|
High resolution geographic imagery and its impact on GIS
Bandwidth
Images by their very nature tend to hog computer resources, taking up many megabytes and even gigabytes of storage and processing. Getting the data to the user is therefore the first problem, as current digital networks cannot cope with this quantity of data. Image compression is one way of getting around this, but it must be noted tha t the current generation of image compression software, such as MrSID are “lossy”, meaning that spectral information from the original image will not be maintained in the uncompressed image. This is acceptable where the image is to be used as a backdrop or for printing, but is not acceptable if the image has to be further processed. Many processing functions are reliant upon the spectral information in the image and if this has been changed, then the results cannot be guaranteed. If the imagery is to be us ed for further processing then the options are relatively limited. The remote sensing industry has long been plagued with data delivery problems, in large part based upon the size of the datasets. Delivery over the Internet is not a solution at present except where dedicated lines such as frame relay are available. Satellite based distribution, such as DirectPC (similar to DirectTV commercial television services) is also an option but is currently limited by availability and also it’s one -way nature. It can be used to deliver the data, but the request and searching functions need to be carried out over a different connection to the web. Both these solutions are expensive and are therefore not practical for the everyday GIS user. That leaves writing to media (CDROM/DVD) and surface delivery, which in many instances takes away the “up to date” quality of the data. A different approach may therefore be to change the location of where the processing takes place and instead of sending the data to the users, a better solution may be for the data provider to provide the processing via the web. ERDAS are currently working on a number of products that will provide processing over the Internet, where users will be able to request processing services from data or servic e providers. This will enable users to request information, potentially based on the expert systems described above, with only the end results delivered to them via the net. Based on the mobility analysis example, a user could request the moisture content for a certain area and just get the basic information to feed into the knowledge base. This “subscription” based approach to processing is some way off but will definitely revolutionize the imaging industry in the medium term, moving the processing overhead from the user to a fee-based service provider. Accuracy Aerial or spaceborne imagery by its very nature is distorted in its raw form, either because of the movement of the sensor, the lensing system or terrain height variations of the area that is being imaged. Happily there are many software packages available which provide capabilities to correct for these distortions and make the imagery GIS ready. ERDAS itself provides a range of capabilities for simple geometric transformation (in products like the Image Analysis Extension for ArcView) through single frame orthorectification (in IMAGINE Advantage) to full rigorous photogrammetric block triangulation (in IMAGINE OrthoBASE). In some instances, where the imagery is being used to look for relative changes or to identify simple features, geometric transformation is used. For example, ArcView users can bring imagery in via the Image Analysis extension and simply warp it to fit their vector database. The imagery can then be used to update attribute information in the vector database. In this instance, it is the relative accuracy that is important. For other more precise mapping application, corrections for the look angle and terrain height variations are critical and in these instances, rigorous photogrammetric processes must be used. In the past, photogrammetry has been viewed as somewhat of a black art by the GIS community, with orthophoto production traditionally being left to specialist service providers. Products such as IMAGINE OrthoBASE are changing this approach however, with wizard based user interfaces enabling many GIS users to create their own orthophoto databases. Overall, the user, dependent upon their own application, must choose the correct level of accuracy. In general, most data providers now offer highly accurate, orthorectified data as standard with simple geometric rectification products becoming less common. Whilst orthophotos can be used to digitize 2D features relatively easily, the ability to get accurate 3D data from imagery has been more difficult. If a very high accuracy DEM is already available, then 3D coordinates can be applied to the 2D features captured from the orthophoto. It is however rare that such a DEM exists. 
The other alternative has been to use stereo imagery and photogrammetric software to digitize 3D features directly. However, this has always been viewed as a specialist capability and hence was uncommon in the GIS world. With the launch of Stereo Analyst, ERDAS is attempting to bring this 3D capability into the GIS world by hiding the complexity of the process beneath a wizard based user interface. With stereo data becoming more readily available from companies such as Space Imaging (subject to certain restrictions) and with many aerial survey companies already providing stereo air photos, GIS users now have the ability to:
Many GIS applications do not require just a single “snapshot”, but are focused on maintaining an up to date inventory of land cover or land use. Because imagery is taken at different times of the year and with differing weather conditions, it is important that some form of “normalization” is applied to the imagery or taken into account during processing. The expert systems described above allow for this and can take into account the different spectral responses of land cover at different times of the year, the effect of sun angle differences and also the atmospheric corrections needed for haze, fog and airborne pollutant removal. Future trends The remote sensing and photogrammetric industries are going through a massive change, becoming more closely integrated with a fast growing and competitive GIS industry. What is clear is that imagery and the technology associated with preparing it for GIS and extracting information from it is becoming a key part of GIS systems worldwide. It is important that GIS software changes to take account of this new and extensive user requirement and that the industry as a whole begins to provide services that match the demands of these new users. What we shall see over the next 2- 3 years is:
Page 3 of 3
| Previous | |