Web Exclusive: Establishing Control in Mapping
In my last column (September 2000) I discussed many of the issues related to the establishment of photo control for mapping projects. What I did not cover, however, was the different procedures that can be used to determine the three-dimensional position of these photo control points.
The method used to establish the position of control points depends on the accuracy required in the project mapping. Obviously, the cost of establishing this control can vary significantly depending on the method employed. Therefore, it is important to pick a process that satisfies the accuracy requirements of the project while minimizing the cost to establish the control.
Control OptionsThere are a number of options available to surveying and mapping professionals for controlling a mapping project. These options range from selecting control from existing maps of the project area to various applications of GPS surveying. Conventional surveying methods can be used to establish control for projects, however, they typically are not cost effective and generally apply only to very small projects. Thus, this discussion does not include any in-depth information regarding conventional surveying procedures.
Historical MapsSelecting control from existing mapping of the project area is typically the least costly option for establishing photo control positions. As you might expect, it also generally provides the least accuracy in determining these positions.
For example, USGS 7.5 minute quadrangle maps are often used as the source of control for projects that do not require high accuracy in the completed mapping. These quad maps are relatively small scale (1"=2000'); therefore, road intersections are the most common feature selected for control positions. The maps are readily available in both paper and digital formats. They generally include information for geographic, state plane and UTM coordinate projections in the margin of the maps.
What kind of accuracies should be expected from control selected from quad maps? Horizontal positions can be determined to an accuracy of 40 to 50 feet. This error budget is really a factor of both the accuracy of the original mapping and the degree of care that is used in scaling or determining the position from the map. Vertical accuracies can typically be determined to an accuracy of a few feet, particularly when road intersections are selected where a spot elevation is shown on the map. Spot elevations are considerably more accurate than interpolated elevations from the map’s contours.
Larger scale historical maps can provide significantly more accuracy than USGS quad maps. But they are not always available for a project area. Even when they are, it can be difficult to locate them and gain access to the valuable information they contain.
The advantages to using historical maps should be obvious: they generally provide the fastest and most cost-effective method of determining control positions, and their greatest disadvantage is the relatively low accuracy they provide.
Differential GPS ControlThe next step up in accuracy and cost is the use of differential, or code-phase, GPS observations. Typical differential GPS observations can provide accuracies in the range of 1 meter horizontally and 2 meters vertically. Differential GPS (DGPS) is cost-effective because the equipment required is relatively affordable and the observations can be conducted by a one-person crew.
Differential observations require a base station for post-processing or a correction signal for real-time correction. But, there are many base stations that collect data 24 hours per day and place the necessary observation files in the public domain on the Internet. This data can be downloaded and used for differential correction for free. The Continuously Operating Reference Station (CORS) network is an excellent example. Data for CORS stations is collected and immediately placed on the National Geodetic Survey (NGS) website at www.ngs.noaa.gov. Most project sites are covered by public base stations, as differential observations can tolerate baseline lengths of 300 kilometers and still provide sub-meter horizontal accuracies.
Moreover, many differential GPS receivers have the ability to accept a real-time correction signal. Several commercial services broadcast such a signal. You can subscribe to their services for just a few hundred dollars per year. Accuracies gained from differential observations can provide a cost-effective method for controlling projects that do not require the utmost in map accuracy.
Real-Time Kinematic ControlReal-Time Kinematic (RTK) GPS can provide an efficient, yet accurate method of establishing control. This method is better suited for projects of limited size due to the nature of RTK observations. RTK is somewhat limited as constant communication between a base station and the roving GPS units must be maintained. Points can be determined very quickly, but the equipment required to conduct RTK observations is relatively expensive.
Accuracies for control points established with RTK procedures are very similar to those gained from static or rapid-static GPS observations—generally a few centimeters should be expected for both horizontal and vertical positions. But, be very careful any time you employ RTK procedures for establishing control positions. In its most basic application, RTK surveying results in little or no redundancy for the control positions. It is much easier to make mistakes using RTK observations than with static or rapid-static GPS surveying. You should always build in redundancy and quality control checks in any RTK efforts. For example, you may choose to occupy all control points twice with the base station set on different known control points each time. This can provide a high degree of reliability.
Static or Rapid-Static GPS ControlThe most accurate and most costly method of determining control for mapping projects is static or rapid-static GPS observations. The traditional use of these observations results in the construction of a strong network that ties known control points with all of the new control points that are being established. Sophisticated carrier-phase GPS receivers are used during simultaneous observations in the field. Most observations make use of a three- or four-person crew for observing sessions. The observation data is then processed in the office, which results in accurate baselines, or three-dimensional vectors, between observed points.
Positions can be established in the range of one centimeter horizontally, and two centimeters vertically when proper planning and observations are employed. This method also results in considerable redundancy and therefore high reliability in the newly determined control positions. This method is commonly employed for larger projects where strong accuracy is required for the project mapping.
Airborne GPS ControlAirborne GPS control is a relatively new method of controlling aerial photography. This method uses kinematic observations in the airplane during the time the photography is acquired. A GPS antenna is placed on the fuselage of the aircraft directly over the photogrammetricÂ camera.Â TheÂ GPS receiver is linked to the camera. The camera sends an electronic signal to the GPS receiver at the midpoint of each photo exposure. The GPS receiver precisely logs this signal as an event in time.
At the same time, the GPS receiver logs kinematic carrier-phase GPS data at very short intervals—typically one half or one second epochs. This airborne observation data is processed against a ground-based GPS receiver that collects simultaneous data in the project area. This results in the accurate position of the aircraft during the entire flight mission as a function of time. This positional data is then used along with the logged events (individual photo exposures) to interpolate the position of the camera at the midpoint of each exposure. The result is accurate positioning of the camera for all photos. These positions can be used to provide accurate mapping from the collected photography.
Airborne control procedures do not totally eliminate the need for ground control. But the number of ground control points can be dramatically reduced, particularly for large projects. While airborne control can be very cost-effective for large projects, it is not typically used on smaller projects or ones that are more linear in nature.