PLS - EXAMINATION, PREPARATION AND REFRESHER COURSE
TOPOGRAPHY / PHOTOGRAMMETRY
Definition:
As drawn from the Greek language Photogrammetry may strictly be defined as drawn or written measurements made from light.
a) Photo Greek for light
b) Gramma Greek for drawn or written
c) Metron Greek for to measure
The American Society of Photogrammetry in 1975 rewrote this simple and concise definition into the politically correct statement of the English language to satisfy today's wide range of disciplines being studied by professional photogrammetrist.
Photogrammetry is the art, science and technology of obtaining reliable information about the physical objects and the environment through processes
of recording, measuring, and interpreting photographic images and patterns of
electromagnetic radiant energy and other phenomena.
Earth Measurements
In this class we will confine our discussion to that portion of the Photogrammetric Science delving in land surveying and the measurement of the Earth's surface from stereoscopic point exposures.
In the photogrammetric process: the object is to determine the location of, and the horizontal and vertical azimuths of at least two geometrically strong stations (camera exposures in this case) so as to be able to compute the coordinate location of any visible point on two or more photos by ray intersection from these stations. This basic principal of photogrammetric measurements is not unlike that of resection and or intersection for the land surveyor on the ground from two known stations. Photogrammetricly, this process is performed through complex computations from known ground control points, image coordinates, and camera geometry. When completed, this process allows the photogrammetrist to compute the coordinate of any point visible on any two images through ray intersection. Millions of these image points, when intersected make up the map detail visible on standard aerial map manuscripts.
Equipment
There are two basic instrumental components in the photogrammetric process of measuring the earth's surface. The first is the aerial camera, which gathers a geometrical rigid image of the earth's surface on aerial film. The second is the stereo plotter which recreates a full restitution of two or more such photographs, and allows for the measuring and recording of the image data in three dimensional model space.
In addition to these basic data gathering and measuring equipment essential to the science of photogrammetric mapping , there is considerable support equipment required to complete the photogrammetric process. This equipment includes but is not limited to: airplanes, film processors, printers for contact prints and diapositives, photo enlargers, photo scanners, computers, software, pen and raster plotters and other asundery equipment.
The Photogrammetric Camera
The metric photogrammetric camera is unlike your standard camera in that it has a fixed focal length, calibrated lens, and flat film platen system with centering marks called fiducials. It is these three characteristics that make the photogrammetric camera a precise instrument which records image data in a rigid geometrical manner. The precise calibration of photogrammetric cameras is required prior to use in any photogrammetric measurements. It is the strict geometry of the focal length, lens distortion and centering marks that enable the photogrammetrist the ability to make accurate measurements from these photos in the stereo plotter.
The standard photogrammetric camera used for aerial mapping weighs about 320 pounds and measures about 2' x 2' x 2'. It consists of a camera cone with lens, a removable film magazine a leveling mount and view finding system.
The Camera Magazine
The camera magazine contains the aerial film with take-up spools, a precisely ground flat platen and a vacuum system. When the film transports between exposure the vacuum is released the platen rises slightly while the film moves for the next exposure. Once the film has moved the vacuum draws the film up against the platen and the platen drops into contact with he precision ground frame on the camera cone, thus establishing a fixed principal distance at the time of exposure. The standard image format for aerial photographic film is 9 inches by 9 inches. The film itself, is abut 10 inches wide and comes in 250' standard rolls.
The Camera Cone
The camera cone is the heart of the geometric system in that it contains the camera lens and the precision ground focal plane surfaced which the film rests against at the time of exposure. This system is a precisely aligned system where the camera axis is perpendicular to the focal plane. The lens and camera is designed so that its optimum focal distance matches this projection distance for sharp imagery and solid geometry. There are 4 to 8 fiducial marks placed on the precision ground surface which forms the base for the aerial film. These marks are illuminated and show up on the aerial film. It is these fiducial marks which the photogrammetrist uses to define the center of the camera axis when photo measurements are later made in the stereo plotter.
