| Two Images are Better than One
Firms see aerial and satellite imagery as complementary
By Kevin P. Corbley If Mark Twain were in the aerial survey business, he would probably say, "The rumors of our demise have been greatly exaggerated."
With the launch of the remote sensing industry's first 1-meter (m)-resolution satellite only a few months away, a serious misconception has begun circulating among GIS users that aerial surveying will soon be a thing of the past, replaced entirely by eagle-eyed satellites.
Nothing could be further from the truth, agree aerial survey firms and imaging satellite operators alike. In fact, both see new opportunities and expanding markets in digital mapping thanks to the coming 1m resolution satellites. More importantly, these aircraft and satellite companies expect GIS users, especially city and local governments, to benefit from having a larger selection of better quality mapping products and services.
"The misunderstanding arose over the term 'high-resolution' which has a distinctly different meaning in the satellite industry than it does in the aerial mapping business," said Shane Imwalle, director of marketing for Woolpert LLP, a large aerial mapping and photogrammetry firm in Dayton, Ohio. "High' is a relative term."
When Space Imaging of Thornton, Colorado, launches its IKONOS 1 satellite this fall, it will be the first commercial satellite to acquire images with 1m spatial resolution - a huge increase in image detail over previous commercial imaging plat- forms. One meter is considered high resolution for satellites, but it is on the low end of resolution for conventional aerial photography, notes Imwalle.
Aerial surveyors have built a billion-dollar industry worldwide providing photography and mapping services primarily at spatial resolutions better than 1m, usually at the 6-inch to 1-foot level. Coarser-resolution mapping requirements have traditionally been satisfied by space imagery providers like SPOT, which offers 10m resolution imagery, and Landsat with its 30m detail.
Imwalle and others agree the coming satellites will bridge that 1 to10m spatial resolution gap that has long existed between aerial photography and space-borne imaging. And this convergence in capability for the first time adds a complementary aspect to the products and services offered by both industries.
The potential advantages these complementary mapping data sets make possible have not been lost on Space Imaging. Three years ago, it began forging partnerships with major U.S. aerial survey and photogrammetry firms, such as Woolpert, whereby each would sell the other's products. The partnership is called the Mapping Alliance Program (MAP).
"These partnerships provide GIS customers with complete product continuity in their mapping projects," said Space Imaging vice president of product research and applications, Lynwood Givens. "The customer can get digital map data at any resolution or scale from a single source, either through us or one of our MAP partners." Photos, Images Add Value to GIS
In a city, county or state GIS, aerial photographs and satellite images often serve as the sources of feature layer data that populate the GIS or as basemaps upon which those layers are stacked. In the basemap role, the photo or image is orthorectified, which means that distortions due to terrain variables or camera angle have been removed. The resulting basemap has the same qualities of a map with all the content of a photograph.
"An orthorectified basemap adds incredible value to a GIS because it can be used to make accurate measurements of distances," said Jeff Liedtke, a GIS specialist at Space Imaging.
Air photos and satellite images have proven to be an extremely cost-efficient means of populating GIS feature layers. Photogrammetric processes can extract a GIS user's choice of natural or man-made features from the image or photo and accurately tie them to their correct locations on the GIS.
"Many long-time aerial photo customers may not realize that the same photogrammetric processes we have applied for years to air photos can be applied to satellite images," said Bob Brinkman, director of photogrammetry at Woolpert.
Although aerial photographs and satellite inages have many similarities, they are not entirely interchangeable in some GIS applications. Specific end-use requirements must be considered in determining whether satellite images or aerial photos are used for any given mapping project. The most important factors taken into account usually will be the level of map detail required, and the overall project coverage size, and the geographic location.
"Some projects will clearly require air photos, while others will favor satellite images," said Brinkman. "And some city and county GIS projects will benefit from a combination of both data sets." Applications of Photos, Images:
Mapping experts from Woolpert and Space Imaging offered the following examples of common city, county, and state GIS applications that benefit from using either aerial photos or satellite images.
Government Planning - City planners and engineers will be major consumers of aerial photo products because so many of their planning projects require locating, inventorying and mapping very small features and tying them to a GIS basemap. Many city and county planners, however, will utilize high-resolution satellite imagery in projects involving larger assets and infrastructure. Engineering Design - Most engineering design work conducted in a GIS will continue to be carried out using aerial photos because of their extremely high accuracy.
Parcel Mapping - Tying properties to multiple GIS layers has proven to be one of the most valuable applications of GIS technology among local governments. In the urban environment where properties are often quite small, parcel mapping is expected to rely heavily on aerial surveys as it has in the past. Outside the city center, however, in areas where property sizes are larger, satellite images are extremely economical for mapping land ownership boundaries and building footprints.
Natural Resource Mapping - High-resolution satellites have a distinct edge in mapping most natural land cover, including park land, agricultural and forest areas.
Disaster Management - Local governments have long relied on high-altitude views of natural or man-made disasters, and both aircraft and satellites fill this role, satellites for large areas and aircraft for rapidly changing situations. Mapping with Both Sources
Those applications that can conceivably use a combination of aerial photos and satellite images, will probably get by using one or the other. A few GIS applications, however, will benefit from using both types of map data.
Change Detection - Planners at all levels of local government want to identify and quantify changes in land use and land cover. Woolpert already is seeing some of its customers use satellite imagery to find rapid growth areas, especially at the urban-rural interface. Once pockets of growth have been spotted, they zoom in for a closer look with aerial photos at higher resolution.
Corridor Mapping - A similar zoom-in approach is being implemented by many transportation planners. They use satellite images to identify land cover types along potential new transportation routes. Once the specific route has been chosen for a new highway or rail corridor, the planner hires an aerial survey company to photograph just the chosen area.
Woolpert's Brinkman expects some clients will choose satellite images first, because of how quickly they can be acquired, as a means of jump starting a GIS program. As they develop the GIS over time, they will fill it in with more detailed aerial photos as applications arise. You Don't Need to be an Expert
From the perspective of the city planner or county surveyor, doing business with aerial survey firms and satellite operators should remain the same as in the past. The only major difference will be the expanded line of products each offers, and the company's plans to provide customers with guidance in choosing the right product.
"Clients normally don't come to us with a specific spatial resolution in mind," said Brinkman, "They tell us what applications their GIS will be used for, and we determine the resolution, accuracy and map scale they need."
The bottom line, added Space Imaging's Liedtke, is that in many cases "the GIS user will not know - or necessarily care - whether their map data came from a satellite or aircraft, as long as it is accurate and cost-effective." About the Author:
Kevin Corbley is a freelance writer specializing in remote sensing, GIS and GPS. He is located in Denver and may be reached at 303-722-0312 or by email at [email protected]. Back |
