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Higher quality photos and images boost demand for accurate ground control
By W. Brant Howard Generating accurate digital maps from aerial photographs and satellite images has never been a quicker, more affordable, and accessible procedure than it is today. As a result, the need for cost-effective and accurate ground control point collection has never been greater.
The growing popularity of digital mapping owes itself to advancements in at least two important technologies. First, soft copy photogrammetric packages have brought the power of automated orthorectification to the desktop computer. And second, improvements in the resolution and accuracy of airborne and spaceborne imaging platforms have made remote sensing the preferred map data source.
Contrary to popular belief, higher quality aerial or satellite imagery will not replace the need for ground control. In fact, ground control point collection becomes increasingly more important as image quality improves. Fortunately, collecting ground control is now a much faster, more accurate and cost-effective process, thanks to a variety of GPS-related tools and techniques.
GPS technology is rapidly replacing traditional survey methods for acquisition of the ground control that is necessary to rectify image frames. Not only is GPS less time consuming and more reliable, it requires a smaller field crew. In some cases, one GPS technician can collect dozens of points in a day.
In addition, GPS is far more flexible. Points can be surveyed before or after the photos are collected. This has enabled several digital map vendors to forego acquisition of new images and simply create orthorectified products from archived photos and up-to-date ground control. The results are highly accurate maps created at a fraction of the cost.
The flexibility of GPS offers many advantages. It is a technology that can be applied in many ways to produce different levels of accuracy to accommodate varying specifications. The required accuracy of GPS-derived ground control is typically dictated by the quality of the source images, needs of the end user, or intended applications of the orthophotos that will be generated.
Care must be taken to select the proper ground control collection technique for any given project. Predictably, the cost of ground control increases with accuracy specifications, but the technology now allows experienced crews to achieve very accurate results at minimal expense if proper GPS collection techniques are applied. Assessing Accuracy Requirements
The list of geospatial data vendors that utilize ground control for the generation of products includes numerous well known GeoTech companies such as Space Imaging, Analytical Surveys, SPOT Image, HJW/Map Factory, Vexcel, and others. CompassCom is a GPS-services firm that collects ground control for many major digital map vendors in both the aerial and satellite remote sensing industry. In each case, the GPS survey method varies slightly depending on the specifications supplied by the vendor.
As mentioned, accuracy requirements depend on many factors. For the vast majority of digital map uses related to cadastral, topographic, and planimetric mapping, an end product accuracy of about 1-meter is sufficient. Vexcel, ImageScans, and others have launched very successful off-the-shelf product lines with 1m accuracy and resolution. To create these products, ground control points must be surveyed with sub-meter accuracy, usually in the 1/10m to 1/2m range.
A few products, however, demand ground control surveys to the centimeter level. For example, The MapFactory's need for x, y, and z ground control with 2cm to 10cm accuracy is driven by the end use of its AlphaMap Telecom Product line. These are DEMs and building height files used by telecommunications engineers to design wireless Local Multipoint Distribution Systems (LMDS).
"LMDS carries a large volume of voice and data communication in a [radio] wave the size of a thumbnail," explains Michael O'Connell, vice president of Marketing for The Map Factory, headquartered in Walnut Creek, Calif. "X, y, and z data [used to create the DEMs] must be extremely accurate because a feature as small as an air conditioning vent on a building can block the signal."
Another important variable to keep in mind is the quality of the air photos or satellite images from which the orthorectified digital maps will be generated. There is a limit to the final accuracy that can be derived from any source data, and therefore, money will be wasted by acquiring ground control with accuracy that exceeds the capabilities of the data.
"A good rule of thumb is to collect ground control at an accuracy that is roughly 1/4 to 1/5 the resolution of the source data," says Harold Spradley of Survey Resources International, Inc., a Houston-based geodesy and photogrammetric consulting firm which has contracted CompassCom for several projects. Picking Points
For projects involving existing air photos or satellite images, control point locations are typically selected on the hard copies before the field crew is dispatched. The key to selection is finding photo-identifiable features, which simply means the points can be seen in the image and on the ground. In addition, points should be on sidewalks, parking lots, or other permanent public features that a field crew can readily access.
CompassCom often uses the corner point where a walkway from a house meets a sidewalk at the street. The painted T on a tennis court serving box also works well. Both of these examples offer high contrast, which is important in projects where ground control is obtained before the images. Contrasting surfaces such as cement and grass are easier to find later on the hard copies.
The control point should also be a suitable GPS survey location, free of overhead obstructions that may block the satellite signal. For this reason, vendors should give survey crews some leeway in choosing points because sometimes a feature found in an archived photo is obscured by trees when the crew arrives.
