|This is the third article in a series about writing good Request for Proposals (RFPs) to ensure that high quality geospatial services are acquired. In the previous issue, we discussed accuracy specifications. In this edition, we will address what ground control is and what factors to consider for on your next project.|
Acquired aerial photography has no ground coordinates. It is simply a pretty picture. Photogrammetrists use the science and art of photogrammetry along with ground control points, possibly airborne GPS/IMU, and aerotriangulation to assign ground coordinates to the imagery using a particular coordinate system. Once done properly, the imagery can be overlaid with a map in your GIS system and it will always be placed in the same location.
How Much & What Type Of Control Is Needed?
The type and amount of ground control must be matched with the needed map accuracies. There are several aspects of controlling aerial photography. Each has its roll depending on the goals of the project and needs of the user base. Specifics will vary for each project, so it is highly advisable that the buyer consult professional photogrammetrists for assistance.
- Conventional Targeted Control: Aerial photography is controlled using surveyed and targeted (or paneled) control points. The structures in the ground may be temporary (i.e. rebar) or permanent monuments (i.e. Bernsten monuments). Permanent monumentation is generally the most expensive option because of the extensive field work required to set the monuments. However, if there will be additional users of the monumentation (such as city, county, or private surveyors), then the added cost may be well worth it.
The target placed on each point must be large enough to be observed in the aerial photograph and accurately measured. The type and size of the targeting material will vary based on the altitude the aircraft is flying when the photography is acquired.
|AGL (Feet)||Width of Material (Inches)||Total Length (Feet)|
|12,000′ or above||24″||15|
|The table summarizes acceptable target specifications given example above ground level (AGL) altitudes.|
- Photo-Identifiable Control Points: Many aerial photography projects can be controlled merely by using well-defined features visible in both the photography and on the ground. The advantage to using this type of ground control is that field targeting does not have to occur before the aerial acquisition. Field crews can go to the field before, during, or after the photography is acquired and take GPS measurements on these well-defined features. These coordinates can then be used to control the photography.
When ground control points are used to control aerial photography they are normally targeted so the point can be observed in the aerial photograph and accurately measured. The type and size of the targeting material will vary based on the altitude the aircraft when acquiring the photography.
- Airborne GPS: When the aerial photography is acquired and a GPS receiver is integrated with the camera system, Airborne GPS coordinates (x, y, z) are captured at the time of exposure. This is referred to as Airborne GPS. This does not eliminate the need for targeted ground control points, but reduces the total number required. Therefore, it can reduce the cost of control overall.
- Full Airborne GPS: Full Airborne GPS is possible when an inertial measurement unit (IMU) is added to the Airborne GPS equipment. This tool enables the recording of the precise angles (roll, pitch, and yaw) in which the aerial camera mounted to the aircraft is pointing. This is also referred to as the “pointing data.” When the pointing data is combined with the positional data (x, y, z) from the Airborne GPS then, in theory, no ground control points are required because all the required information to control the photography is obtained inside the aircraft. In practice, however, minimal ground control points are used to verify the airborne/IMU equipment was functioning properly. Often, high accuracy projects also require supplemental ground control so their special positional accuracy requirements are achieved. Full airborne GPS is quite useful is when acquiring aerial photography in locations where it may be difficult or costly to survey ground control.
Control In Your Request For Proposal
Now that you understand control, what important aspects should be defined in your RFP’s Scope of Services?
- Horizontal and Vertical Datum: A datum is a 3D model of the surface of the earth based on a surveyed network of physical points (i.e. as a sphere). The datum is the basis for a planar (flat) coordinate system. For example, the North American Datum for 1983 (NAD83), a horizontal datum, and the National Geodetic Vertical Datum of 1983 (NGVD83), a vertical datum, are commonly used for mapping projects in North America.
- Survey Units: Specifying ground units should be easy, either feet or meters. Unfortunately, it is more complicated because Americans have complicated this with a unit called U.S. Survey Feet. The difference amounts to only 32cm per 100 miles. Of course, to ensure accuracy, this is an important difference. The surveyors and engineers in most states use either meters or U.S. Survey Feet. Because of this confusion, it is important to clearly specify which unit is expected.
- Permanent or Temporary Ground Control Points: If these are marked with a permanent monument they are permanent control points. If they are marked, but cannot reliably remain at a location for a long period of time, they are temporary control points. Either type of ground control point can be used to assign ground coordinates to aerial photography. As discussed, the advantage of permanent ground control are they can be relocated by other surveyors and engineers in the future and/or used to tie other future surveys into your project. The number and placement of the control points is critical. The geospatial service provider should dictate these criteria to ensure the mapping meets the required positional accuracy specifications.
Look For Part 4 In The Next Issue
In the Part 4 of this RFP series, we will discuss aerial acquisition of data. Learn how these processes work and what aspects are important in your next RFP.
Questions Or Comments?
If you have questions about this article, feel free to contact Aerial Services, Inc.’s experts for advice with no obligation. We are here to help you.