By Jeff Specht, publisher, Earth Imaging Journal (www.eijournal.com), Greeley, Colo.
With search engines and other online sources trying to distinguish their offerings with ever-greater amounts of free content, managers in public agencies find themselves asking, Why aren't we just using free Earth imagery off the Web? After all, they're accountable to taxpayers for what can be considerable sums of money for a custom imagery project.
This question is understandable, especially in a time of strained budgets, when nearly everyone is being called upon to maintain current service levels but with reduced funding. It's also understandable in the context of legislators, executives and senior managers who are trying to carry out their responsibilities as best they can but who may see imagery only as a visual display, without understanding how the imagery's technical aspects are critical to fulfilling intended requirements.
Accuracy Is a Prime Factor
The easiest way to contrast the two imagery sources is to examine the accuracy issue. It's easy to illustrate that aerial authoritative imagery can be precisely overlaid with road centerlines, parcel information or other data to show it's much more than a picture. The imagery has gone through a rigorous orthorectification process, which uses ground reference targets, precise positioning and orientation information for the camera sensor at the time of each exposure.
In addition, the process uses a 3-D model of the terrain surface to run a sophisticated computer program that removes distortions from the imagery and positions it correctly in a chosen coordinate system such as latitude and longitude. Search engines and other online businesses, which largely are based upon selling advertising through the use of visualization, and doing it all for free, may not be able to economically produce imagery that meets the business requirements of state and local government agencies.
In many cases, public agencies use authoritative imagery to derive other products such as building footprints, road centerlines or utility inventories. If the currency and accuracy aren't known, the derivative data are questionable and can be challenged.
Purchasing imagery to a required specification and validating it mitigates this issue. Public and private entities that use authoritative imagery and its derivative products frequently make crucial decisions based on this information, many of which significantly impact the public. Applications as diverse as the design of transportation infrastructure, code enforcement and property tax assessment are just a few examples of how these data are used.
For many applications, lives depend on accurate imagery, and government officials must be able to back up their decisions. Free or low-cost referential imagery may look great, but those who disseminate it online don't control the specifications for creating the data, probably don't know what those specifications were, and most of the time can't even find out.
Authenticity and currency are significant issues related to accountability in public agencies. If the online company has masked areas to satisfy a user via legal request, the end user has no way of knowing what changes were made to the data. The time period in which the imagery was collected isn't readily available, and in many cases it's unknown. This makes online imagery useless in a court or legal proceeding.
Exploring the Value of Authoritative Imagery
In return for their cost, custom aerial mapping projects provide the ability to dictate standards and specifications as well as confirm product ownership. You regulate the quality-control procedures and documentation, and your products are backed by a guarantee.
The needs of today's geospatial professionals are complex and demanding, and imagery that can stand up to such tasks is often equally so. Some of the more critical imagery specifications are as follows.
How accurate does the imagery need to be for your requirements? This dictates how well it will be positioned in global space and how precisely end users can overlay their data.
The spatial resolution of the imagery should be based on the smallest objects that need to be identifiable with the imagery”e.g., manhole covers or utility poles.
Coordinate System and Projection
The imagery must be in compatible formats, so it can be overlaid with other datasets for which end users often have invested considerable sums of money.
Time of Acquisition
The time of year imagery is acquired can be critical. For most government applications, imagery must be acquired in leaf-off, snow-free conditions to see as much built infrastructure beneath tree canopies as possible before it becomes obscured. The time these conditions exist in some places is measured in a few short weeks, with weather often delaying acquisition.
The time of day the imagery must be acquired for certain applications is critical. Shadows from high buildings and trees can render large portions of imagery virtually useless because the shadows obscure key features that must be mapped.
Virtually all modern sensors collect the near-infrared band with the conventional red, green and blue bands that compose natural color imagery. Modern processing methodologies often make such data available for little if any additional cost. Near-infrared imagery can be tremendously useful for a wide variety of applications, e.g., wetlands identification.
Professional firms can collect and process imagery within a variety of different resolutions, accuracies and other specifications to meet the varying needs of end users while still keeping costs to a minimum.
Derivative Data Products
Ownership of authoritative aerial imagery provides access to fully controlled stereo imagery that can be used for terrain modeling, planimetric data extraction, production of 3-D visualization products, and other products and processes.
The odds of any off-the-shelf imagery product, free or not, meshing with even a small number of these specifications are extremely low. While budgetary constraints may not always allow an organization to procure everything it wants in terms of imagery and derivative data products, the costs pale in comparison with potential lawsuits arising from injury, property damage or even loss of life resulting from the use of inaccurate data.
Commissioning a custom project from a professional aerial mapping firm provides the ability to dictate imagery specifications that meet the needs of the largest number of end users possible.
Referential Imagery vs. Authoritative Imagery
Several important issues distinguish referential imagery from authoritative imagery.
Most government agencies must be able to use data in a legal context. Referential imagery may have been modified by the online provider for a variety of reasons. There's no way to validate whether or not the imagery has been altered.
Referential imagery has no guaranteed temporal specification. It could be current, one year old or five years old. In general, government must be able to validate the time the imagery was collected, and the requirement is often for current data.
Referential imagery may not be produced to an appropriate accuracy specification. Where there are specifications, they may be relaxed, with no independent validation.
Is there documentation that defines rights of use for the imagery? Can derivative products be made from it? Who owns these products? Are royalties required? This is only a sample of the issues that must be addressed.
Publisher's Note: Thanks to the National States Geographic Information Council and Learon Dalby, vice president, geolocation solutions, Sanborn, for assistance with this article.