maps. GCPs are used in conjunction with block bundle adjustment to establish a geometric relationship among the images in a project, the sensor model and the ground so accurate 3-D data can be collected from imagery. Photogrammetry requires a minimum amount of ground control, and the number of GCPs will vary from project to project. A tie point is a point with unknown ground coordinates, but the point is visually recognizable in the overlap area between multiple images. Tie points are used to create geometric harmony among the images in a project so they're positioned correctly relative to one another. ERDAS software uses Automatic tie point collection to automatically identify and measure tie points across multiple images and strips of imagery. Once GCPs and tie points are collected, block bundle adjustment can begin. Block bundle adjustment (triangulation) is essential to determining the information required to create orthophotos, DTMs, digital stereo models (DSMs) and 3-D features. An output report can provide detailed statistical reports outlining the accuracy and precision of the derived data. Once the block is ready and solved, the end user can then proceed directly to create /update the 2D or 3D GIS database. If Orthophotos need to be produced and printed for other applications, then this can be accomplished at this stage as long as a DEM (Digital elevation model) is available. Orthorectification is the process of removing geometric errors inherent within photography and imagery. Using sensor model information generated by block bundle adjustment and a DTM, GIS users can remove errors associated with sensor orientation, topographic relief displacement, Earth curvature and other systematic errors. Measurements and geographic information collected from an orthorectified image represent the corresponding measurements as if they were taken on Earth's surface. 3-D data and information can be collected from Digital Stereo Models (DSMs). Using the block file created in the triangulation step, two overlapping images of a DSM are automatically aligned, levelled and scaled to produce a 3-D stereo effect (with appropriate stereo viewing hardware). A DSM allows GIS users to interpret, collect and visualize 3-D geographic information from imagery and is used as the primary data source for collecting 3-D GIS data. Geographic imaging systems like ERDAS Stereo Analyst directly collect true, real-world 3-D geographic information from a DSM, using a 3-D floating mark or cursor, and don't require an additional DTM (Digital Terrain Model). During data collection, the 3-D floating cursor commonly floats above, below or rests on Earth's surface or the object of interest within the DSM. To ensure the accuracy of the data, the height of the floating mark is adjusted so it rests on the geographic feature being collected. ERDAS Stereo Analyst uses a automatic Terrain-following cursor to automatically place the 3-D floating cursor on the ground so a user doesn't have to manually adjust the height of the cursor every time a feature is collected. For updating a GIS, existing vector layers are commonly superimposed on a DSM and then edited and re-shaped to their accurate real-world positions. Two-dimensional vector layers can be transformed into 3-D geographic information. During data collection, the spatial and non-spatial attribute information associated with a vector layer can be edited, and the attribute tables can be displayed with the DSM. Automated attribution techniques simultaneously populate a GIS during the collection of 3-D data. Additional qualitative and quantitative attribution information associated with a feature can be input during the collection process.

Why move to a 3-D GIS database for urban planning?
With the evolution of GIS software packages, Urban planners can now have access to 3-D data in visualisation, spatial modelling and analysis applications. Output products created by 3-D geographic imaging techniques include orthorectified imagery, DTMs, DSMs, 3-D features, and spatial and non-spatial attribute information associated with a geographic feature. Using these primary sources of geographic information, additional GIS data can be collected, updated and edited. DSMs created from high-resolution imagery are useful for the following:

• Identifying and categorizing urban and rural land use and land cover. 3-D topographic information such as slope, vegetation type, soil characteristics, underlying geological information and infrastructure information can be collected as 3-D vectors.
• Accurately interpreting and collecting data such as soil type, slope, soil suitability, soil moisture, soil texture, surface roughness, etc. DSMs are useful for determining the suitability of a given development (i.e., highways, building foundations, etc.).
• Estimating population using 3-D high-resolution imagery to identify the number of dwelling units for various household types. Determining the height of buildings is important to generate an accurate estimate.
• Deriving house size, lot size, building density, street width and condition, driveway presence/absence, vegetation quality and proximity to other land use types for housing quality studies.
• Identifying and inventorying geographic information for site selection applications such as planning of transportation routes, sanitary landfills, power plants and transmission lines.

Each application requires accurate 3-D topographic representations, geological inventory, soils inventory, land use and vegetation inventory, etc. Analysing the extent of urban growth in change detection studies requires photography collected from various time periods. Land use and land cover information is categorized for each time period and subsequently compared to determine the extent and nature of land use/land cover change. Accurate 3-D building polygons can be derived via digital stereo models with ERDAS Stereo Analyst



Conclusions
The backbone of a GIS requires a strong underpinning of geographic control that makes pinpointing an exact location, along with its attributes, possible. A 3-D GIS powered by digital photogrammetry presents a new paradigm that offers greater accuracy and precision in data collection, and preserves the investment made in a GIS by any Urban Planning authority or local government. The methodology discussed in this paper is currently used by a number of local government planning agencies to manage the accuracy of their geo-spatial databases with excellent results. ERDAS photogrammetric solutions are designed for the GIS user with minimal or no photogrammetry training so they easy to learn and use and importantly offer a cost effective way for local governments to maintain healthy landscapes.

References

• Stojic, Mladen, 2000, 3-D GIS: Unleash the power, Geo-Europe November 2000.

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