The gap between these two usually complementary mapping technologies is getting smaller as both satellite and aerial imaging systems have benefited from great technology improvements in the last decade. These technologies can even overlap in terms of resolution, meaning that issues such as accuracy, acquisition and processing time need to be taken into account . It is more of a challenge to make informed decisions about which technology to choose for a mapping project. Nevertheless, each technology is alive and well and possess unique characteristics and strengths which need to be highlighted.
Without going into the detail of specific sensors, this article presents the main parameters to consider when making a choice between high resolution satellite and aerial imagery. These parameters are:
- Data type
- Location accuracy
- Location accessibility
- Speed and coverage
With the development of new generation large format digital aerial cameras, it is possible to capture a project area with resolution up to 25mm (2.5cm), suitable for engineering survey type projects. Despite the fact that very large amount of data has to be acquired, large format aerial cameras can capture larger frames than ever before which implies less runs are needed to map an area. Aerial imagery resolution typically ranges from 90cm at the coarse end up to an impressive 2.5cm with a large demand in the range 15cm to 10cm.
Most of new earth observation satellites can now capture images at sub-metre resolution. Restrictions apply for civil use and 50cm is usually the highest resolution available. Efforts are being made to decrease this limit to 30cm in the coming years but physical and technology limitations will keep satellite resolution around 30cm.
It is important to note that “very high-resolution” satellite imagery falls in the mid to low end range of digital aerial imagery resolutions.
Aerial acquisition aircraft offer the flexibility of being fitted with a wide range of sensors such as multispectral, hyperspectral / thermal and other survey sensors. Most aerial cameras offer a fourth near-infra-red band of imagery as well as standard R,G,B bands. Aerial acquisition benefit from the fact that newly developed technologies can be adapted very quickly to an aircraft.
Normal aerial surveys will be captured with full stereo mapping capability (60-80% forward overlap between images), and 30% side overlap between runs. This enables a wide range of value-added products to be generated to a high degree of accuracy including Digital Elevation Models (DEMs), Digital Surface Models (DSMs), contours, orthophotos and 3D GIS feature data capture.
The range of data available from earth observation satellites is increasing but non-standard sensors still suffer from lower resolution and high capture costs. However, newly launched high resolution satellites possess additional optical bands designed for specific applications such as vegetation analysis. Stereo imagery is usually not included in satellite image capture and the generation of other value-added products usually relies on external data sources.
A big advantage of using aerial imagery is the flexibility to plan data acquisition according to the local weather conditions and take every opportunity to fly in cloud-free conditions. It is also possible to fly under cloud cover with minor corrections to be applied during post-processing. This guarantees cloud-free data delivery. Due to low altitude acquisition, aerial data doesn’t suffer from atmospheric effects which can impact the quality of the data in satellite imagery.
With the exception of SAR (synthetic aperture radar) satellites which can acquire data regardless of cloud cover and illumination conditions, most satellite providers do not guarantee completely cloud-free imagery. An image is considered valid if cloud cover is less than 10-15%. Cloud free coverage, especially in the tropics, can be very difficult to obtain. Corrections also have to be applied to remove atmospheric effects and haze.
With improved quality of large-format digital aerial cameras, airborne GPS and post-processing techniques, the absolute accuracy of aerial imagery is getting better and better. A typical horizontal accuracy of two pixels can be expected for aerial imagery. 3D features and DEM data can be determined to an accuracy of approximately one pixel in height.
High resolution satellites location accuracy is usually around 10m to 20m without ground control points but this value is improving with new satellites coming up. With a network of GCPs the accuracy can be improved to a few metres.
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