Buying Optical Satellite Imagery - continued page 4

8. Collection Capacity

Because high-resolution imaging satellites are able to shoot off-nadir, their revisit times—ability to repeat coverage of the same AOI—is three days or less. Employing multiple high-resolution satellites on the same AOI means that intraday revisits are theoretically possible. However, in the real world, clouds and competition for satellite time are significant obstacles.

A satellite’s collection capacity is determined by its swath width (see table on page 1), agility to point and shoot at multiple targets or capture multiple adjacent strips for large-area mapping on the same orbital pass, onboard storage and downlink capacities. Pléiades 1, GeoEye-1, WordView-1 and WorldView-2, and other satellites offer significantly improved collection capacities than their predecessors, while the older satellites, such as IKONOS and QuickBird, may have better available capacity to collect an AOI in regions where the newer satellites have demand backlogs.

A satellite such as Pléiades 1, which doesn’t have a U.S. government backlog, also may be able to offer faster commercial tasking. Tasking two or more satellites for the same AOI improves the ability for imagery to be captured. For example, DigitalGlobe offers constellation order fulfillment (COF) where QuickBird and WorldView-2 can be tasked to acquire the same AOI with no cost uplift. RapidEye further leverages the constellation concept by employing a system of five identical satellites to facilitate country-and regional-scale mapping and saturating coverage of areas with persistent cloud cover.

When placing a new collect order, a feasibility assessment typically will be run to estimate the turn-around time. The feasibility takes into account potential cloud cover for the region and competition for satellite time in the area. Because collection queues can change daily and cloud cover is uncontrollable, feasibilities are only estimates, not guaranteed collection times. Most satellite operators offer priority tasking for an additional charge. Areas that have both high demand for imagery and persistent cloud cover are challenging to collect regardless of time of year and are likely to require lengthy turn-around times.

It’s important to note that due to sun-synchronous orbits, there’s little control over the time of day an AOI is imaged. Collects, which are made only on the descending orbit, typically are made around 10:30 a.m. local time over a desired AOI. Because high-resolution imaging satellites orbit Earth 15 times a day, an interesting exception to this is polar latitudes. Because Earth narrows at extreme latitudes, intra-day collects via the same satellite are possible, presuming an acceptable sun angle.

Project Example: Ortho Mosaics

Most satellite imagery comes georeferenced but not orthorectified. Orthorectification is a process that corrects inherent distortions in the optics and viewing geometry. The process incorporates a satellite orbital model, a digital elevation model (DEM) and optionally photo-identifiable ground control points (GCPs). Orthorectification is the most reliable way to correctly georeference all points in an image.

In addition to improving the absolute accuracy of the imagery, orthorectification also improves the relative spatial match across adjacent scenes and strips. However, if the source scenes have different or opposite look angles, some spatial mismatch may be unavoidable, especially in high-relief areas.

If an area of interest (AOI) comprises more than one scene/strip, the imagery also can be tonally balanced. Tonal balancing matches colors that otherwise would vary across adjacent scenes due to a variety of factors, including atmospheric conditions and vegetation seasonality across multiple imagery acquisition dates. In cases of extreme seasonality differences, it may not be possible to create an entirely seamless tonal match.

ortho mosaic tonal source and outcome

    Multiple overlappping scenes can be used to output a tonally balanced ortho mosaic with cloud patching

Typically, tonal balancing only is applied to land areas, not water, as the appearance of water features, especially saltwater, can vary greatly across multiple dates/scenes. If a visually pleasing or realistic picture is desired, water areas can be manually manipulated with software tools such as Photoshop, but image intelligence, such as shallow water depth, would be lost.

When creating orthomosaics, limiting the number of scenes facilitates processing and, if applicable, helps reduce the total number of optimal Global Positioning System (GPS) ground control points (GCPs). In the United States, public-domain products, such as imagery from the National Agriculture Imagery Program (NAIP), can be used as control to improve the ortho accuracy of imagery with lower native accuracy. Outside of the United States, if GPS GCPs aren’t available, ideally the next-generation imaging sensors with higher native accuracy, such as GeoEye-1, WorldView-1 and WorldView-2, should be favored.

Consideration should be given to the DEM being used. In the United States, 10-meter (1/3 arc) National Elevation Dataset imagery is the best public-domain option. However, there are also high-quality commercial DEM options. Outside the United States, commercial and/or local government DEM options are available. However, if DEM selection is limited to public-domain options, despite the resolution difference, 90-meter (3 arc) Shuttle Radar Topography Mission data will likely yield better results than a 30-meter (1 arc) ASTER GDEM, especially in areas with high cloud cover and/or low contrast (snow, dense vegetation, etc). Also, imagery that has already had a DEM applied to it, including the Standard format from Digital- Globe, isn’t suitable to be orthorectified—the Ortho Ready format should be selected instead.

To the extent possible, limiting the seasonality differences across strips helps improve tonal balancing. If an AOI comprises multiple scenes, some satellite operators will crop out part of the overlap area to reduce the file size. From an image processing perspective, however, especially where imagery needs to be cloud-patched, the overlap is desirable and should be available at no extra charge, as long as it’s requested when the order is placed.

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