Spatial Technologies-New Angle on Mapping30 Jun, 2005 By: James L. Sipes
Oblique imagery enhances GIS
The Advent Of Aerial and satellite photographs has given us tools to see the earth's surface through new eyes. This ability to view geospatial information and identify features on the ground makes it easier to make decisions about communities and natural resources. A big limitation, though, is that vertical images, which are basically overhead views of the earth, show the tops of buildings or natural features, but not the sides. Overlapping vertical air photos can be viewed with stereoscopic pairs to produce 3D views, but even these views are limited in what they can show. Oblique images—photographs taken at an angle, typically greater than 20°—allow us to see the sides, and that's why there's so much interest in integrating these types of images with GIS applications. The big advantage of oblique images is that they're easier for most people to recognize because they more closely match how we normally see places, structures and features. The problem is that oblique aerial photographs contain image distortions caused by the angle of the camera and elevation changes of landforms. Until recently, rectifying these distortions has been beyond our technological limitations. A couple of companies have finally solved this problem, so now integrating oblique images into GIS is a reality.
Figure 1. A GIS layer showing parcel centerlines can be added to oblique imagery and used by tax assessors to minimize field reconnaissance. Image courtesy of Pictometry International Corp.
Aerial Photographs and Satellite ImageryTraditionally, the images used in GIS are created via aerial photographs or satellite images. Aerial surveys were first used in this country by the USGS (U.S. Geological Survey) in the 1930s and 1940s to support soil conservation and forest management activities. These types of low-altitude photographs cover smaller areas and produce a greater level of detail than high-altitude satellite photographs. Governmental agencies and private companies are contracted to generate aerial photographs for specific areas. Oblique photos are typically shot with helicopters and slow, low-flying planes, while vertical views are shot with higher-flying planes and satellites.
Satellites orbit hundreds of miles above the Earth and use electronic scanners to produce images of the Earth's surface. Satellite imagery was originally used during the Cold War for military purposes, but today is also used for more peaceful purposes. According to NASA's Goddard Space Flight Center, the United States has launched more than 4,000 satellites over the years, and several hundred are still active. National mapping depends increasingly on satellite imagery, but aerial photography is effective for site-specific projects.
Digital Orthophoto QuadranglesFor aerial and satellite images, the DOQ (digital orthophoto quadrangle) is the standard format used for national photographic coverage. DOQs are typically produced by scanning aerial photographs, comparing the aerial photographs to corresponding DEMs (digital elevation models) and then adjusting the photographs to correct any distortions caused by the angle of the camera or elevation changes in landforms. The USGS produces three types of DOQs: 3.75-minute (quarter-quad) DOQs, 7.5-minute (full-quad) DOQs and County DOQs. A 3.75-minute DOQ is typically black-and-white or color, and has a scale of 1:12,000 and a pixel resolution of 1 square meter.
DOQs can be obtained directly from USGS or from any of its commercial partners. The National Aerial Photography Program, which began in 1987, provides aerial photographs for the continental United States. The USGS EROS Data Center oversees several million frames of historical aerial and satellite photographs dating back to the 1930s. The National Digital Orthophoto Program develops and maintains national orthogonal imagery coverage in the public domain. USGS and the National Geospatial Intelligence Agency are acquiring high-resolution orthogonal imagery for major metropolitan areas as part of the National Map project. The National Agricultural Imagery Program makes images available to help with decisions about upcoming agricultural uses. The NASA Landsat Data Collection, which dates from 1975, consists of selected scenes acquired by NASA investigators for use in a variety of projects.
Oblique ImageryThe uses for oblique imagery are varied because the images provide a level of visual detail and information that is not available in orthophotos and can be accessed quickly and effectively.
Possible users include law enforcement, fire departments, emergency services, homeland security, tax assessment (figure 1), planning agencies and utilities. Law enforcement can use oblique images to determine the best place for a stakeout, emergency services can use visual landmarks to find the quickest way to an accident scene, and community planners are better able to determine how well a new design proposal will fit with existing structures (figure 2). A number of communities are using homeland security grants to acquire oblique imagery.
Figure 2. Engineers and architects can conduct pre-inspections of existing structures to gather information on building layout, construction type, and landscape details. Image courtesy of Pictometry International Corp.
The ability to georeferenced oblique images is a relatively new technology, so only a couple of companies offer products that provide these features.
PictometryThe industry leader in oblique imagery is Pictometry, whose patented information system combines aerial imaging with a software system called EFS (Electronic Field Study). Pictometry's software enables users to easily access up to twelve different high-resolution views of any property, building, highway or other feature in a county. The software also enables users to measure distances, height, elevation and area directly from the imagery (figure 3). The imagery is high resolution, so it's easy to select specific features such as doors, windows, sidewalks, roads and other objects. Pictometry's libraries contain orthogonal images, but more than 80% of Pictometry's images are oblique.
