Full text: Technical Commission VII (B7)

International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B7, 2012 
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia 
U. Beisl* 
* Leica Geosystems AG, 9435 Heerbrugg, Switzerland — Ulrich.Beisl@leica-geosystems.com 
Commission VII, WG VII/1 
KEY WORDS: Atmosphere, Modelling, Radiometric, Calibration, Processing, Multispectral, Digital, Camera 
The image quality of photogrammetric images is influenced by various effects from outside the camera. One effect is the scattered 
light from the atmosphere that lowers contrast in the images and creates a colour shift towards the blue. Another is the changing 
illumination during the day which results in changing image brightness within an image block. In addition, there is the so-called 
bidirectional reflectance of the ground (BRDF effects) that is giving rise to a view and sun angle dependent brightness gradient in the 
image itself. To correct for the first two effects an atmospheric correction with reflectance calibration is chosen. The effects have 
been corrected successfully for ADS linescan sensor data by using a parametrization of the atmospheric quantities. Following 
Kaufman et al. the actual atmospheric condition is estimated by the brightness of a dark pixel taken from the image. The BRDF 
effects are corrected using a semi-empirical modelling of the brightness gradient. Both methods are now extended to frame cameras. 
Linescan sensors have a viewing geometry that is only dependent from the cross track view zenith angle. The difference for frame 
cameras now is to include the extra dimension of the view azimuth into the modelling. Since both the atmospheric correction and the 
BRDF correction require a model inversion with the help of image data, a different image sampling strategy is necessary which 
includes the azimuth angle dependence. For the atmospheric correction a sixth variable is added to the existing five variables 
visibility, view zenith angle, sun zenith angle, ground altitude, and flight altitude - thus multiplying the number of modelling input 
combinations for the offline-inversion. The parametrization has to reflect the view azimuth angle dependence. The BRDF model 
already contains the view azimuth dependence and is combined with a new sampling strategy. 
Originally, photogrammetric camera systems were used for 
metric purposes in the geometric domain, i.e. for measuring 
distances, areas and angles. This was possible with the analog 
film cameras which provided sufficiently good contrast and 
sharpness. Attempts were made to use the wet film technology 
for radiometric measurements using densitometers, but the 
radiometric resolution was poor and the results were only stable 
within one film roll due to the influences of temperature and 
developer during film development. 
With large format digital sensors becoming affordable for 
photogrammetric users ten years ago, new application areas 
have developed quickly. Remote sensing applications which 
could be handled only with calibrated satellite images can now 
be solved with airborne images, too. In addition, a new mass 
market for cheap high resolution images for use in internet 
based mapping systems has emerged. In addition to a minimal 
geometric accuracy the new applications require a balanced 
radiometry and removal of atmospheric artefacts. 
When digital cameras appeared on the market the analog film 
data workflow had already turned digital by using film scanners. 
Therefore the geometric calibration algorithms could be easily 
transferred to the digital image data workflow. The radiometric 
processing of digital camera images had long been dominated 
by a mere relative calibration of the lens falloff. 
However, the large field of view (FOV) and the varying flying 
height of airborne cameras introduce strongly varying effects of 
atmospheric stray light, giving rise to a blue hue, increasing 
towards the borders of the images. Furthermore the effects of 
bidirectional ground reflectance (BRDF) cause varying 
brightness within the image, the most prominent ones being 
sunglint in the water and a hot-spot in the image at high sun 
elevation. To address these radiometric aspects an EUROSDR 
project was initiated (Honkavaara, 2011). 
In order to correct those environmental artefacts in airborne 
images, methods from satellite and hyperspectral airborne image 
workflows were adapted to the needs of high-resolution 
photogrammetric images. Those methods use physical models 
which require an absolute calibration of the airborne sensors. 
The Leica ADS40 camera was the first commercial 
photogrammetric camera that provided an absolute radiometric 
calibration (Beisl, 2006). This was the prerequisite for applying 
an automated atmospheric correction in the photogrammetric 
workflow, which was implemented together with a BRDF 
correction (Beisl et al, 2008). The atmospheric correction 
option for ADS image data has become the standard setting in 
the image workflow for XPro users (Downey et al., 2010). A 
validation of the reflectance calibration has been presented by 
(Markelin et al., 2010) and (Beisl et al., 2010). 
The Swiss Federal Office of Topography (swisstopo) is now 
using absolutely calibrated ADS images to produce two quality 
image products in an operational way (swissimage standard 
product and remote sensing basis product) (Schläpfer et al, 
This paper gives an outline, how to extend the ADS radiometric 
correction algorithms for use in frame sensors like DMC (Ryan 
et al. 2009) or RCD30 (Wagner, 2011), (Tempelmann, 2012). 

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