The impact of 5 years of high resolution satellite imagery on image acquisition


Adel Hallak
Regional Sales Manager GCC – States
MAPS geosystems,
UAE
Email: katherine.thiemicke@mapsuae.com

Abstract
Since its introduction about 5 years ago, high resolution satellite imagery has become a major challenger to aerial photography for many GIS and mapping projects. Based on the experience of a multitude of projects, this presentation discusses the pros and cons of both data sources, not only from a technical point of view, but by also taking into consideration the non technical aspects that affect the decision whether to use satellite imagery or aerial photography. The presentation also highlights the usage of satellite imagery in non topographic applications which provides an unprecedented insight into our environment on a popular level. By describing how satellite imagery, nowadays, is embedded in development- and planning processes both, in the public- and civil sector, the presentation will demonstrate the ground-breaking effect, the accessibility of high resolution satellite imagery has had on the civil sector.

In spite of its many advantages, high resolution satellite imagery can not completely replace aerial photography. The horizontal and vertical accuracy achievable by aerial photography remains to this day unmatched and opens up the imagery for further processing such as large scale mapping. The stereo coverage which by default is available for aerial photography is difficult to achieve with high resolution satellite imagery. Aerial photography has therefore advantages for projects requiring accurate terrain heights. For projects along narrow corridors such as roads or pipelines the flexibility provided by aerial photography has advantages. Additionally in areas predominantly covered by cloud or haze, the ability to take photography at selected times can be critical in obtaining the required imagery.

1. Introduction
Until five years ago, digital orthophotos produced from aerial photography had been the main data source for mapping and GIS applications. The low resolution Satellite imagery available at that time was mainly used for remote sensing applications where statistics and classifications play a more important role than geometric accuracy and interpretation. The introduction of hi-resolution satellite imagery (HRSI) to the commercial sector, in 2001, has increased the availability of data in an unprecedented way.

Today, anybody can acquire images showing details which, until 5 years ago, had been accessible only to the most sophisticated military agencies. In the following the impact that high resolution satellite imagery had on the government- and private sector will be examined, and satellite imagery (QuickBird) and aerial photography will be compared to each other in terms of different aspects.

2. The impact of satellite imagery
Over the past 10 years, digital orthophotos produced from aerial photographs have become predominant in providing the land-base requirement in GIS applications, especially relevant in the Middle East and Africa, where existing vector mapping is scarce in comparison to Europe and the US. During the last few years it has become clear that data derived from imagery that has been acquired from high-resolution sensors on satellites, like QuickBird or Ikonos, has the potential to replace aerial photographs, some day. This applies particularly for rather remote areas like Africa and the Middle East, that haven’t been properly mapped in the past and thus will benefit from the quick and uncomplicated image acquisition through high resolution satellites.

2.1 Government Sector
There are two reasons why the demand for high resolution satellite imagery will increase noticeably within the next few years

First of all, the rapid urban development in regions like Asia and the Middle East has led to the need to update existing maps more frequently. Secondly, the popularity of satellite imagery, people are more accustomed to it. This familiarity and the easy accessibility to high resolution satellite imagery leads to an increase in the demand for satellite imagery.

Unsurprisingly, government agencies such as military survey departments, municipalities, as well as oil and gas companies are the major customers in this market, as these organizations are the traditional users of aerial photography imagery and have the biggest budgets, the sufficient number of qualified staff, adequate facilities, and are authorized to carry out such tasks.

2.2 Private Sector
The introduction of high resolution satellite imagery has great benefits for the private sector. It enables small companies to buy imagery for the study and approximation of their projects without the having to get permissions from respective governments these projects are located in.

3. QuickBird imagery vs. aerial photography
Although at present, most satellite imagery, like QuickBird imagery, and aerial photography are of equal use for most areas of application, satellite imagery has some remarkable advantages compared to aerial photography.

Restrictions that are connected to the technical limitations of aircrafts don’t apply to satellite imagery. Unlike an aircraft, a satellite is operational 365 days of the year, regardless of holidays, weather conditions or local flying restrictions. As it is orbiting the earth anyway, there are no extra costs involved in attempting more than one capture of an image. Because of the re-visit time of approximately 4 days the chance of acquiring imagery under similar weather conditions is fairly high. The need for block adjustment and the creation of image mosaics is reduced, due to the large area footprint that is acquired through satellite imagery. As Air Traffic Control restrictions don’t apply for satellites, imagery of restricted and/or remote areas can be acquired easily. To acquire imagery by satellite no aircraft, cameras or other expensive equipment are needed. On the financial side, satellite imagery has a clear advantage over aerial photography when it comes to working on small areas. Because of the licensing scheme, images can be sold multiple times and therefore one image is less expensive than the same image acquired by aerial photography would be.

Despite these great assets, satellite imagery still has some drawbacks in terms of accuracy. In temperate regions of the world there is always a strong possibility of cloud cover. Therefore completely cloud-free images will be very rare. Although the large footprint is a real asset concerning the elimination of extra work through image mosaicing and block adjustment, it is a drawback concerning Corridor AOIs. High resolution satellites are not suitable for acquiring imagery from dense urban areas and areas with deficient DTMs, because the typical off-nadir viewing angle of up to 25° is not acceptable for those areas. Finally, the image resolution is very low, compared to large scale aerial photography where resolutions of 05 – 25 cm are common. This drawback, however, may only be temporary, because satellites with higher resolutions are being developed, promising resolutions up to 30cm in the near future.

There are also some disadvantages concerning the further processing of satellite imagery. New equipment and more training will be required, because the production processes for high resolution satellite imagery differ significantly from those of traditional photogrammetric data capture. It is also relatively expensive and difficult to achieve stereo coverage from satellite imagery.

4. Conclusion
There is no real competition between Aerial Photography and Satellite imagery, yet. In fact, there is a strong synergy between these two image acquisition technologies from which the users and the GIS Community can benefit. But in the future the proliferation of image data will be spearheaded by satellite images. Procedures for acquisition, processing and distribution of images will develop very fast, driven by man’s desire and need

to understand and manage his environment. The image is the natural way for us to conceive our environment. Therefore it is only logical to conclude that – by the time the implementing technologies are sufficiently advanced – images will be used as transmitter for information that reaches far beyond what meets the eye.

Right now, the impact of Satellite Imagery, in the Middle East, is probably greater than elsewhere, because through imposed flying restrictions only a very small group of companies has the capacity to acquire aerial photography, in this region.

In spite of some of the disadvantages of satellite imagery, after 5 years of high resolution satellite imagery it can be ascertained that, although there is still room for improvement concerning accuracy and resolution, high resolution satellite imagery is leading the way to convenient and cost- efficient acquirement of image data for everyone.