Photogrammetry or aerial photography is the branch of surveying that deals with production of maps such as planimetric or topographic maps by compiling number of photographs taken in that area.
Photogrammetry consists two branches:
- Terrestrial photogrammetry
- Aerial photogrammetry
In terrestrial photogrammetry photographs are taken from a fixed position on ground while in the aerial photogrammetry, an aircraft with camera setup is used to take photographs from the air flying over the ground. In this article we will discuss about the aerial photography.
Terms Used in Aerial Photography
The terms or definitions used in aerial photography are:
An exposure station is the point at which aircraft is positioned in space with camera for instant of exposure by camera lens.
The height of exposure station above datum is called as flying height.
Altitude is the vertical distance between the aircraft and earth’s surface or ground.
Tilt is an angle produced by the rotation of aerial camera about line of flight (vertical axis).
Swing is angle produced by the rotation of aerial camera about horizontal axis which is perpendicular to the line of flight. Swing is also called as tip.
When aerial camera captures a photo of ground, the optical axis of camera will intersect at some point (center) in the aerial photograph. This point of intersection is called as principal plane.
Bisector of angle of tilt will intersect somewhere on the photograph (at a distance of f tan (t/2)). This point is known as isocenter.
Nadir point is a point on an aerial photograph pierced by the plumb line when dropped from front nodal point.
The combination of points demonstrating ground points and photo points are known as homologous points. This combination happened between original plane and projecting plane.
Procedure of Aerial Photography
- Establishing control points
- Flight planning and photography
- Photo interpretation and stereoscopy
- Parallax and measurement of parallax
- Construction of map and cartography
Establishing Control Points
Control points are points established on ground with known relative positions. The photograph captured is observed by setting these control points as boundaries. So, the points should be established in such a way that they should be easily identifiable on photograph.
There should be minimum of 3 to 4 control points are need in one photograph. The establishment of control points depends upon the scale of map, flight control, and cartographical method of mapping.
Flight Planning and Photography
Flight planning is nothing but knowing the height to be maintained by flight while taking photos, area to be covered in each photograph, number of photographs, no of strips, and time interval b/n exposures. This planning mainly depends upon the following factors
- Area to be surveyed
- Focal length of camera
- Scale of photograph
- Ground Speed of aircraft in still air
There are some formulae are available for different parameters as follows.
Altitude of Aircraft
It can be computed from
Flying height H = contour interval x C
Where C varies from 500 – 1500
Area covered by one photograph
Area covered by one photograph = (length x scale) x (width x scale)
Number of photographs required
No. of photographs required to cover all the given area are = N1 x N2
Where N1 =L1/((1-Pl)Sl ) + 1 & N2 = B1/((1-Pw)Sw) + 1
Where N1 = number of photographs in each strip
N2 = number of strips
L1 = length of photograph (in direction of flight)
L2 = width of photograph (perpendicular to the direction of flight)
Pl = longitudinal overlap
Pw = Side overlap
Sl = scale in length wise
Sw = scale in width wise
Photo Interpretation and Stereoscopy
Photo interpretation is done by the instrument called stereoscope which contains magnifiers. So, one can observe the three-dimensional model of area through it and it also ease the drawing of maps of photographed area. For accuracy, control stations, elevations, length of lines should be sufficiently available.
So, we can say photo interpretation will enable the significance of objects in photograph. Coming to stereoscopes, there are four types of stereoscopes are available which are used for the photo interpretation.
They are namely
- Lens stereoscope
- Mirror stereoscope
- Scanning mirror stereoscope
- Zoom stereoscope
Lens and mirror stereoscopes are majorly used for photointerpretation. Apart from these, some characteristics should be maintained for good photo interpretation. The characteristics should be as follows.
Shape is an important property for an object in photograph. The outline or configuration will deliver the shape of an object. So, one can easily recognize from the shape of an abject in the map or photograph.
Size is also an important factor in photo interpretation. The size should fixed to some scale and properly interpreted on the photograph. Then only the observer can feel the difference between large objects and small objects. For example size of major river and drain should be interpreted in different sizes.
The arrangement of objects in the photograph should be done in a good pattern in such a way that they should be easily recognizable without any overlapping confusion.
Sometimes top view may create confusion about object type, shadows of that type of objects will help to find the profile of an object.
Texture of an object in the photograph is produced by the whole combination of its shape, size, shadow, tone etc. so, it is dependent of scale of photograph and considered mainly in large scale photographs.
Site is nothing but location of an object. The location of an object can be easily identified based on its surroundings.
Parallax and Measurement of Parallax
An aerial photograph can be studied to get the location of an object by its co-ordinates in the photograph.
Similarly, to know the third dimension of same object, there should be minimum of two points of observation is needed from different angles.
Parallax is nothing but a displacement of an object in the photograph when point of observation is shifted to another angle.
In general Measurement of parallax can be done by two ways as follows:
- Floating marks
- Parallax bar
Construction of Map and Cartography
After collecting all photographs, it’s time to create or plot the map. There are several methods available to plot the details of map and they are
- Radial line method
- Slotted template method
- Stereoscopic method
Radial Line Method
Radial line method is a graphical method of plotting the map from vertical photographs. By this method we can prepare a planimetric map. Some kind of perspective properties are used in this method. Those are, a principal point is fixed in the photograph and the objects near this point are free from error of tilt.
The change is position of an object due to tilt and ground relief are measured outwards from the principal point. The location of point in two overlapping photographs can be corrected by the intersection of three rays from known points.
Slotted Template Method
Slotted template method is mechanical method of plotting. In this method templates are prepared which are nothing but enlarged images of standard scale photographs. These templates are made of transparent celluloid sheets or cardboards etc.
The steps involved in plotting the map by slotted template method are as follows.
- Preparation of templates
- Transfer of principal points
- Selection and transfer of minor control points
- Selection and transfer of lateral control points
- Centre punching
- Assembly of templates
- Completion of plotting
The equipment used in slotted template method is uneconomical than graphical method but however it is rapid and more accurate.
In Stereoscopic plotting method an instrument called stereo plotter or multiplex is used for preparing maps. The maps prepared by this method is of high precision.
In this method, the instrument will help to view the overlapped area in three dimensional which help to view the spatial model. Then the model is measured and orthographically projected as map. It is more accurate and this method is used by large mapping organizations.