Where Do Maps Come From? – Source 2: Aerophotography
Aerophotography is one more important source of data for topographic maps used by modern online map services along with satellite photos. Both satellite maps and photos from flying machines taken at lower heights supplement each other very well allowing creation of maps with a large zoom degree. They also allow creation of 3D models of the earth surface objects used by some map services today.
Essentially, aerophotography represents shots taken by cameras installed on an atmospheric flying machine like a plane, helicopter, dirigible, or on any other similar machine or pilotless aircraft (drone).
The flight altitude accessible for such machines varies from several hundred meters to tens of kilometers. Thanks solely to aerophotography, we have access to map details shown with zoom at a degree of 0.63 meter per pixel. In particular, industry requirements for topographic maps require the photo resolution be no less than 15 cm/pixel. Larger objects can also be determined by satellites (as we wrote in the previous article, QuickBird satellite provides precision shooting at a level of 0.61 meters per pixel and LandSat satellite up to 20 meters per pixel).
It won’t be long before satellites will be able to “see” objects of even less magnitude, based on the direction. Satellite photography is developing now. For example, in 2016, the launch of the third-generation GeoEye satellite is expected. It will have precision shooting at a level of 0.25-0.34 meters per pixel.
However, whether or not “very far-sighted” satellites enter the arena, aerophotography will still remain a very popular source for cartographic data owing to the following advantages:
- relatively low cost compared to satellite photography – even small companies and private individuals can afford photo shooting from aircrafts;
- mobility and independence – anyone with the necessary equipment* can create maps ;
- increasing expansion of drones suitable for photoreconnaissance.
How is the aerophotographic process?
In general, aerophotography is carried out either by a human onboard an aircraft or automatically by drones.
The aerophotography process for both includes the same steps:
2) Flight and shooting
3) Taking photo files from camera and processing them.
Each step in detail:
1) Preparation includes defining the district on the earth’s surface to photograph and composing the flight route based on relevant details.
Aerophotography may be vertical one and oblique.
Vertical aerophotography produces shots when the camera object-glass is looking steeply down to the earth and “sees” objects strictly from above.
With oblique aerial photography, the camera object-glass is set at an angle to the earth’s surface either capturing the horizon line (so-called high-oblique aerial photograph) or not capturing the horizon (low-oblique aerial photograph).
There can also be a panoramic aerial photography.
Aerial photography may also be general coverage photography (within ranges of a predefined territory) and strip aerial photography (made within a certain direction).
Sometimes the area to be covered by aerial photography requires a series of parallel flight routes to cover the whole desired terrain.
If several routes are required, shots of adjacent lands should contain the same areas. This requirement is called “fore-and-aft overlap”. The percentage of the overlapped area may vary from 30% to 60%, depending on the requirements to the photos.
2) Flight and shooting step is increasingly being done by drones because:
- it is very easy to set the route just by uploading the necessary GPX file to the drone and/or correcting the route manually from the ground during the flight;
- piloted aircraft (helicopters, etc.) have definite limitations for airspace and locations where they are allowed to fly, while drones are more portable because safety regulations applying to people are more restrictive than those applying to pilotless machines.
- it is very difficult to take shots manually – even with a professional camera – when flying on board a plane. Just imagine: the aircraft is moving, noise surrounds you, it is often uncomfortable and physically impossible to take the right foreshortening or to hold the camera steady in your hands to get a sharp image. Such problems do not exist with drones where shooting requires no human intrusion and the whole process, including camera stabilizing, is automated.
Also, drone manufacturers embed all devices and equipment necessary for an accurate definition of flight parameters and photo-shooting points into their aircrafts, such as:
• Altimeter – fixes flight altitude based on the air pressure;
• Radio altimeter – defines altitude of photo shooting with regard to the terrain;
• Radio geodetic stations – allow defining the distance from the aircraft to stations on the earth’s surface located at the points with precise geodetic coordinates.
Data from the devices listed above is used to calculate the right coordinates of the territory on the photos. And, to guarantee the accuracy of the future map, the following parameters are very important: angle the camera took, camera rotation around the axis during the shooting process, and height of camera above the ground.
The required geolocation accuracy for photos is achieved when the coordinates of the photo centers are measured with the help of high-precision GNSS-receivers within range of a referential network or by using a ground control network with points connected with an accuracy when greater than 30 cm.
3) Final step includes taking photo files from the camera and processing them on terra firma.
Currently, the processing (decoding) of geoshots is performed by special computer complexes—so-called Digital Photogrammetric stations— such as Intergraph ImageStation or PHOTOMOD. While processing, additional corrections of perspective, distortion and other optical misrepresentations are performed along with color and tone correction. The mounted photoplan is clipped into a single image, added to the general catalog and combined with existing cartographical material. It is also included in the general Geographic information systems.
The level and complexity of the post-shot image processing for photos taken by drones depends on the drone equipment: quality range differs dramatically.
Simple drones with basic, “garden variety” cameras with no gyroscope reference system, resulting in optical axes deviating from the vertical by several degrees, makes the initial processing quite complicated. However, such disadvantages do not present a serious problem for up-to-date photo mapping software. Moreover, the development of photo mapping digital methods has called into being some programs and program complexes capable of processing even such “poor” data of aerial photography in a highly automated mode with minimal intrusion of an operator; for example, Agisoft PhotoScan, a program that represents a universal tool for generation of 3D model of subject surfaces based on the subject photos.
However there are already enhanced drones on the market that are equipped with all the hardware and software tools necessary for taking precise, professional shots, such as drones with tri-axis digital stabilization carrying sensors for accelerometer, pressure, gyroscope and magnetometer along with a vertical camera.
Processing shots taken by such drones requires minimal additional processing.
In general, it is possible to predict that reduction of prices for drones and their further technical improvement ensure that drone aerial photography will increasingly become a significant source of earth surface photos. Aerial image processing drone software is also developing very quickly; right now quite complex image processing may be performed with the help of free drone mapping software, while paid services are also developing in parallel.
I think that capabilities of the modern drones in the area of aerophotography deserve a dedicated article – so please watch for updates .
* Necessary equipment: flying machine with camera, photo processing software. The cost for this may vary from thousand dollars to hire a plane for a photo session to hundred dollars to buy drone and exploit it. Photo processing software may be free or paid (ten, hundred dollars)
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