The Camera View Finder
In order to know where and when the exposures are to be made the modern camera crews utilized a closed circuit TV with monitor. This camera is aligned with the aerial camera to have coinciding axis. A monitor is mounted next to the pilot for easy viewing. Rheostats on the monitor regulate the sequence of exposures by adjusting the speed of a traveling bar on the screen to coincide with the image of the ground as it passes by. Dials allow the operator to set the desired overlap and camera focal length.
Camera Geometry:
1) Scale
d = photo distance f = focal length
D = ground distance H = flight height above mean terrain
h' = mean terrain elevation H' = elevation of flight above sea level
Scale = d/D = f/(H'-h') = f/H
example:
Aerial Camera is a 6" focal length
Mean terrain is 400' above sea level
Flight Height is 3400' above sea level
Expressed as a Ratio:
use same units for focal length and elevation above mean terrain
S = f/(H-h) = (6/12)' /(3400-400)' = 0.5/3000 = 1/6000
or 1:6000 photo scale
Expressed as Inches per Foot
express focal length in inches and elevation above mean terrain in feet
S = f/(H-h) = 6"/(3400'-400') = 6"/3000' = 1"/500'
or 1"=500' photo scale
Photo Ground Coverage:
Where D = d/Scale
letting d = 9" for total coverage of standard 9" aerial film
and using Scale = 1"=500' as in the above sample
Photo Coverage d(max) = 9" /(1"/500') = 9" x 500'/1" = 4,500' on the ground
2) Image Displacement: Photos are perspective pictures not orthogonal maps
a) Relief displacement
d = relief displacement h = height of object
r = radial distance to image H = flight height above mean terrain
The relief displacement on a photography radiates about the center of the photo on a vertical photograph. For objects above the mean terrain the displacement is always out ward from center of the photo whereas objects below the mean terrain the displacement is inward. This results in an inconsistent scale throughout the photograph.
Relief Displacement d = rh/H
From this equation you can see that the displacement is minimum at the center of the photo and is maximum at the outer edges of the photo. Also, it can be seen that the height of the object directly relates to the relief displacement.
b) Tilt Displacement
The effect of a tilted photo is to displace an image as seen above, again this causes an inconsistent scale throughout the photo.
It is these two displacements, relief and tilt, that separate a perspective photo from a true orthogonal map projection. It is not possible to obtain reliable scale distances from a photograph or photographic enlargement because of this. Photos should only be used for visual purposes, preliminary planning, or rough measurements because of this.
Stereo Coverage:
For the process of aerial mapping, continuous stereo overlap is required of the photos. Along the flight line it is desirable to plan on 60% to 65% forward overlap. This assures coverage, and provides sufficient triple overlap area for good control placement, aerial triangulation and some leeway for terrain change and flight line deviation. Side lap consideration is applicable for projects large enough to require more than one flight line. A designed 20% sidelap is desirable for most mapping projects. This normally assures coverage considering flight line deviations, ground relief, and ground control requirements . In mountainous terrain these percentages may be increased to allow for terrain changes.
Forward Overlap Side lap between flights
When planning a flight line, a standard 60% forward overlap would correspond (1-60%) or 40% forward progression between photo centers. similarly a 20% side lap would correspond to (1-20%) or 80% flight line separation.
Base Between Photos Flight line Spacing
For standard 9" aerial film and using a photo scale of 1"=500' with a standard 60% forward overlap and 20% side lap, these distances would be.
