Aerial Photogrammetry & Control
After an accurate geodetic control is established, aerial photography is the critical next link in the land base chain of events. The acquisition of aerial photography must be planned carefully. The parcel map will be built from and referenced to features that are visible in the aerial photography. For a successful project, several factors must be taken into consideration for aerial photography planning, including overall project timing, size of the project area, and scale.
Timing
The project must be scheduled to allow for a time lag between contract initiation and the land base completion. Flights should be planned for those times of the year when: trees are absent of foliage; the ground is free of snow; the sun is at the highest possible angle; it is not cloudy, raining, or snowing; streams and rivers are within their banks; and air traffic control permits the mapping aircraft to enter the project area.
Allow at least a six-week window for the flights. It may be necessary to re-fly the project area if there is unexpected interference with picture clarity. Once the flights are completed, adequate time should be allotted for processing of the aerials before the land base is completed.
Project Size
The size of the project area will affect both the time for completion as well as cost. Typically, multiple flight paths with photographs taken at controlled intervals are used to produce overlapping aerial photos for any given area. Therefore, mapping a large land base would be relatively more time consuming and significantly more expensive than a small land base.
Scale
The goals and needs of each project directly influence the scale for which aerials should be flown. Scale is the ratio comparison between measurements on a map, or photo, and actual ground distance. Scale can be expressed as a ratio (1:10,000, where 1 inch on the map equals 10,000 inches on the ground) or as an equation (1 inch = 12,000 feet). During the flight itself, scale varies as a function of the plane's altitude, such that a lower level flight will produce a larger scale aerial photograph. The smaller the number represented by such a ratio, the smaller the scale of the map and the larger the area shown. Thus, a scale of 1" = 40,000' is smaller than a scale of 1" = 12,000'. In general, larger scales such as 1" = 12,000' might be warranted for engineering level parcel mapping, while smaller scales may be adequate for planning activities. The scale of the aerial photo affects the size of features that can be seen and recognized. For example, smaller features such as manhole covers might be discernible on a large scale (1" = 500') photo, but would not be visible at a smaller scale such as 1" = 24,000'. As a practical matter, flight altitudes relative to land base detail and map scale are well established, as indicated by the following table:
| Base Map Scale and
Contour Interval (CI) |
Aerial Photography
(Negative) Scale |
Flight Altitude Above
Ground Level (AGL) |
| 1"=20', 1" CI |
1"=200' |
1200' |
| 1"=50', 1' CI |
1"=350' |
2100' |
| 1"=100', 2' CI |
1"=700' |
4200' |
| 1"=200', 5' CI |
1"=1600' |
9600' |
The mixing map scale and contour interval will result in adjustments to aerial photography scale and flight altitude. For example, a 1"=50' land base with a 2' CI would most likely be flown at 3,000' AGL. This would result in 1"=500' scale photography (also called the "negative scale" of the photography). The negative scale and map scale relationships displayed in the table are relative to stereo photogrammetric plotter capability.
Control Surveys
Control surveys are survey methods that establish the exact location of points on the ground. The purpose of control surveys for aerial photography and photogrammetry are to determine the exact position of the aerial camera at the instant of exposure and to establish known reference points for parcel mapping.
As an airplane flies over the terrain, it snaps many exposures at specified intervals. Without control surveys there would be no way of determining exactly where the aircraft was at the instant of exposure. Control surveys are used to determine the precise geometric relationship between the physical position of the aircraft camera (its altitude and attitude) and the internal spatial geometry of the camera itself.
To accomplish this task, targets are physically placed on the ground at specified locations within the project area. These targets are then surveyed to establish their location. For proper visibility, targets must be sized according to the photo scale necessary.
The number of targeted control points and their overall positions within the project area is relative to the flight altitude. Lower aerial photography requires more control points than higher aerial photography.
In some cases the targeted points used for the aerial photography may also be established points used for parcel boundaries or other survey applications. In these cases the targeted control points should be permanently monumented. There may also be cost considerations, since establishing a permanently monumented point costs more than a non-monumented point.
Another approach to establish control for aerial photography is to use the technique of Airborne GPS. Airborne GPS involves the use of GPS receivers mounted on board aircraft. The GPS receiver records the position of the aerial camera at the time of the photo exposure, enabling the photography to be controlled from the air instead of on the ground. As a result, fewer ground control stations are required.
Although this is a relatively new technology, Airborne GPS has several positive benefits. It is an unobtrusive and non-invasive method of determining positional information without targeting control points. Targeting takes time and the presence of the targets in a project area may require obtaining landowner permissions.
Before ending this discussion on survey control, it may be helpful to look at analytical aerotriangulation, a most cost effective method for reducing survey control requirements. This technique, often called analytics (or AT) for short, is a computerized, mathematical photogrammetric procedure for densifying survey control for base map compilation. Measurements based on the precise geometry of the camera and basic survey control can be employed to calculate X, Y, and Z coordinates for use as photo control. The root mean square (RMS) error of analytically generated photo control points should be no greater than 1:10,000 of the flight altitude. For example, if the flight altitude is 2100' AGL, the maximum allowable error in the X, Y, and Z coordinates of any analytically generated control point should be no greater than 0.21'. The use of AT reduces the total number of required targeted points.
Stereo Digitizing the Land Base
The third critical link in the chain of compiling a GIS land base is the stereo photogrammetric digitizing process. The major component of this phase is the stereo photogrammetric plotting instrument. At this point in the aerial photography process, the actual capturing of the land base occurs.
Elevations and Contours
The collection of contours is another matter. Contours can be collected by either of two methods: stream digitizing or electronically through a digital elevation model (DEM). In the stream digitizing mode, the stereo plotter operator follows his or her interpretation of the physical elevation of the ground with the digitizing dot, thus creating an isoline (contour) of coordinates that exist at the same elevation. This is not a task that can be done quickly or without a great deal of care.
Cartographic Editing
Upon completion of the land base compilation, the data must be cartographically edited. This process occurs on a digital edit station which consists of a computer, a digitizing surface, and a digitizing puck. At this stage, any overprinting of data is corrected, contours are smoothed, and street and place names are inserted. In general, the land base is given final cartographic quality and completeness. At this point in the process, the data are translated from the photogrammetric collection software to either the final GIS file format or a generic file format.
Field Editing
A field edit is completed by personnel physically walking throughout the project area. Survey crews use hardcopy of the land base and measurement tools, such as steel tape, a measuring wheel or even GPS, to compare the hardcopy of the land base to the ground, visually locating any missing detail. The missing items are positioned by measuring from features that have been properly located on the land base. The missing items are then scaled into their actual position and physically plotted onto the hardcopy land base. Back at the office, the printed map is electronically registered to the land base at a digital edit station and the detail obtained from the field edit is transferred into the digital land base via a digitizing puck.
The field edit process is both time-consuming and costly. The need for and extent of field edits should be evaluated carefully. Some spot checks are important to establish the accuracy and completeness of the land base.
Summary of Photogrammetry
Photogrammetry is an important tool for base map development and parcel mapping. Digital Orthophotography, which is a special type of aerial photography product than can be used to support parcel mapping.
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