Surveying points for satellite image rectification vary only slightly from that of air photos. CompassCom worked closely with Vexcel Corp. in collecting control that Vexcel processed and supplied to Space Imaging of Thornton, Colo., for use with its IKONOS satellite data. In that project, control points were selected in the middle of many features such as sidewalk squares or tennis courts.
"When images are acquired by digital satellite sensors, there is often a bleeding between the edges of pixels," says Rob Ledner, director of Aerial Photogrammetry at Vexcel in Boulder, Colo. "When the edge of a sidewalk coincides with a pixel border, the location of the edge can be displaced in the image, which makes for a poor control point target."
Space Imaging sought to eliminate any potential error caused by this bleeding by asking CompassCom to set up GPS points on the centers of sidewalk blocks or parking lots.
Density of control points is another factor that varies according to specifications of orthorectification software and scale of photos. Generally, vendors request points established on 1- to 2-square mile grids in the overlap areas of stereopairs. At least six should be surveyed within an overlap region, and if possible, one point should be located within three overlapping photos of a flight line. Spacing of points will be closer for photos taken at lower altitude. Collecting Points
For most projects requiring sub-meter accuracy, CompassCom uses Trimble Pathfinder Pro XRS units. These are 12-channel devices integrated to receive position signals from the GPS satellites and differential correction signals in real time from a subscription satellite service or a beacon antenna transmission.
CompassCom typically uses the OmniSTAR or Racal differential service which is broadcast by communication satellites. These services require no additional hardware attached to the receiver and are accessible to anyone who purchases a subscription. In this process, differential correction signals from a network of antennas at established positions across the country are transmitted to the relay satellite for broadcast.
"An advanced version of the differential correction technique supplied by Racal and OmniSTAR is the so called 'Virtual Base Station' mode. This adds another level of accuracy to the differential results, increasing them from sub-meter accuracy to about 30cm in x and y and about 50cm in z," says Ledner. "This method is cost-effective since only one person is required, and it is also very accurate."
With the Pro XRS mounted on a hand-held monopod and operating in its code phase processing mode, the field technician places the tip of the pod directly on the selected control point. For sub-meter accuracy, a 10 minute residence time with the unit collecting one reading per second is sufficient. The accuracy of these control points varies from one meter to a decimeter.
To ensure the equipment is getting satisfactory results, CompassCom locates one or more first-order National Geodetic Survey (NGS) points in the project area and collects readings on them. If the results are not within project specifications, CompassCom may set up a second GPS unit on an NGS point for differential processing on the computer at a later time.
For clients requesting greater accuracy than one decimeter, CompassCom makes several changes to the survey procedure. The much more precise Trimble 4800 receiver replaces the Pro XRS and is used in its carrier phase operating mode. Residence times on site are doubled to 20 minutes for each point, and the units are set on free-standing tripods for added stability.
Real-time differential correction with a satellite subscription service is substituted with a base station and post processing. The real-time services cannot provide the sub-decimeter accuracy due to slight signal distortions that result from atmospheric disturbances.
The geodetic base station, usually a second Trimble 4800, is established at a first-order NGS point within 50-kilometers of where the ground control will be collected. Post processing is performed in Trimble Pathfinder Office software which outputs the x, y, z files in a variety of coordinate systems and datums.
The horizontal and vertical accuracy of control points collected in this method are 2cm to 10cm. Costs associated with this technique are higher because a crew of at least two is required and they stay in the field longer due to doubled residence times. The survey grade GPS unit is also more expensive. Adding Details
Every digital map vendor has developed its own photogrammetric processes to produce high quality orthorectified images. Often these techniques require additional information than is contained in the control point file. To accommodate these needs, CompassCom supplies as much extra information about the control point collection site as possible.
For each point collected, the field technician marks its precise location on a copy of the actual air photo or satellite image provided by the vendor so there can be no confusion about which sidewalk corner was used. In addition, the technician keeps a written log of physical conditions around the point and then takes photographs of it from two different directions with a digital camera.
In situations where the vendor is especially sensitive to elevation point coordinates in their final products such as the telecom products generated by The Map Factory and Vexcel, the CompassCom technician uses a laser range finder from Laser Technology of Englewood, Colo., to shoot distances from control points to the tops of nearby buildings. Vendors can use this to calibrate building height DEMs made from the air photos.
All of this information is sent along with the ASCII file of point coordinates to the vendor who uses them to tie object coordinates to the ground and compensate for local datum variations. The results are very accurate digital map products produced at relatively low cost from either new or archived air photos or satellite images. About the Author
W. Brant Howard is the president and founder of CompassCom, Inc., a provider of GPS products and services specializing in ground control collection and implementation of Automatic Vehicle Location systems. More Info CompassCom; Tel: 303-680-3221; Web: www.compasscom.com Back |
