Revolution in Satellite Imagery
A new satellite is now providing remarkable 1 m panchromatic (black & white) and 4 m multispectral (true color and infrared) images. Space Imaging (Thornton, Colo.) launched IKONOS, its 1 m bird, on September 24, 1999. If you are wondering about the derivation of the name, IKONOS comes from the Greek word for “image.” The images available from IKONOS have many applications in the professional arena. These uses include land development, planning, cadastral, utility, hydrologic and GIS applications. Previously available satellite images with lower resolutions were only marginally functional in these areas.
The Basics of Satellite ImageryCommercial remote sensing satellites collect imagery as they circle the earth in a near-polar, sun-synchronous orbit. Obviously, they follow the sun in order to collect information during the daylight hours. Each orbit takes around 100 minutes to complete. In the case of IKONOS, each orbit is completed in 98 minutes at an altitude of 423 miles above the earth. The earth’s rotation reveals new terrain for image capture with each new orbit.
Digital satellite images are made up of individual pixels of information. Each pixel contains visual information about a specific area of the ground. The size of the ground area represented by each pixel is known as the spatial resolution of the imagery. This resolution is specified as the linear distance along one side of the pixel.
Imagine dividing the subdivision where you live into a grid of perfect squares of specific dimension. In a digital satellite image, each of these squares will be represented by a single pixel of information. The predominant color of that square on the ground will be represented in the digital image. Your lawn will have a color value very different from your driveway, your house and the trees growing in the yard. The ability to see detail in the imagery increases as the resolution increases. However, an increase in file size and imagery cost accompanies this increase in resolution.
The resolution of the panchromatic imagery available from IKONOS results in pixels measuring 1 m along each side. Houses, cars, sidewalks and even paint striping can be seen in satellite images. The multispectral (color) imagery has less resolution than its panchromatic counterpart. However, the value of multispectral imagery can be significant for a number of applications—particularly in the environmental arena.
How does the digital imagery make its way back to the earth? The information is transmitted by the satellite to ground receiving stations located around the globe. The collected information is then available for processing and use by the professional community.
Imagery CorrectionThe raw satellite images contain a number of errors. Geometric distortions are present primarily due to the earth’s rotation while the imagery is being collected and by the elevation variation of the earth’s surface that is being captured in the imagery. The latter error is known as relief displacement. Additionally, radiometric and atmospheric errors affect the visual quality of the image.
Most of the distortions in the imagery can be corrected. The price of the imagery increases as more corrections are made to it. An entry-level price for satellite imagery normally includes radiometric and geometric corrections. In other words, the imagery is corrected for the earth’s rotation and for general radiometric errors. Orthorectified images are corrected for the effects of elevation variation and represent the most accurate imagery available. Orthorectified images also cost more than other correction types.
What accuracy should be expected in the satellite imagery? Space Imaging claims a 12 m horizontal and 10 m vertical accuracy for its images when ground control is not used in the image processing. These predicted errors decrease to 2 m horizontally and 3 m vertically when ground control is added to the image processing. Not bad for data collected 423 miles up.
Comparisons to Aerial ImageryMany questions may come to mind when comparing satellite imagery to conventional aerial photography. When is it appropriate to use satellite imagery instead of aerial photography? How does the accuracy of satellite imagery compare to that gained from conventional airborne photography? Are there still applications that are better suited to the conventional approach? Will the satellite industry result in the eventual failure of conventional photogrammetric firms?
Satellite imagery is ideally suited to a number of projects, particularly projects that are more “block” as opposed to “linear” in nature. This is primarily because satellite images are collected in rather large blocks (a nominal image from IKONOS is 11 km by 11 km). The resolution and accuracy of satellite images cannot match that gained from traditional aerial photography. Therefore, conventional aerial images are still used for projects where high accuracy and resolution are important.
Other factors must be considered in comparing satellite to aerial images. Aerial photography projects can be tasked at a particular time of the year with very specific requirements—like a cloud-free day. Satellites are only able to capture imagery of a certain area when the satellite is overhead. Each satellite has a revisit frequency—the amount of time that passes between successive orbits over a certain area. For IKONOS, this revisit frequency is 2.9 days for the 1 m product. An otherwise perfect day for image collection can be missed if the satellite does not pass overhead while the weather is good.
Most experts do not see a major threat to the traditional mapping firm created by improvements in the satellite industry. In fact, the overall demand for mapping and imagery products is growing exponentially thanks to the changes driven by technology. The increase in demand is good for both the satellite and conventional mapping industries.
Obtaining ImageryHow can you acquire satellite imagery? You can look for resellers in your area by visiting Space Imaging’s website at www.spaceimaging.com and navigating to their “find a reseller” menu bar. You will need to provide the geographical limits for your area of coverage. You will also need to provide details regarding the accuracy, resolution and image type (panchromatic vs. multispectral) that you require.
In general, prices range from $30 to $300 per square mile for the digital imagery. The pricing obviously increases depending on the required processing and correction of the imagery. A minimum order of $1,000 applies to all imagery of North America; a minimum of $2,000 applies for other geographic areas.
The successful launch of IKONOS by Space Imaging has created a revolution in the satellite industry. The new high-resolution satellite images have many applications in the mapping, surveying, GIS and engineering fields. Consider their application for your next project. c
All satellite images provided by Space Imaging (Thornton, Colo.) For more information, visit their Web site at www.spaceimaging.com.