Satellite imagery is an alternative to aerial photography but there is a lot of confusion as to what is actually satellite and what is aerial photography. Many people think that the data seen in Google Earth is taken from a satellite when in reality it is high resolution digital aerial photography. Similarly, many television shows, such as the BBC's Spooks, refers to their imagery of towns and cities as satellite data when it is actually aerial photography supplied by Bluesky! So, what are the main differences between the two and which would be most suited to your project?
Below is a list of benefits and weaknesses for both aerial photography and satellite imagery. This information should help you make an informed decision on the type most suited to your requirements.
Satellite imagery
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Aerial photography
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 Speed
Satellites are capable of collecting large amounts of data in relatively small amounts of time. A modern satellite can be moved into position in less than 3 days and can take the photographs quickly once locked on to an area. The size of these pictures are very large and allow the complete area to be captured using less images and therefore, in a shorter space of time. This is necessary with satellites as, because of their limited numbers, the satellite may be required in a different location very soon after.
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Speed
Aerial photography used to be a slow and time consuming process. The amount of time taken to capture an area depends greatly on its size and shape. As airplanes fly back and forth in 'runs' a series of overlapping photographs are taken. The dawn of digital aerial photography cameras has made the acquisition of airborne photography considerably quicker. However, the newest cameras are making this faster still by recording strips of data rather than individual frames.
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Level of detail
Satellites generally reside several hundred kilometres above the earth's surface. Although satellite imagery has improved greatly over the years it is still lower resolution than aerial photography. High resolution satellite imagery as high as 50cm per pixel is readily available, up to 41cm in the case of GeoEye-1 (however, the U.S. Military requires resampling the imagery to 50cm for all customers not explicitly granted a waiver by the U.S. Government). Military satellites more than likely have a higher resolution but as yet this imagery has not become publically available. Most off-the-shelf satellite imagery is between 250m and 50cm in resolution.
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Level of detail
Aerial photography has the distinct advantage of having the lens closer to the subject, in this case the land. By adjusting the flying height of the aircraft capturing the data it is possible to improve the detail that can be captured. Most aerial photography was flown with a resolution of between 50cm to 12.5cm per pixel. With the new technology of aerial photographic cameras it is not unusual for newer imagery to be captured at 10cm per pixel. In some cases this can be as low as 5cm or 2.5cm per pixel. However, these ultra-high resolution datasets consume enormous amounts of storage space and consists of many times the number of photographs than lower resolution datasets. Also, having a lower flying height enables airborne acquired multi-spectral and hyperspectral data to maintain a higher resolution increasing its potential uses.
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Weather conditions
Both satellite imagery and aerial photography can suffer from environmental conditions. Being higher up in the atmosphere means that satellites have more weather conditions to cope with. Thin cloud that may not stop aerial photography can still have a large effect on the quality of satellite imagery. Due to its position in orbit, and other requirements for different areas the satellite may be needed for another, clearer area. In this case the window of opportunity for photography may pass and it may be some time before the satellite can be repositioned. It is worth noting that RADAR data is not affected by weather conditions so can be collected at any time.
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Weather conditions
Although aerial photography can be affected by adverse weather conditions there is still the possibility of photographing areas in thin or high level cloud which might stop the use of satellites. This has a small bearing on the quality of the final imagery and can normally be rectified during the post-processing stage. There are also far more planes available for taking aerial photographs than there are satellites so if one plane is needed where the weather is clearer then another may be available to take its place. RADAR is cloud penetrating so it is not affected by adverse weather.
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Types of data
Many modern satellites can collect a variety of data. These include standard photographic imagery, colour infrared and in some cases LiDAR, thermal, radar data. This variety of datasets makes satellites very versatile. However, non-standard data sources such as thermal, LiDAR, multi-spectral and hyperspectral imagery will likely be expensive to capture or may not even be publically available. The main problem with satellite data types is that when new or improved technology is released it is very difficult to change the sensors and cameras in a satellite but in a plane or helicopter it is simply a case of removing the old device and replacing it with the new one.
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Types of data
Most aerial acquisition aircraft are fitted with a mount that allows a number of different cameras or sensors to be attached to the same aircraft. This allows the swapping of capture devices but only when the plane is not airborne. Some aircraft can mount multiple cameras or sensors and some of the latest aerial cameras capture different types of data simultaneously, such as standard imagery, radar and colour infrared. Other datasets such as thermal imaging requires the flying to be performed at night so a plane that was capturing standard imagery during the day can be converted during the evening and then sent out again at night to capture thermal data. Having a removable sensor allows aerial acquisition to keep abreast of the latest developments in new technologies and the cost of fitting these is much lower than in satellites orbiting many miles in orbit.
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Location
The major advantage of satellite imagery is that the satellite can be positioned to take imagery of anywhere on the planet. It does not have to cross borders or go through the many passport controls that could delay a visiting acquision team. Being above the earth, it is more efficient for a satellite to allow the earth to revolve below it than to physically move to the necessary location. Satellites still need to take into account the location of the sun to acquire visible light surveys so many satellites try to remain in sun-synchronous orbit.
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Location
Location can cause a delay for aircraft acquisition teams. To guarantee that the survey can be completed an acquisition team must remain on stand-by near to the area to ensure that a weather window is not missed. Luckily there are many aerial companies around the world who specialise in photographing their own country. This means that for a company in Austrailia to acquire photography of the UK they only have to contact one of the companies that specialise in data of that area.
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Post-processing
Imagery acquired by satellite usually consists of far fewer "shots" than that taken by aircraft. The extra distance means that more area can be covered in one pass - at the deficit of detail. Satellites usually capture data in strips (similar to a continual video of the area) and allow a larger amount of data to be acquired per digital file. This requires far less post-processing than if it were to capture individual "frame" images.
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Post-processing
Early aerial photography was made up of tens of thousands of individual photographs. When the aerial photography industry adopted ditigal cameras each digital file was a copy of what was captured using analogue cameras. Recent developments in aerial camera design have moved away from these traditional images and moved towards the satellite's method of capturing strips of imagery instead. This has greatly reduced the amount of post-processing work required but, with it's lower flying height, there are still more images than with the higher flying satellites.
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