Control is established by relating objects found in the photography to their position on the ground. These objects can be normal features found in the everyday landscape, such as fence corners, utility poles or road intersections. These naturally occurring features found in the photography are known as photo-identifiable control points. Control points can also take the form of artificial features constructed specifically such that their location is clear in the photography. These are commonly known as panel points or control targets.
Targets and Photo IdentitiesWhy are targets used for some projects while other projects rely solely on the use of photo-identifiable features for control? Under the right project conditions, both methods can be acceptable for controlling a project. However, there are advantages and disadvantages to each method that must be considered.
The advantages to setting targets at the project site prior to the aerial flight are many. One main advantage is that the control points can be placed precisely where they are needed for most projects. Targets also generally afford the best accuracy for project mapping as they provide sharp, crisp identification of the control point.
However, the disadvantages to setting targets can also be significant. First the targets are highly visible, and therefore, serve to alert everyone in the area that an aerial flight and mapping project is in progress. Moreover, painted targets on roadways or parking areas can last for years. With the exception of these painted targets, maintenance is required from the time the targets are constructed to the time the aerial flight is accomplished. High winds, children playing or livestock can obliterate targets prior to the flight. Control points are rendered useless if the target is missing when the photography is flown.
The alternative to using targets for control purposes is the use of photo-identifiable features found in the photography. These features are commonly utility poles, fence corners, sidewalk intersections, paint striping, driveway-road intersections or building corners. The advantages to the use of photo-identifiable features resolve the issues that make targeting disadvantageous. The features are part of the normal landscape, thereby avoiding unwanted publicity. The aerial flight can be commissioned immediately when photo-identifiable features are used for control-no need to wait for the placement of targets. Finally, there is no requirement for maintaining the features.
However, the disadvantages associated with photo-identifiable features can be significant. They are less accurate than targets. More photo control points are often required when photo-identifiable features are used as compared to the placement of targets. This results from difficulty in selecting photo-identifiable features at ideal spacing in the photography. Furthermore, the use of photo-identifiable features is not appropriate for some projects when the project area lacks enough features to provide adequate control. This problem is more common for rural projects as urban areas generally provide lots of opportunities for control selection.
The type of photo-identifiable features that are appropriate for project control will change with the scale of the photography and the project's accuracy requirements. What is visible in low altitude flights can be lost at higher altitudes. For example, small private fences can be crisp and clear in low altitude photography but disappear completely in higher altitude photography. The same can be said for utility poles.
Control TargetsThere are several simple rules that will result in better project results when targets are used. The size and configuration of the targets will normally be specified by the project photogrammetrist. Targets must have good contrast with the area where they are constructed. Normally, they are constructed as white targets in dark areas (or in grasses) or black targets in light areas. There are several common target configurations used for photogrammetric projects. The most common are the Tee, Chevron and Cross (See Figure 1).
Notice that the diagram also illustrates the most common location of the control point in relation to the target configuration. The size of these targets will vary depending on the required flight altitude. Higher altitudes require larger targets. The target must be larger for these smaller scale photographs so that it can be readily seen during the photogrammetric process. Most targets are constructed with leg widths of 12" or 24". The length of the legs can vary from a few feet to more than 40' depending on the flight altitude. Make sure that you have clear directions as to target configuration, control location and target dimensions prior to embarking on a photo control mission.
It is imperative that the elevation of the control point coincides with the elevation of the constructed target. Remember that the photographic measurements will be made to the point that is seen in the photography. When control points are located below or above ground, it is important to make an accurate measurement from the control point to the ground elevation at the target or photo-identifiable feature so that the elevation differential can be added or subtracted to the elevation of the control point to determine the appropriate photo control elevation.
Offset PointsSometimes it is impossible to directly occupy some photo-identifiable features selected as control points. Some areas can be void of features that can be occupied directly with GPS-yet this same area might have a number of features that would be otherwise suitable for photo control. For example, utility poles, busy road intersections or tall fence corners can serve as good control points, yet they can prove impossible to occupy directly.
It is acceptable to set offset points near these features for most projects (first check with your photogrammetrist)-then determine the direction and distance from the offset point to the photo control point to determine accurate positioning. Acceptable methods of measuring the direction and distance can vary significantly depending on a number of factors-primarily the offset distance and the required accuracy.
Many times, particularly when the offset distance is kept to a minimum, it is acceptable to measure the direction from the offset point to the photo point using a magnetic compass. Offset distances of 3' or 4' from many photo points (such as utility poles) are sufficient to allow accurate GPS observations. Care must be exercised when making the distance and direction determinations. Carefully read the magnetic direction. Don't forget to apply the current magnetic declination to correct from magnetic to true bearings. The USGS has software that you can download free from their website at www.usgs.gov for determining the declination at your project site.
It might be necessary to measure the direction to a much greater precision than that available with a magnetic compass when the offset distance is large or when the accuracy requirements for a project demand especially high precision. A theodolite or total station can be used for angular measurements for these higher accuracy requirements. However, a new problem is introduced. Intervisible points are not commonly present when GPS surveys are used to determine photo control positions.
There are several methods for measuring the direction in this scenario. A second GPS point could be established within line of sight of the first. One point would serve as an occupation point while the second acts as a backsight point. The angle between the backsight point and the photo control point is then measured precisely using a traditional surveying instrument. The advantage is accuracy-the disadvantage is the additional effort (and therefore money) required to establish the second GPS point. Sun shots or star shots can also be used to determine the direction from the occupation point to a backsight point.
The offset should always be accompanied by a sketch and an accurate, unambiguous description of the offset (See Figure 2). The sketch should illustrate in very general terms the location of the offset point, the photo control point, the distance between the two and the general direction of north as it relates to the sketch. Keeping the offset distance as short as possible is especially important as the amount of error in the photo control point position increases linearly with increased offset distances.
CommunicationCommunication is an extremely important part of the photo control effort. Most problems in the photo control process arise from a breakdown in communication between the surveyor and the mapping professional. Remember that the field crew often serves as the eyes and ears for all other project professionals. Many project professionals all too often do not get the opportunity to visit the project site first hand. Therefore they rely on the information transferred from the surveyors by way of survey reports, field sketches and other lines of communication (See Figure 3).
What things can be critical in terms of project communications? Certainly any changes from the photography noted during the surveying effort should be communicated-these changes sometimes prove critical to the project's success. They could be in the form of new construction (buildings, roadways, etc.) or maintenance operations (e.g., pavement resurfacing or utility reconstruction). Obviously these types of changes will be more prevalent when the field work is carried out after significant time has passed since the acquisition of the photography.
Finally, the importance of proper communications is well illustrated by the simple task of promptly notifying the photogrammetrist when all targets have been placed for a project (so the aerial flight can be scheduled). Also, it is critically important to clearly note any points that were moved from the originally selected point and the reason for this move. All too often, this simple task is overlooked.
The success and accuracy of mapping projects depends on the quality of the control provided early in the project. You can help ensure this success by gaining a clear understanding of the photo control process.
Target ConstructionWhen constructing targets, it is important to select locations that are:
- on relatively flat terrain to allow for more accurate vertical determination in the photography.
- in areas where minimal amounts of grasses and weeds allow the target to lie flat on the ground.
- in areas with an open sky to ensure good visibility in the photography and good GPS satellite conditions.
- easy to access and provide a safe environment for conducting the control sessions.