Abstract:
The correct and regular assessment of the status of the urban environment is a fundamental towards the understanding of the transformations of local, regional and global ecosystems. Extension of built-up areas and the loss of green space both within and around urban areas threaten bio-diversity as well as the quality of life of the residents of urban area. In order to aid urban planners make accurate predictions and plans, it is imperative to have an accurate and sustainable spatial database. This database should be accessible to the planners via a Geographic Information System (GIS). Imagery of the Earth's surface, both aerial and satellite, has become an integral part of today's GIS and desktop mapping systems. While a traditional Vector Map is a representation of the landscape as defined by a cartographer, an image map shows information from the 'Real World'. Imagery is no more just a pretty backdrop but a crucial information extraction tool in the Geo-Spatial world. Image-based information products are valuable tools for urban planning. The true usefulness and usability of a GIS stems from the accuracy of the data. Most of the existing maps, topographic sheets and therefore GIS databases existing today are becoming rapidly obsolete with due to rapid changes and inaccuracies. Traditionally digital photogrammetry has been used by mapping firms to create the base maps to update the GIS databases. These systems have been for non-GIS experts and such systems have cost more than what most GIS projects could afford. ERDAS has released desktop digital photogrammetry solutions (IMAGINE OrthoBASE and Stereo Analyst) which will aid urban planners maintain the base maps in urban geo-spatial databases accurately and cost effectively.
Introduction
A large population of India lives in cities and towns, and urban areas are therefore the places where the environmental problems touch most the quality of life of citizens. Indian cities are experiencing an increasing use of energy, natural resources, and living space. The demand for residential areas, social facilities, infrastructures , health facilities put pressure on available living space of cities. Today's urban planners have to understand the complex of relationship between people, resources, environment and development. Given these circumstances, many planning agencies throughout the world using complex Geographic Information System (GIS) for data capture, storage and analysis for deriving more realistic models for development. GIS is a tool for integrating large volumes of spatial and other data sets on different themes for such analysis, and offers excellent tools for understanding urban development in the past and running prediction models for the future. Collecting geographic data is vital to creating and maintaining a GIS, because inaccurate or outdated information will not reflect true, real-world scenarios. Today's GISs are built using information derived from various types of geographic data, chiefly vector-based data. The increasing number of new data sources requires new tools that can bring into clear view today's complex world of 3-D objects, features and spatial interactions, and help build vital infrastructure for the future. However, data currency and accuracy poses challenges to all GIS professionals. How can the GIS be updated and corrected?
- The costs and time required to prepare and collect GIS data from existing sources of information can be high. For example, geo-rectifying 500 photographs to map an entire county may take up to three months before data collection begins.
- Digitising hard-copy maps is time- and labour-intensive, and it introduces errors into the system.
- Most of the original sources of information in a GIS provide only 2-D information (the X and Y coordinates).
- Outsourcing core
digital mapping and routine updates to specialty
shops is expensive and time consuming.
Fortunately geographic imagery provides the ideal solution for deriving current and accurate 3-D information, and has proven to be a cost-effective tool for updating a GIS. Digital photogrammetry is an essential imaging tool for creating useful 3-D data and building a 3-D GIS. The idea of integrating digital photogrammetry and GIS has intimidated many within the GIS community. The accuracy of photogrammetry is well known, but the cost and learning curve associated with it have forced many GIS managers to select less accurate methods in their production. Many local governments have resorted to outsourcing their projects to specialty photogrammetric production shops. Fortunately, advancements in the development of geo-spatial tools at ERDAS, Inc. are bridging this long-standing gap. It's now possible to precisely identify a geographic location in 3-D space and link that location with its information attributes through the synthesis of photogrammetry, remote sensing, GIS and 3-D visualization. This combination of geographic imaging techniques, fuelled by the precision and accuracy of photogrammetry, are the ideal tools for building the 3-D GIS of the future.
From Imagery to a 3-D GIS
Transforming imagery into accurate 3-D data involves several processes. The data and information required for building and maintaining a 3-D GIS includes orthorectified imagery, digital terrain models (DTMs), 3-D features (i.e., vectors) and the non-spatial attribute information associated with the 3-D features. Using various processing steps, the 3-D data can be automatically extracted and
collected from imagery. Digital photogrammetric techniques offer unlimited access to the various types of photography and imagery that can be used to collect accurate GIS data. Although traditional photogrammetry applications use aerial photography (commonly nine by nine inches in size), new photogrammetry tools from ERDAS, Inc. easily handle any type of imagery, including satellite, digital camera, video camera, and 35-millimeter camera photography. Hard-copy photographs must be scanned or digitised to the desired resolution. For highly accurate mapping projects, calibrated photogrammetric scanners will scan to high precision (i.e., microns). If high-end micron accuracy isn't required, more affordable desktop scanners (i.e., A3 size to accommodate nine- by nine-inch photography) can be used. Conventional photogrammetric applications such as topographic mapping and contour line collection use aerial photography, but current photogrammetry applications extend the processing to include oblique and terrestrial (i.e., ground-based) photography and imagery. Various image file formats can be used, including TIF, JPEG, GIF, Raw and Generic Binary, and compressed imagery, along with various software-specific file formats.
GIS Update Workflow for Urban Planners
Traditionally most photogrammetric / mapping houses users have updated GIS databases by first creating Orthophotos and then digitising 2D vector from those Orthophotos. ERDAS software can take the urban planner from raw imagery to 2D/3D GIS database without the need for creating Orthophotos or DTMs. The straightforward and linear process for this includes several steps:
- Sensor model definition
- Ground control point (GCP) measurement
- Automated tie point collection
- Block bundle adjustment (i.e., aerial triangulation)
- 3-D feature collection
and attribution
The workflow is scaleable and doesn't need to be repeated for every application scenario. For example, a block bundle adjustment doesn't need to be performed every time imagery is processed for 3-D feature collection. If the end user does need to create Orthophotos, this can be accomplished after the block bundle adjustment step.