Photo Base Flight Separation
b= (500 x 9) x 40% = 1,800 feet d= (500 x 9) x 80% = 3,600 feet
Camera Focal Lengths:
Aerial cameras come in varying focal lengths, however, four of the most common are the 12", the 8 1/4", 6", and 3 1/2 ". Their focal lengths determining their use because of geometric considerations and resolution. Longer focal length cameras are used for reconnaissance because of low radial displacement and high resolution characteristics. On the other hand, shorter focal length cameras are used for mapping because of their strong geometric strength of the angle of intersection on stereo images and increased relief displacement. This can be see by revising the previous equations of relief displacement.
where relief displacement d = rh/H and Scale S= f/H or H= f/S
by can rewrite radial displacement by substitution for H
d = rhS/f
From this equation, it can be seen that the displacement (d) caused by the height (h) of an object is inversely proportional to the focal length of the camera. It is for this reason that longer focal length cameras are more suitable for reconnaissance purposes. Objects are not displaced so much due to differences in heights on the ground. In addition, the longer the focal length of the cameras, the flatter the lenses are, and the easier they are to maintain a precisely ground surface. This provides cameras with higher resolving power throughout the field of view, which is another requirement of reconisence cameras.
On the other hand, this equation shows that the shorter focal length cameras have greater image displacements due to changes in height. It is this larger displacement that provides the photogrammetrist with the ease of measuring elevations in a stereo model. As such, the shorter focal length cameras are those used for topographic mapping. The standard aerial topographic mapping camera is the 6 inch camera. It has sufficient relief displacement and its angle of intersection is large enough to provide accurate vertical measurements. The lens is sufficiently flat enough to maintain minimal lens distortion and a very high resolving power. The 3 1/2 possesses excessive relief displacement and lens distortion and usually only used in flatter desert lands.
Although it is important for the surveyor to understand these differences , it is probably of most importance to remember that it will be the 6" aerial camera that will be used for 99% of all aerial mapping projects. If circumstances dictate otherwise your photogrammetrist will advise you as to the reasons.
Camera Distortions and Calibration
The heart of the theory of photogrammetry lies in the aerial cameras geometric stability. It is imperative that the lens has minimal lens distortion over the full image, and that the resolving power of the lens remains sharp over the entire image area. Aerial cameras are sent in for calibration every three years to assure their variations are within acceptable tolerances. Modern aerial cameras have distortions ranging from about +- 4 microns, and resolving powers from about 90 lines per mm at the center of the lens to 40 lines per mm at the outer edges. Such cameras carry certificates of calibration, and it is from such certificates that the photogrammetrist can apply mechanical or mathematical corrections within his stereo plotter to correct for these calibrated camera distortions.
Ground Control:
It is necessary for individual stereo models to have enough ground control in order to scale and level it to the earth's datum. To this end, it is essential for the Land Surveyor in charge to understand the needs of the photogrammetrist.
Purpose of Ground Control:
Scaling the Stereo Model:
In order for a stereo model to be scaled it is necessary to have a minimum of 2 points to determine the scale facto and rotation. This is a minimum and not desirable since there is no check on this data. For this reason photogrammetrist will require a minimum of three horizontal points in each individual model so that there is a horizontal check on the map being prepared.
Leveling the Stereo Model
Leveling the stereo model is liken to establishing a vertical plane for the photogrammetrist to measure from. To define a plane it is necessary to have three points which are not on a straight line. Again this is a minimum number of points to define this vertical plane, and not really acceptable since the photogrammetrist has no way to check this data. For this reason a minimum of four points are desirable to have a check on the vertical datum the map will be prepared from.
Optimal positions of these points are always sought for these control points. The greater the separation of the data the better the scaling and leveling are. It rather like building a table. If the legs are all in the center of the table the table is not very stable. On the other hand if the legs are near the corner of the table it is a very stable surface. Because of toadies surveying equipment and capabilities, usually it is just as easy to obtain horizontal and vertical data on all four such points. This is highly advisable and provides a very stable model to build on.
Types of Photogrammetric Control:
Full Ground Control:
This method of ground control requires the ground surveyor to put out enough control to satisfy the above requirements for every planned stereo model covering the project. Usually this will be a minimum of 4 to 6 points per planned model, to assure satisfying the scaling and leveling requirements of each model discussed above. Good for small projects consisting of one to three models.