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High resolution geographic imagery and its impact on GIS
John W Allan ERDAS Inc., Telford House, Fulbourn Cambridge CB1 6DY, United Kingdom john.allan@erdas.com
Introduction
The era of 1-meter satellite imagery presents new and exciting opportunities for users of spatial data. With Space Imaging’s IKONOS satellite already in orbit and satellites from EarthWatch Inc., Orbital Imaging Corp. and, of course, ISRO scheduled for launch in the near future, high resolution imagery will add an entirely new level of geographic knowledge and detail to the intelligent maps that we create from imagery. Geographic imagery is now widely used in GIS applications worldwide. Decisions made using these GIS systems by national, regional and local governments, as well as commercial companies, affect millions of people, so it is critical that the information in the GIS is up to date. In most instances, aerial or satellite imagery provides is the most up to date source of data available, helping to ensure accurate and reliable decisions. However, with technological advancements come new opportunities and challenges. The challenge now facing the geotechnology industry is twofold - how best to fully exploit high-resolution imagery and how to get access to it in a timely manner It is very easy to show high-resolution imagery in new and innovative applications and many papers are already being presented at Map India 2001 that show this. However, it is also very easy to focus on purely the “artistic” side of the imagery in the application and to lose sight entirely of any commercial issues that will help or hinder the application from being commercially successful. This paper will explore these issues and will provide an objective view of problems that the industry has to overcome before it can achieve true commercial acceptance. Is high-resolution imagery making a difference? There is no doubt that the GIS press has been deluged with high-resolution imagery for the last 12 months. Showing an application with an imagery backdrop provides an immediate visua l cue for readers. Without the imagery backdrop, the context is lost and the basic map, comprising polygons, lines and points becomes more difficult for the layman to interpret. It is the context or visual clues that provide the useful information and it is this information that is the inherent value of the imagery. The higher the resolution of the imagery, the more man made objects that can be identified. The human eye – the best image processor of all – can quickly detect and identify these objects. If the application is therefore one that just requires an operator to identify objects and manually add them into the GIS database, then the imagery is making a positive difference. It is adding a new data source for the GIS Manager to use. However, if the imagery requires information to be extracted from it in an automated and semi automated fashion (for example, a land cover classification), it is a different matter. If the same techniques that were developed for earlier lower resolution satellite imagery are used on the high-resolution imagery, (such as maximum likelihood classification), the results can actually create a negative impact. Whilst lower resolution imagery isn’t affected greatly by artifacts such as shadows, high-resolution data can be. Lower resolution data also “smoothes” out variations across ranges of individual pixels, allowing statistical processing to create effective land cover maps. Higher resolution data doesn’t do this – individual pixels can represent individual objects like manhole covers, puddles and bushes - and contiguous pixels in an image can vary dramatically, creating very mixed or “confused” classification results. There is also the issue of linear feature extraction. Lines of communication on a lower resolution image (such as roads) can be identified and extracted as a single line. However, on a high-resolution image, a road comprises the road markings, the road itself, the kerb (and its shadow) and the pavement (or sidewalk). A very different method of feature extraction is therefore needed. Figure 1 shows the range and variety of information contained in a high-resolution image and the problems caused by shadows, overhanging trees and parked cars. It’s not just the spatial resolution that can affect the usage of the imagery. With 11 bit imagery becoming available, the ability of the GIS to work with high spectral content imagery becomes key. 11 bit data means that up to 2048 levels of grey can be stored and viewed. If the software being used to view the imagery assumes it is 8 bit (256 levels), then it will either a) display only the information below the 255 level (creating either a black or very poor image) or b) try to compress the 2048 levels into 256, also reducing the quality of the displayed image considerably. Having 2048 levels allows more information in shadowy areas to be extracted as well as enabling more precise spectral signatures to be defined to aid in feature identification. However, without the correct software, this added “bonus” can easily turn into a pr oblem. One other area that needs to be addressed in terms of usage is the actual availability of data to the end user. Application papers tend only show us the finished results without giving any indication of the actual project itself and the problems that may have been encountered in the actual running of the project. In many instances, availability of data is limited, especially from spaceborne sensors and users have to look elsewhere for data. An increasingly common source of image data is therefore existing aerial survey photographs. With the massive improvement in scanning technology and orthophoto production software, these old photo archives can be readily made available to GIS users. No licencing fees are required (as the organization generally ow ns the photography) and the data can easily be made available internally within the organization. The only downside is the question of how recent the imagery is. Contrast this with the high-resolution satellite data. If it is not archived data, then the data has to be acquired, which is dependent upon both the weather and other demands on the satellite. If it is acquired then it has to be processed and shipped out via tape or CD/DVD (as bandwidth is limited) and finally, it usage is limited by licencing – single user, multiple user, site usage etc. pricing is therefore a key issue. The message here is clear. High-resolution satellite data will not replace other sources of data -it will in fact only complement them. 
Finally, the issue of digital versus analog is also being addressed in this new digital age. Old airphotos need to be scanned to convert them to a digital format. New digital airborne cameras get around this step, providing high quality airborne imagery at any user defined resolution. Depending upon the application and the levels of accuracy needed, cameras ranging in price from the hundreds to the millions of dollars can be used. The drop in price and increased availability of GPS units is also aiding the growth in the use of low cost digital cameras for GIS applications. Attached to remotely controlled aircraft or helicopters, they can provide very high-resolution, targeted aerial surveys for specific applications.
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