aerial photography

These images from the Megantic and Orford region show a simulation of what can be seen by looking at two three-dimensional aerial photos, without stereoscopes or glasses to see in 3D!

- The images on the left are mosaics of about twenty color aerial photographs at a scale of 1:15,000, which show a large area in one piece.

- The images of the center are called Digital Elevation Models (DEMs) and were produced from the contour lines found on the 1/20,000 scale topographic maps. These MNA present, in images, the relief of these regions.

- On the right, the result of the combination of the MNA and photo mosaics: Mont Mégantic and Mont Orford in perspective and in all their splendor!

Mont Mégantic
Mont Orford
Another way to see aerial photos in 3D and, this time vertically, as you would see them with a stereoscope, is the anaglyph process. This word is taken from the Greek word anaglyphs which means "carved" or "embossed".

In these examples, it is an overlay of two consecutive aerial photographs (the area where there is a longitudinal overlap) of Mount Orford on the left and the Hautes-Gorges-de-la-Rivière-Malbaie on the right. The use of two complementary colors, here red for the left image and cyan (blue-green) for the right image, allows the relief to be reconstructed by looking at the composite image through "3D" glasses equipped with colored filters. Each eye sees only the image that suits it and the brain merges the two images to give the illusion of a single relief image.

Note: Red-cyan anaglyph 3D anaglyph glasses are available in different locations, such as photoshops, or on the Internet. But maybe you had some with your latest DVD? in 3D? You can also make your own glasses using red and cyan acetates, quite simply!

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The different types of aerial photos

Aerial photographs are images of the territory taken on a given date and are used to interpret the forms and details that characterize it. The interpretation of shapes, shades of color or grey and textures makes it possible to distinguish the details present and their nature. For example, it can be interpreted:

•  hydrography (lakes, rivers, streams, ponds, ponds);
•  vegetation (woodland, wetland, crops);
•  inhabited areas;
•  transport infrastructure (roads, forest roads, trails);
•  the bonfires and burns.

Generally, 1:5,000 scale shots are reserved for urban areas, while 1:15,000 and 1:40,000 scale shots are generally used in the southern part of Quebec, south of the 52nd parallel of latitude, for topographic and forest mapping purposes.

The raw image, as taken on board the aircraft, can be developed and printed on film or slide paper.

The following are the types of aerial photographs most commonly used by departments and agencies for land and resource management:

•  Aerial photographs in black and white at scales of 1:15,000 and 1:40,000

Black and white aerial photography is stored on panchromatic emulsion film (black and white). This emulsion makes it possible to print on photo paper a range of about 50 different shades of grey, which represent different objects on the territory.

• Aerial photographs in color infrared and color at a scale of 1:15,000

- Colour photography can be used for many purposes: forestry, agriculture, land use planning, etc.

When it is digitized and corrected and has the same geometric location properties as a map, it is called orthophotography.

Orthophotography is an aerial photograph:

- which has been scanned digitally in high resolution;

- which has been geometrically corrected and straightened from the ground to eliminate inaccuracies caused by camera tilt at the time of the shooting and due to image distortion caused by terrain;

- to which the coordinates (georeference) have been added;

- which can, therefore, be used in a computer to superimpose other geographical information, measure distances, etc.

Photos of the future...

More and more, aerial photographs are taken directly in digital mode, with digital airborne cameras. These digital airborne images are taken, in the same way as satellite images, by electronic detectors. Therefore, as with satellite images, their accuracy is given in terms of resolution and not scale. In addition, these cameras simultaneously take panchromatic images (in black and white) and color images that are also called multispectral (in red, green and blue). Panchromatic bands are generally of higher resolution.

The great advantage of using airborne digital cameras is that there is no film, so there is no development operation. In addition, the photo does not need to be scanned to produce a digital image and is easier to store. Finally, these images have great qualities, since they are both in good resolution in black and white and in color.

On the other hand, the great advantage of traditional cameras is their wider field (they cover more territory per shot) and their price is much lower than that of large format digital equivalents.

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Why use aerial photographs?

Aerial photography is very rich in information. Among other things, it can be used to identify streams, ponds, trails, and clearings that are not shown on a map. It can also identify bonfires and burns, and determine changes and types of vegetation. However, the undeniable advantage of aerial photography remains the possibility of obtaining a three-dimensional vision of the territory through stereoscopy and thus giving an insight into the relief.

Orthophotography is halfway between aerial photography and topographic mapping. It offers the detail and richness of an aerial photograph, combined with the geometric accuracy of a map. Since it is georeferenced, it can be superimposed with geographic information such as:

•  hypsometry (representation of the relief);
•  toponymy (place names: lakes, rivers, road network);
•  territorial boundaries (administrative boundaries, boundaries of recreational and protected areas).

By their nature, aerial photographs and orthophotographs provide an overview of the territory at a specific time in time. This is why they are used to obtain information about the past or even to plan projects.

Back in the past...

Aerial photography can be used as an archive. Indeed, it allows going back in time by performing the photo-interpretation of a shot taken a few years before.

Here are some examples of the use of old aerial photographs for various fields of activity:

•  justice
•  land-use planning
•  civil security

 justice

When the land on a property is in dispute, old aerial photographs are often used to obtain information on the condition or delimitation of the property before the changes that caused the dispute. In such a case, these photographs become very reliable support for establishing evidence.

land-use planning

Old aerial photographs are often used to redevelop the site as they were in the past. The photographs will then be used to prepare the basic elements of the urban development master plans.

civil security

Aerial photographs can be used to assess the extent of damage caused by a natural disaster.

•  Flooding in the Saguenay
•  The ice storm

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The witness of the evolution...

Aerial photography, when taken in an urban environment, makes it possible to show the stages of urbanization or its spread by the juxtaposition of images captured at different times.

Here are some examples of use for various fields of activity:

•  land-use planning
•  forestry

land-use planning

Aerial photography makes it possible to study the evolution of urbanization over time.

forestry

It is used to obtain information on the physiognomy and groupings of forest species (stand types). It also makes it possible to assess forest potential, since it is possible to inventory forest resources and obtain an approximation of the volume of wood. Thus, the evolution of tree species or cutting areas can be more easily monitored to ensure proper management.

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A vision of the future...

Aerial photographs can be used to conduct site research analyses or plan large-scale projects.

Here are some examples of how to use it for project planning:

•  civil engineering
•  agriculture

civil engineering

Aerial photographs can be used to search for sites (factories, dams, ports, etc.) or to make preliminary network projects (roads, railways, canals, power transmission lines, pipelines, etc.). They can also be used as orthophotographs to take measurements, add layers of information, plan and even make project models.

agriculture

Aerial photographs are used to predict crop yields, estimate crop damage caused by insects, diseases or floods, assess soil water balance, discover areas affected by salinization, etc. They provide good forecasts of agricultural potential.