guide to buying optical satellite imagery

Buying Optical Satellite Imagery? The Top Ten Things to Consider

By Nick Hubing, president, LAND INFO Worldwide Mapping

A little more than a decade ago, the only satellite imagery most people saw was a weather map on a TV broadcast. That changed with the launch of IKONOS in 1999, when the world’s first commercial high-resolution Earth imaging satellite returned stunning images from around the globe. From more than 400 miles in space, the striking details of Egypt’s Great Pyramids and San Francisco’s Golden Gate Bridge suddenly came alive, inspiring a host of uses for the new data. The golden age of high-resolution optical satellite imagery continues as users worldwide tap into this information-rich data for a host of commercial projects and research studies. All satellite imagery, however, is not created equal. A growing number of sensors and sources can make choosing the right imagery for your project seem like a daunting task. This article sheds light on some of the most important considerations when ordering and using optical satellite imagery.

Download a .pdf version of the Resource Guide here.


1. Resolution


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.

Optical satellite imagery

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.

Although digital imagery doesn’t have an inherent scale, higher spatial resolution will support viewing/plotting at a larger scale
(see table below ).

 

Satellite imagery resolution comparison chart
  • 1 at nadir
  • 2 at nadir
    3 Estimated value only, as actual max zoom level prior to pixelization will vary based on collection geometry, size, shape & contrast
       of objects on ground. (If satellite offers multiple resolutions, the max zoom value listed is for the highest available resolution.)
  • 4 Horizontal accuracy CE90 without GCPs (except Rapid Eye), excluding terrain and off-nadir effects
  • 5 Higher elevation angle imagery available at 0.80 meter x 3.20 meters
  • 6 Changed from earlier 16.5 kilometers due to April 2011 orbit raise
  • 7 2.5 meters from 2 x 5-meter scenes
  • 8 RapidEye is the only imagery listed where GCPs (but not a DEM) are used with the Basic (1B) imagery, therefore accuracy is
       higher in areas where higher accuracy GCPs are available, such as the United States.

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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.


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