In Lake County, Ohio, every office in the county uses Pictometry. "We decided to go with oblique imagery because it provides so much more information than orthophotos," says Dick Kotapish, the GIS director for Lake County. The county has an unlimited number of licenses for governmental offices and is using the data across the whole spectrum of government. Currently, the county has set up more than 180 stations with Pictometry, and Kotapish says that number continues to increase.
Figure 3. Oblique imagery can provide measurements such as building height, distance, area, and elevation, as seen in this view of a building in Rochester, New York. Image courtesy of Pictometry International Corp.
Pictometry uses aerial photography to generate both orthogonal and oblique images. For most customers, Pictometry flies an area every two years or so, but sometimes it flies every year or every six months. The images are current enough for assessors to use them to evaluate changes.
"We use Cessna 172s flying at a very slow rate at either 2,000' or 5,000'." says Dan Pennacchia, senior vice-president of sales and marketing at Pictometry. Images captured at 2,000' have a pixel resolution of 6", while those from 5,000' have a resolution of 1' to 2'. "We capture images from east to west in a one-square-mile sector, and we capture orthophotos at the same time we do oblique photos," he says. Pictometry takes photos of images in all four directions, and as a result shoots a total of 12 to 20 views for every square foot of the earth's surface that it flies. This results in a massive amount of data. For example, the database for Los Angeles is around 1.3TB.
Pictometry is only about four years old, but its software is already used in eight out of the ten major population centers, and it has covered close to 20–25% of the United States' area. Pictometry is used in more than 120 counties, and that number continues to grow. Several states also use Pictometry techology. For example, the Massachusetts Highway Department signed a license agreement with Pictometry to provide oblique aerial imagery and software tools at no charge to all communities in the commonwealth. The Boston Metropolitan Area Planning Council is one organization taking advantage of this opportunity. The council acts as the regional distributor of Pictometry software through an agreement with the highway department. The Pictometry technology package includes the EFS software program, two-way oblique imagery for each community, and training materials and free training.
Figure 4. Combining oblique imagery with geospatial data such as contours that show the lay of the land provides planners with a better understanding of grade changes at an intersection. Image courtesy of Pictometry International Corp.
A number of communities use both ArcGIS and Pictometry, but unfortunately there's no seamless connection between the two. Users can export shapefiles to be used in ArcGIS or any other GIS program that supports this file format. EFS can display ESRI shapefiles and can provide the same functionality as ArcSDE, which is used for centralized data management. "One of the difficulties is that no one works with oblique images right now," notes Pennacchia. "When that changes, we will look at a greater interaction between oblique imagery and GIS." Pictometry has created a stop-gap solution by letting users import GIS data into its software or export images to plug into a GIS program (figure 4).
Some may think oblique imagery is too cost-prohibitive for most users, but Kotapish says the technology is cost-effective because of the time saved that would normally be required for field visits. "Our planning department has experienced a 70% reduction in field time. They can pull up the images to answer questions about setbacks, etc.," says Kotapish. When Lake County needed to build a dataset showing all of the nuclear power plant sirens in the county to make sure the sound patterns overlapped, it used oblique imagery. Orthophotos did not provide enough detail, and conducting field visits would have taken weeks. With Pictometry, the project was completed in one day.
Pictometry is also exploring ways to make its software more accessible. Pennacchia notes, "We also sell seats individually as a library, and offer Pictometry Online, where a user can access a myriad of images at a cost of $19.99 a month to look at images, or $69.99 to make measurements. Then you can buy images that you need."
OrthoBASE and OblivisionTwo other software packages that provide oblique imagery are ERDAS' IMAGINE OrthoBASE and Idan Computers' Oblivision. IMAGINE OrthoBASE is digital mapping software that simplifies photogrammetric procedures. An add-on module for IMAGINE Advantage and IMAGINE Professional, it's used to generate digital orthogonal imagery and oblique imagery. IMAGINE OrthoBASE provides highly accurate 3D point positions that can be used to determine topographic change. The software can read and input a variety of imagery sources and types of imagery, including aerial, satellite, video, 35mm and digital CCD cameras.
Oblivision, developed by the Israeli company Idan Computers, also combines orthophotos and oblique images with image-analysis tools. Idan is an established software company that develops CAD, image analysis and geospatial applications. Oblique images are geocoded and overlaid on orthophotos to produce a single data layer. One nice thing about Oblivision is that it is data-source independent. That means it can analyze any oblique image, new or existing, regardless of the settings, position and direction of the camera. These images can come from almost any source, including handheld cameras, scanned images and airborne cameras.
Privacy Issues?Though the added visual details offered with oblique imagery have many potential benefits, they also raise some serious questions involving privacy. The technology for taking oblique images is capable of generating a 6"-per-pixel resolution, and that will continue to improve. "Privacy is a little bit of an issue, and we try to be sensitive to the concerns of our constituents," says Kotapish. "One thing we have to keep in mind, though, is that oblique images don't really provide much more information than we would get if we conducted field reconnaissance."
James L. Sipes is the founding principal of Sand County Studios in Seattle, Washington. Reach him at firstname.lastname@example.org.