What makes the Automated Ground Control Service unique is the Google Earth-based interface on the CompassData website (http://www.compassdatainc.com) where customers can browse the archive for off-the-shelf ground control points (GCPs) in their areas of interest (AOI). If the archive contains points for that AOI, the customer can view ground photos, station diagrams and truncated metadata for those points, and then place an order by phone or email. The points will be delivered electronically within as little as 24 hours.
"We gave our field crews the name 'Control Freaks' because they travel the world to collect quality ground control for our clients," said Kate Schlatter, CompassData chief operating officer. "Their goal is to deliver consistent, highly accurate control points in a standard format whether the points come from the archive or our Control Freaks perform a custom collection for the client."
The archive currently contains 7,500 points for locations within the United States and about 2,500 elsewhere in the world, including major population centers in Europe, South America, Asia, Africa, Australia and the Middle East. If archived points are not available for a specific AOI, a customer can place orders for collection of new ground control, which will meet the end user's exact GCP specifications. CompassData maintains its own GPS survey crews, as well as 30 international partners, capable of acquiring high-accuracy points anywhere in the world in a matter of weeks.
"Accurate ground control is critical in photogrammetric processing of remotely sensed imagery, but collecting the points is often a difficult and a challenging task," said Schlatter. "CompassData's Control Freaks take the hassle out of ground control for our clients by automating the process as much as possible."
A Need for Control
CompassData was established to collect the mapping information needed to build GIS data sets from scratch and to verify the accuracy of existing geospatial data. CompassData assembled several teams of survey and mapping technicians with expertise in the latest GPS technology. In 1997, CompassData began collecting GPS points in support of airborne photography and satellite image processing programs.
The concept of building a ground control archive grew out of the fact that CompassData was receiving multiple requests to acquire new points in the same areas from several of the large raster imagery providers. W. Brant Howard, CompassData founder and CEO, realized the company could reduce the cost to the end user by maintaining ownership of the data sets and then repeatedly licensing control to the geospatial community.
Howard's vision was to continually build the archive and enable organizations to rapidly produce imagery and GIS mapping products without revisiting the same real estate over and over again. As chief Control Freak, Howard has set a goal for CompassData to become the host for the largest ground control library in the world by collecting data and reselling control from qualified partners around the globe.
The concept has caught on in virtually every sector of the image processing and photogrammetry industry. CompassData now counts dozens of satellite imagery and aerial photography firms and photogrammetric service professionals among its clientele.
Ground control is crucial to the rectification process because the surveyed points are used to accurately tie a remotely sensed image to its true location on the Earth's surface. In the orthorectification process, the real world coordinates of all other points in the imagery are calculated based on the locations of the control points. The geometric quality of the entire orthoimage is derived directly from the accuracy of the ground control. This means the quality of any application built upon the orthoimage depends on the control point accuracy as well.
The Microsoft Virtual Earth office in Boulder, Colorado, obtains a large volume of ground control data through CompassData for orthorectification projects, according to Jason Setzer, Microsoft Virtual Earth program manager. "It has been impressive to see them going to other regions of the world to collect points," he said.
"Aerial survey firms are also among the growing users of ground control," said Schlatter. "They use the points as back-up ground control in case their onboard GPS or inertial measurement units (IMUs) fail during flight."
She explained that aerial photography companies buy the points over project areas as insurance against disruptions of the navigation equipment, which can sometimes occur without notice in the air. The cost of purchasing a set of archived control points is much less than the expense of having to mobilize an aircraft to re-fly a project area.
Collecting New Points
Aside from the growing demand for quality ground control, CompassData owes some of its success to the fact that collecting the points is a difficult and sometimes dangerous process. Most geospatial companies simply don't want to deal with the technical and logistical problems that collection entails. In recent years, Control Freaks crews have faced down marauding rebels, angry land owners, suspicious military officials and an uncooperative Mother Nature, all in the name of ground control.
"Access to private property is the biggest obstacle in North America," said Hayden Howard, CompassData field services manager. "But we've literally had crews chased by armed bandits in other parts of the world."
Under ideal conditions, however, a ground control survey begins peacefully at CompassData headquarters. In the first steps of a custom collection, the client sends imagery of the project area. Typically, the client marks the images indicating where it would like the points to be surveyed. For a small town or city, the total number may be 10, but larger metropolitan areas will require up to 50 or more points. In most cases, the target collection zones are selected at even intervals throughout the AOI.
The Control Freaks team then draws a 500-meter-radius buffer around the desired point location and begins analyzing the imagery for photo-identifiable features that will be easily found on the ground. The best features are geometric shapes on objects likely to remain in place for many years without being overgrown by foliage, such as the intersection of two sidewalks on a street corner. At least one feature is selected in the photo within each buffer zone.
For locations in North America, a CompassData Control Freaks crew usually performs the work. Outside of the U.S., the firm has established relationships with GPS survey crews in South America, Europe, Asia, Australia and Africa, which can usually perform the work less expensively due to their proximity. These partners adhere to stringent collection specifications established by CompassData to deliver a standardized product. The in-country teams also have insight into issues relating to local customs, political situations and safety concerns, which greatly facilitates point collection. With assistance from these international partners, the Control Freaks archive contains points from Afghanistan, Myanmar and several African nations that are otherwise inaccessible.
Surveying requires an occupation time of about 20-40 minutes. At the end of each day, the technicians upload the collected location points into a laptop computer for differential post processing. In the United States, differential correction points are usually obtained via the Internet from freely available public GPS base stations or a mobile base station occupying existing control monuments.
"Sub-meter collection results in control points with 0.5-cm horizontal and 0.75-meter vertical accuracy," said Howard. "We also offer decimeter-level accuracy, which requires longer occupation times and different processing methodology."
Before the Control Freaks leave a project site, the crew also collects a GPS reading on a National Geodetic Survey monument. This becomes the corrected process point which enables the team to calculate the quality of the other ground control collected in the area. The technicians generate an accuracy report for each set of points, which comprises the metadata that end users need during evaluation and processing.