Oblique

The company Pictometry has patented an aerial imagery technology based on acquiring oblique digital images and primarily developed as an imagery visualisation tool. Oblique: I know what that is, but where is the novelty?

In this month’s interview, two directors of Blom Group (Norway) proudly talk about their ongoing Pictometry project, currently one of the largest image-acquisition projects in Europe. The aim is to create a standard database of oblique and ortho-aerial imagery covering every town in Europe with a population larger than 50,000 inhabitants, a total of about nine hundred towns. Aerial image acquisition and data processing is being done using a technology developed and patented by US-based Pictometry International Corp, headquartered in Rochester, New York. Under an agreement signed in 2005, Blom has been licensed to apply the technology in 23 European countries for ten years, with the option to renew. A unique feature of the business model is that, anticipating growing interest in geo-information on the part of non-conventional users, and partly induced by the success of Google Earth, Blom first acquires the images and then approaches potential customers. Anyone can then take out a licence to use the standard image library for a price per km2. This is a business model that is gaining in popularity; for example, Cyclomedia, which captures terrestrial photographs from a car platform, has also adopted this capture-prior-to-order model (see interview, GIM March 2007).

Data Acquisition
Looking at the data-acquisition system, the difference with conventional airborne photography is that here not only vertical but also oblique images are taken. The developers have constructed a sensor system consisting of five cameras, one directed nadir, the others viewing forward, backward, left and right. The oblique angle is approximately 40 degrees off-nadir. The flying height for neighbouring images is 3,000 feet (1,000m) and for community images 6,000 feet (1,000m) and pixel size 6 inches (15cm) and 1 foot (30cm), respectively. Geo-referencing is done using data gathered by the onboard, integrated GPS and Inertial Navigations Systems from Applanix. To create orthoimages from the vertical views a Digital Elevation Model (DEM) is needed. For this the existing DEMs are used.

View, Measure, Navigate
The technology consists of the entire processing chain, from image acquisition to customer use. The processed and geo-referenced images are delivered together with a software package called Electronic Field Study (EFS), a tool to view and take measurements, navigate and find a required location. It is possible to take measures in both orthoimages and oblique images. Measures include height of object, ground surface elevation, distance between two points, bearing, area and perimeter. However, the technology has not been developed to replace traditional surveying and photogrammetry for accurate mapping but is a complementary product primarily developed as a visualisation tool and for taking simple measures with non-mapping accuracy. Images and EFS package can be integrated with software products from most major GIS vendors.

Oblique Perspective
Oblique, cavalier or high viewpoint perspective (projection), are century-old terms indicating a technique for representing volumetric objects, such as buildings, on a plain. Initially used to map fortifications, the military named this ‘cavalier perspective’ since that was the way a horseman saw the environment. In today’s era of aeroplanes and satellites, the simple term ‘oblique’ survives. The birth of aerial photogrammetry originates in France and the history of ‘air surveying’ is marked by three important dates: 1783, 1837 and 1855. On 4th June 1783 the first demonstration flight of a hot-air balloon took place in Annonay, France. It was by the Montgolfier brothers, since known as the inventors of the first practical balloon. In 1837 Jacques Mandé Daguerre, by profession a painter of diorammas, obtained the first practical photograph using the process named after him. In 1855 Nadar for the first time in history combined the two technologies by climbing into a balloon with a camera in his hands and taking the first aerial photograph from eighty metres above earth’s surface. The first images from balloons were ‘birds-eye’ view snapshots and Emperor Napoléon III immediately recognised the potential for images taken from artificial ‘high ground’; funds soon became available for developing airborne photogrammetric techniques to produce topographic maps for military purposes.

Sideways-looking
To create accurate maps from vertical photographs many Ground-Control Points (GCPs) were necessary and the photographs had to overlap (usually 60%) in the direction of flight, enabling a 3D-view and x,y,z measuring with use of a stereoplotter. The method was labour-intensive and required many flight lines. Both disadvantages could be mitigated by mounting two cameras in the aeroplane, each facing outwards and sideways from line of flight. This provided the additional advantage of better interpretability and more intuitive view. And sideways and forward-looking is the principle upon which Pictometry technology is based. The processing cycle can now be highly automated because integrated onboard GPS/IMU systems have replaced GCPs and the images are in digital format. Today, 150 years after the invention of aerial photo–grammetry, oblique aerial images may enjoy renewed interest. And this because geo-information is now being discovered by the millions as a convenient and strategic source of information. As long ago acknowledged by Emperor Napoléon III.