Page 1 of 3 Next  Stewart Walker Director of Marketing (Geospatial eXplotation Product)BAE Systems. Photogrammetry is well known to the readership of GIS Development as the workhorse for creating and updating maps and GIS databases. It remains dynamic, however, and its increasing versatility and accessibility increase its value to many users. We review several trends, stressing the rapidly changing hardware for airborne digital imaging. Airborne digital imagers Ever since the launch of the Leica ADS40 and the Z/I Imaging DMC at the ISPRS Congress in Amsterdam in 2000, the photogrammetric world has been buzzing with excitement that digital technology has arrived powerful enough to replace the aerial film camera. These high-end imagers, together with the Vexcel UltraCam, have achieved acceptance in the market-place: the three suppliers have sold more than 150 units, at prices well above those of film cameras, and diverse applications are being tackled and deliverables created in sufficient quantities to quell any doubts about teething troubles or residual systematic image errors. Hundreds of film cameras are still in use, however, and the predominant workflow for map production remains film photography, chemical processing and scanning, yet we must acknowledge that a “paradigm shift” is taking place and the attractions of simpler, all digital workflows are considerable despite high initial investments. It appears that a given map specification can be met using digital imagery with a larger pixel size (ground sample distance) than scanned film, which compensates for the smaller fields of view of the digital imagers. The radiometric content is better too, resulting in longer flying seasons and days. Users should expect operations such as automated triangulation and generation of digital terrain models, which require reliable image matching, to run better with digital imagery. High-end imagers are only part of the bewildering range of hardware available (table 1). However much the spotlight is on these, far more units have been sold of the small- and medium-format cameras. Small digital frame cameras feature in economical, lightweight multiple camera systems, for example the GeoVantage system developed for agricultural monitoring or the several systems involved in widespread production of off-the-shelf oblique imagery that has proved unexpectedly popular with emergency services and other users owing to its easy interpretability – Pictometry now operates more than 70 aircraft! Perhaps 200 medium-format digital frame cameras have been sold by suppliers such as Applanix and Rollei, many integrated with airborne LIDAR sensors. Color is generally achieved by Bayer interpolation, whereby a matrix of tiny filters is placed over the CCD array, so that each cell receives red, green or blue and the two colors it does not receive are interpolated from neighboring cells. The largest CCD arrays incorporated in cameras remain below 100 megapixels, such as an 85-megapixel system from BAE Systems to capture panchromatic imagery for reconnaissance. The high-performance imagers, therefore, use multiple digital frame cameras to achieve sufficient coverage. The Intergraph DMC has four 28-megapixel panchromatic cameras pointing slightly obliquely in a star-like shape and acquiring images simultaneously, which are used to generate a single, synthetic, central perspective image in software. The Vexcel UltraCam uses an ingenious arrangement of nine arrays capturing in a particular sequence to generate a single panchromatic image, which has increased from 86 megapixels in the UltraCamD to 133 megapixels in the recently introduced UltraCam-X model. Both the DMC and the UltraCam have four, lower resolution, multispectral cameras pointing vertically, the imagery from which is used to colorize the panchromatic image. Also in this group is the DiMAC 2.0 from DIMAC Systems, which has two 39-megapixel cameras with Bayer interpolation, pointing vertically downwards but with their focal planes offset relative to the optical axes to increase cross-track coverage. Pushbroom line scanners represent a quite different technology. Based on linear CCD arrays, they acquire continuous strip images through the forward motion of the aircraft. Multiple arrays may be placed on the focal plane, to facilitate restitution of the imagery using the “in-track” geometry, provide stereoscopic coverage and achieve color. The recent introduction by Leica Geosystems of two new sensor heads for its ADS40 is intriguing (figure 1). These incorporate a novel tetrachroid beam splitter so that the same strip of terrain is imaged on four multispectral arrays that are physically separated on the focal plane. Moreover, the sensitivity of the multispectral arrays has been quadrupled so that high resolution imagery can be acquired in less than ideal light conditions. Thus three major perceived disadvantages of pushbroom line scanners are dispelled, i.e. fringing caused by imperfect color registration, resolution limitations in the multispectral, and color stereo.  Fig. 1 Focal plane layouts of the recently introduced SH51 and SH52 sensor heads for Leica ADS40: both are fitted with tetrachroid four-way beam splitter and have multispectral collection in the nadir for minimum long-track building lean and superior “near true” orthophotos; SH52 additionally offers multispectral stereo imagery.
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