A method of collecting information by utilizing aerial photography, or from remote sensing technology using other bands of the electromagnetic spectrum, such as infrared, gamma-ray or ultraviolet.
Photogrammetry:
It is an art and science of obtaining reliable information from photographs.It encompasses two major areas of specialization.
.metrical:
It is applied in determining spatial information including distances, elevations, areas, volumes, cross sections, and data for compiling topographic maps from measurements made on photographs.
.interpretative:
Interpretative photogrammetry involves recognizing objects from their photographic images and judging their significance.These area of photogrammetry identifying shapes sizes, patterns ,shadows, tones and texture of the images.
N0TE:
(The term, remote sensing, is generally applied to the interpretative area of photogrammetry because sensing and imaging devices such as multi-spectral scanners, thermal scanners, radiometers, and side-looking airborne radar are used in the interpretation.These instruments sense energy in wavelengths beyond those which the human eye can see, or standard photographic films can record. They are often carried in aircraft as remote as satellites.)
Use of photogrammetry:
Photogrammetry presently has many applications in surveying and engineering.
- It is used in land surveying to compute coordinates of section corners, boundary corners, or points of evidence that help to locate these corners.
- Large-scale maps are made by photogrammetric procedures for many uses, one being subdivision design.
- Photogrammetry is used to map shorelines in hydro-graphic survey.
Photogrammetry is playing an increasingly important role in developing the necessary data for modern Land and Geographic Information systems.
Photogrammetry is also being successfully applied in many non-engineering fields.
- In geology, forestry, agriculture, conservation, planning, archaeology, military intelligence, traffic management, and accident investigation.
Advantages of Photogrammetry:
(1)Speed of collecting spatial data in an area.
(2) Relatively low cost.
(3) Ease of obtaining topographic details, especially in inaccessible areas.
(4) Reduced likelihood (probability) of omitting details in spatial data collection.
Aerial Cameras:
Aerial mapping cameras are the most important photogrammetric instruments, since they expose the photographs.
Component:
The principal components of a single-lens frame camera are,
- Lens (the most important part), which gathers incoming light rays and brings them to focus on the focal plane.
- Shutter to control the interval of time that light passes through the lens.
- Diaphragm to regulate the size of lens opening.
- Filter to reduce the effect of haze and distribute light uniformly over the format.
- A camera cone to support the lens-shutter-diaphragm assembly with respect to the focal plane and prevent stray light from striking the film.
- Focal plane the surface on which the film lies when exposed.
- fiducial marks four or eight in number, which are essential to define the geometry of the photographs.
- camera body to house the drive mechanism that adjust the shutter, flattens the film, and advances it between exposures;
- Magazine, which holds the supply of exposed and unexposed film or houses the digital storage device.
How to carry out aerial survey:
In order to start an aerial survey the very first you need are ground control points (GCPs). Ground control points are marked points on the ground that have a known geographic location . You have to make GCPs on the ground at an area you required to perform a survey. But the thing which you must keep in mind while marking GSD(ground sampling distance) is that, you shouldn’t have to mark them at a very beyond distances because it will lower the accuracy of your survey . The bigger the valueof image GSD, the lower the resolution of the image as a result of which less visible details can be recognized in that photograph. When you have marked these points then you have pick an instrument for an aerial survey which obviously be the flying one and the application instrument you have chosen must be known by you. Now a days drone survey is very efficient and time saving way to conduct aerial photographs. Now fly your drone up in the air at certain altitude that it will not increases your GSD value. Then just take the photographs of your marked GSDs in such a way that your camera must be in horizontal direction to the surface of the earth.
Types of photograph:
Vertical Photographs:
A vertical photograph is taken as the camera pointed as straight as possible.allowable tolerance is usually +3° plumb line to the camera axis.
- It cover relatively small area.
Scale of vertical photographs:
The scale of a vertical photograph is the ratio of a photo distance to the corresponding ground distance.Since a photograph is a perspective view, scale varies from point to point with variations in terrain elevation.
S =f/H-h
Photo scale:
photo scale = photo distance * map scale/map distance
Relief displacement:
Relief displacement is the shift in an object’s image position caused by its elevation above a particular datum due perspective rays.
Terms:
1.Exposure station (O) :
The point in the atmosphere occupied by the center of camera lenses at instance of photography.
2.flying height :
The vertical distance between exposure station and mean sea level.
3.Flight line:
Line traced by exposure station in atmosphere (track of aircraft).
