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
- Terms Used in Aerial Photography
- Procedure of Aerial Photography
Terms Used in Aerial PhotographyThe terms or definitions used in aerial photography are:
Exposure StationAn exposure station is the point at which aircraft is positioned in space with camera for instant of exposure by camera lens.
Flying HeightThe height of exposure station above datum is called as flying height.
AltitudeAltitude is the vertical distance between the aircraft and earth’s surface or ground.
TiltTilt is an angle produced by the rotation of aerial camera about line of flight (vertical axis).
SwingSwing 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.
Principal PointWhen 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.
IsocenterBisector 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 pointNadir point is a point on an aerial photograph pierced by the plumb line when dropped from front nodal point.
Homologous PointsThe 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 PointsControl 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 PhotographyFlight 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
Photo Interpretation and StereoscopyPhoto 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
ShapeShape 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.
SizeSize 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.
PatternThe 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.
ShadowSometimes top view may create confusion about object type, shadows of that type of objects will help to find the profile of an object.
TextureTexture 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.
SiteSite is nothing but location of an object. The location of an object can be easily identified based on its surroundings.
Parallax and Measurement of ParallaxAn 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 CartographyAfter 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 MethodRadial 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 MethodSlotted 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