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Buying Optical Satellite Imagery? The Top Ten Things to Consider
By Nick Hubing, president, LAND INFO Worldwide Mapping
Download a .pdf version of the Resource Guide here.
It’s the first number we look at and the one that grabs the headlines. Resolution, however, can refer to multiple parameters. For example, temporal resolution measures how frequently a satellite can image a target. But more commonly spatial resolution is used to describe the level of detail. An image with 1-meter spatial resolution, where each pixel represents a ground distance of 1 meter x 1 meter, has higher resolution—is more detailed— than a 5-meter resolution image, where each pixel represents a ground distance of 5 meters x 5 meters. The native ground sample distance (GSD) of images varies based on collection geometry, but images are subsequently re-sampled to a uniform resolution.
Five-meter imagery (left) can be ideal for mapping larger areas, but it won’t show the same level of detail as high-resolution imagery (right).
Zoomed out far enough, high- and medium-resolution imagery looks the same. The difference becomes apparent when zooming in closer, as the high resolution imagery—typically 1 meter or less—will display greater feature detail and show smaller features.
Resolution selection often is driven by size of the area of interest (AOI). Due to cost and technical considerations, high-resolution imagery usually is selected for AOIs smaller than 500 square kilometers, whereas medium-resolution imagery can offer a cost savings for AOIs 500 square kilometers and larger. Besides higher cost, disadvantages of high-resolution imagery include larger file size (there’s an exponential relationship between resolution and file size) and smaller swath widths—the width across a single scene/strip of imagery.
2. Spatial Accuracy
Although there’s typically some level of correlation between spatial resolution and accuracy there are notable exceptions. For example, compared with DigitalGlobe’s QuickBird satellite, the company’s WorldView-1 and WorldView-2 satellites offer only a moderate enhancement to spatial resolution, but because they employ new technology they achieve significantly improved native accuracy. Most satellite imagery is delivered georeferenced or georectified, but not orthorectified, which is a process that improves absolute accuracy by correcting for terrain displacement. Therefore, the accuracies listed in the table are exclusive of terrain displacement, which is significant in areas of high relief. Typically, horizontal accuracy is expressed as CE90 (Circular Error 90 percent), but it may also be expressed as RMSE (Root Mean Square Error) or as a scale. For example, to comply with U.S. national map accuracy standards for 1:12,000 scale, an orthorectified image would need to achieve 10-meter CE90 accuracy.