4.Photo principle plane (k):
It is point on photographs obtained by projecting camera axis to intersect at a point on photographs.
5.Ground principle plane(K):
Camera axis extended up to the ground. The point obtained at ground is called ground principle point.
6.Photo nadir point(n):
It is the point on photographs obtained by dropping vertical line from camera center.
7.Ground nadir point(N):
Plumb line extended up to the ground give ground nadir point.
8.Horizon point (h):
It is the point of intersection of horizontal line through center of lenses and principle line on photographs.
9.Principle plane:
Plane defined by exposure station (O) ,Ground nadir point and Ground principle point .i.e(plane NOK)
10. Principle line:
Line of intersection of principal plane with photographs plane.
11. Azimuth:
Clockwise horizontal angle measured about ground nadir point from true north to the principal plane of photographs.
12.Swing(s):
Angle measured in plane of photographs from +y-axis clockwise to photo Nadir point.
13. Isocenter:
Point on photographs where bisection of tilt falls on photo(dotted line in sketch).
2. Oblique photographs:
A oblique photograph is taken as the camera inclined at angle >3°.
- It cover relatively large area.
- No scale is required and distance cannot be measured.
3. Convergent photographs:
These are the photographs taking with pair of camera with axis inclined.
These are done with single twin lens
wide angle camera or with two single
lens wide angle camera couple rigidly
with the same mount.
4. Trimetrogen photographs:
These is an assembly of 3 photographs taken at the same time,1 vertical and 2 high oblique in a direction at right angle to the line of flight.
Stereoscopic parallax:
- parallax:
The displacement of an object caused by the change in the point of observation is called parallax.
stereoscopic parallax is caused by taking photographs of the same object but from different point of observation.
Stereo-pairs :
Adjacent but overlapping pairs are called stereo-pairs and are needed to determine the parallax and 3D viewing.
stereoscope:
A stereoscope is a device for viewing a stereoscopic pair of separate images, depicting left-eye and right-eye views of the same scene, as a single three-dimensional image.
TECHNIQUES TO TAKE AERIAL PHOTOGRAPHS:
LIDAR Acquisition & Processing:
Light detection and ranging (LIDAR) has revolutionized the acquisition of digital elevation data for all scales of mapping application.
The LIDAR sensor is integrated with airborne GPS and an inertial measuring unit (IMU) to simultaneously capture light pulse information, location of the sensor and angle or orientation of the sensor. The measurement by LIDAR is direct and does not require post-processing. LIDAR technology can be embedded easily on a drone thanks to its lightweight.
| SPECS | Fox6 LiDAR Package |
| Number of lasers | 16 single beams and dual echo |
| Measurement | Range up to 100m |
| Accuracy | ± 3cm |
| Field of view | 360° (Horizontal) / 30° (Vertical) |
| Rate | Single Return Mode: -300,000 points / sec Dual Return Mode: -600,000 points / sec |
| Protection | IP67 (Dust and Water) |
| IMU | Roll & Pitch 0.025° PPK |
| Typical scanning area | Up to 50 ha (123 acres) |
ORTHOPHOTOGRAPHY:
An Orthophotograph or orthoimage is an aerial photograph or image geometrically corrected such that the scale is uniformed. Unlike an uncorrected photograph, an Orthophotograph can be used to measure true distances because it is an accurate representation of the earth’s surface.
Orthophotographs are commonly used in Geographic Information System(GIS) as a map accurate “background image.” A digital elevation model is required to create an accurate orthophoto as distortions in the image need to be corrected.
REMOTE SENSING:
In general, remote sensing can be defined as any methodology employed to study the characteristics of objects using data collected from a remote observation point. More specifically, and in the context of surveying and photogrammetry, it is the extraction of information about the earth and our environment from imagery obtained by various sensors carried in aircraft and satellites. Satellite imagery is unique because it affords a practical means of monitoring our entire planet on a regular basis.
Remote sensing imaging systems operate much the same as the human eye, but they can sense or “see” over a much broader range than humans. Cameras that expose various types of film are among the best types of remote sensing imaging systems. Non Photographic systems such as multispectral scanners (MSS), radiometers, side-looking airborne radar (SLAR), and passive microwave are also employed.
UAVs:
An Unmanned air vehicle is also used to take image. An unmanned aerial vehicle, commonly known as a drone, is an aircraft without a human pilot aboard. UAVs are a component of an unmanned aircraft system; which include a UAV, a ground-based controller, and a system of communications between the two .
aerial surveying 