Table of Contents for Drones in Construction- Aerial Survey
- Introduction to Aerial Survey & Drones
- Procedure for Aerial Survey with Drone
- Preflight Planning
- Establishment of Ground Control
- Flight Planning
- Flight & Image Capturing
- Image Processing
- Scale of Photograph
- Actual distance measurement from an aerial photograph
- Points to consider in aerial survey
- How aerial photographs are used?
- What are the uses of drones in construction?
- Regulations in India
- Final Words
Introduction to Aerial Survey & Drones
The aerial survey is a science and technology of surveying by aerial photographs. The complete process involves the capturing of images, measurements, and interpretations. The camera mounted on an aeroplane or a UAV captures the ground images. The image processing generates cadastral maps, archaeological maps, mining details, and 3D models, etc.
The Unmanned Air Vehicle (UAV), also known as drones have gained popularity in aerial surveying. The advancement in technology has led to the development of lighter, smaller and cheaper drones. The accuracy of the survey work is up to one centimetre.
Aerial survey is also known as aerial photogrammetry. The first use of aerial photogrammetry dates back to 1858 when Gasper Felix Tournachon took a photograph of a French village from a hot air balloon. Later in the first World War, the countries used aerial photographs to survey the strategic positions. Following that, in 1920, Talbert Abrams made the first commercial move in aerial photography. He is the father of aerial photography.
Procedure for Drone Survey
The procedure for a drone survey comprises five steps.
Checklist for flying- Pre-flight Planning
The most important aspect before flying is to check for the local regulations. The local authority may not allow drone flight in its’ area. Therefore, obtain the necessary permissions and approvals before the flight planning.
Another thing to notice is the purpose of the survey and understanding of the survey requirement. Different uses may need separate data.
Observe the local terrain, image resolution requirement, general weather etc. These observations help in selecting the most economical drone type.
The selection of photograph scale is a necessary thing for the details required in the map. The higher scale has more details. The scale of the photograph has been discussed in detail later in this article.
Establishment of Ground Control
Ground control points are established on the ground to act as a reference for image processing. The location (coordinates) of ground control points is known. The number of control points required depends upon the scale of the photograph, flight control and the processing system. At least, 3-4 ground points should appear in every photo. Existing structures with known locations can also be used as control points.
The ground points help correct the photograph distortions due to the ground elevation change, angle of exposure of photograph, tilt etc. The ground features in images are measured for their location and location about the ground control points.
The flights are made in parallel lines. The distance between each line is the same. The distance between the lines is such planned that two adjacent photos overlap 30% laterally (also known as side overlap).
Along each parallel line, the points of image capture are planned. The image capture points are such planned those two adjacent photographs 60% overlap longitudinally (also known as forward overlap).
The two overlapping images are called stereo pairs and are necessary for tilt correction.
Another aspect of flight planning is the scale of the image. The scale of the photograph depends upon the focal length of the camera and the camera elevation. For a pre-decided scale, and the camera focus, the flying altitude is calculated.
Upon decision on flight lines, image capturing points and flying altitude, the site visit is made to the ground to check if the proposed flight is free of obstructions.
Flight & Image Capturing
On the flight day, the weather should be clear of fog, mist, smog, snowfall etc. Plan the flight at noon as the shadows are minimum and the light is maximum for the day.
Check the wind before the flight. The wind limits are specified for each drone by its’ manufacturer on the user manual. In addition to it, check the drone’s battery level before the flight.
Look for the obstruction in the flight plan and adjust accordingly. Remove the camera lid, and fly the drone as per plan if there is no obstruction.
Check the images once before leaving the site. If the photos are unclear due to any reason, fly the drone again.
The images collected in flights are uploaded on photogrammetry software that analyses them. The geo-tag of each photo defines their point of image capture, and hence their position on the model.
The software converts the perspective view of the image into an orthographic view. The ground control points act as a reference for the correction of the distortions and tilt.
Scale of Photograph
The scale of a photograph is the ratio of measurement on a photo to the ground. For example, if a 100 m long road stretch is 10 cm on the image, the scale of the photograph is 10 cm:100m. However, the scale representation is 1:x. The conversion to the representation form by changing the numerator and denominator to the same unit. In the above example, the scale of the photograph is 1:1000.
The higher denominator means the coverage of a larger area has in the photograph.
The formula for the average scale of an aerial photograph is-
S = f/h
Where f is the focal length of the camera lens and h is the average altitude of the camera lens from the ground.
Actual length calculation from an aerial photograph using coordinates
The aerial photographs help calculate the actual length. The scale is not uniform at all points of the image due to the difference in ground elevation. The coordinates are a reliable method to calculate the horizontal distance between two points.
Let us consider two points A and B on a photograph. We shall calculate the actual distance between A and B. The photo coordinates of A & B are (xA, yA) and (xB, yB), respectively.
Calculate the actual coordinates of A (XA, YA) and B (XB, YB) by the given formula.
Where H is the drone altitude above MSL,
f is camera lens focal length,
hA & hB are the elevations of points A & B above MSL.
The actual distance between points A and B is-
The above formulas are valid for orthographic images only and not for tilted images.
Points to consider in an aerial survey
- The drone should fly horizontally throughout the flight. i.e., the drone altitude should remain constant.
- The ground control points should be firm, and their locations should be well known and cross-checked.
- Correct geotags helps in placing the images in their place on the map. Also, verify the image orientation in the map.
- Discard the blur images as it hampers the processing.
How aerial photographs are used?
The aerial photograph processing creates the following results.
- Interpretive Map for identification of ground features
- Metric map for measurement of ground feature
- Contour map
- Mining map
- Archaeological map
- Digital cadastral map
- Time-Lapse details
What are the uses of drones in construction?
Drones provide a wide range of benefits to the construction industry. Their application is throughout the construction process also the pre-construction and post-construction. Drones save time and cost in surveys.
Some of the uses of drones are as follows.
- Site surveys.
- Routing and alignment work for highways, railways, airstrips, ports etc.
- Construction progress monitoring.
- To enforce better safety practices by sending drones into hazardous places for inspection.
- Traffic monitoring and planning.
- Construction site planning. For example, material storage, vehicle movement, concrete production, tower crane locations.
- Environmental protection such as forest cover, coastal protection, wild-fire monitoring, wildlife migration.
Regulations In India
The Ministry of Civil Aviation has released the new policies in August 2021 for drones that states the need for a license for the drone pilot. However, tiny drones need no security permits. The policy also defines the green, yellow and red zones for flying. Read more about the policy here.
In June 2021, NHAI made the drone survey mandatory for all the national highways in India. The monthly drone survey is compulsory for the construction and operation phases.
Final words for aerial survey
Drones are offering undeniable advantages to the world. Drones have been in use for a long time in the industries, but the construction sector is relatively a new space for drones. Also, the construction sector has welcomed it with both hands. The uses of drones in construction are increasing day by day.
The drone policy in India shall further encourage drone technology adoption. The drone deployment in government projects is a big boost and a token of confidence for its’ reliability.
Aerial photogrammetry with a drone paves a new career stream as professional drone pilots, flight planners and image processing experts. Drone pilot jobs are on the rise due to the increase in the adoption of drones.
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