The ITG digital terrain model generation system


ITG (O'Neill and Denos, 1996, Denos 1991, Otto and Chau, 1989) is a fully automated system which generates digital terrain models [DTM's] fromoptical stereo imagery. Currently, the system works with SPOT imagery (O'Neill, 1992). A Tsai (Tsai, 1987) central perspective camera model has also recently been added to the system. This allows it to be used with aerial photography. The ITG system has been used by Geophysicists in Oxford Department of Earth Sciences to characterise terrain in the area of active fault zones in Greece and Turkey. The system has alsobeen used by the Cardiff Geophysics Consultancy terradat to in both gravity surveys and intervisibility studies for windfarms.



Digital terrain model of the Seville area of Spain produced from SPOT-PA imagery [1:100000] scale, using the Tumbling Dice ITG stereo DTM creation facility.



SPOT-PA stereo pair of the Seville area of Spain used to produce DTM.

Components of the ITG terrain model generation system


The ITG system consists of a number of pipelined components. This pipeline is generally set up automatically ITG driver. The pipeline generated by this program consists of the following components:

  1. An autoseeder.
  2. An (area correlation) optical stereo matcher.
  3. A camera model.
  4. A set of co-ordinate conversion filters.
  5. An interpolator.


The autoseeder is used to generate a set of (approximate) stereo correspondences which are used to seed the optical stereo matcher. The autoseeder is divided into two parts; cascade which uses an annealer to generate a small number of random stereo correspondences and cheops which applies a resolution pyramid to extract those stereo-correpondences which correpond to actual matches.

These stereo correpondences are used to seed the pslam area correlation stereo matcher. This is sheet growing adaptive least squares pattern correlator which is based on the work of Prof. A. Gruen (ETH Zurich) and Paul Otto and Tony Chau (Alvey MMI-137 Project, University College London). Pslam extends the basic sheet growing ALSC [Adaptive Least Squares Correlation] technique, enabling it to work tolerably well on images which are:

  • very large
  • of poor contrast

The stereo matcher produces a dense array of stereo-correpondences. These are either output producing a digital disparity model, which can be further processed using tools like disp2ddm or they are passed to the camera model which transforms them from image space to ground space, producing an ungridded array of (x,y,z) co-ordinates in a geocentric co-ordinate system. These points are converted to an appropriate local vertical system using the co-ordinate conversion filters. Currenlty filters are available to transform between geocentrics and:

  1. Universal Transverse Mercator [UTM]
  2. Lambert Conformal Conic (zone 3) ["French Lambert"]
  3. Geographic (long,lat,height)

Finally a regularly gridded digital terrain model DTM is produced by interpolating the set of points produced by the co-ordinate conversion filters. The interpolator used by the ITG system, krige_spb, is based on the Kriging interpolator (Delfiner and Delhomme, 1975) implemented by Tim Day for the Alvey MMI-137 Project at University College London (Day 1990). It has been extensively modified to both improve it throughput and to allow it to efficiently interpolate datasets containing millions of points on mid range Pentium Personal computer's and similar hardware. In addition to the Kriging interpolator a simple nearest DTM cell gridding algorithm fndem is also available. This is typically used to generate quick look DTM's at coarse resolution.

In addition to producing DTM's, a pipeline generator gti is provided which generates orthoimages using the DTM generated by the ITG subsystem and one of the images (used by ITG to generate the DTM. gti automatically sets up an orthoimage generation pipeline consisting of the fnxyz, spot1m (inverse), cocon (inverse), npraw filters. ITG may also be used in stand alone mode a change predictor. Coupled with appropriate imagery from a scanning radiometer, for example the ATSR-1 instrument carried by the European Remote Sensing Satellite, ERS-1 or the Japanese ASTER satellite (O'Neill and Dowman, 1993), it can function as part of an automated system to assess the health of large-scale ecosystems.




Orthoimage of the Seville area of Spain produced from SPOT-PA imagery [1:100000] scale, using the Tumbling Dice ITG stereo DTM creation facility.

Obtaining the ITG stereo matching software

If you are interested in more information about ITG contact Mark O'Neill (mao@tumblingdice.co.uk),

References

    Day 1990: Day T, Semi-variograms Kriging and Stereo Matching Alvey MMI-137 Final Report, Chapter. 18.

    Delfiner and Delhomme, 1975: Delfiner P. and Delhomme J.P, Optimum Interpolation by Kriging, in Display and Analysis of Spatial Data J.C Davis and M.J. McCullagh (Eds), pp.96-114, Wiley.

    O'Neill 1992: O'Neill Mark A, A Kinematic Model of the SPOT-1 Sensor, Ph.D Thesis, University of London.

    O'Neill and Denos 1996: O'Neill Mark .A and Denos Mia, Automated System for Coarse-to-Fine Area Corellation Stereomatching Image & Vision Computing 14(3):225-236.

    Denos 1991: Denos Mia, A pyramidal scheme for stereomatching SIR-B imagery Int. J. Rem. Sens. 13(2):387-392.

    O'Neill and Dowman 1993: O'Neill, Mark A. and I.J. Dowman, A simulation study of the ASTER sensor using a versatile general purpose rigid sensor modelling system (Advanced Spaceborne Thermal Emission and Reflection Radiometer). Int. J. Rem. Sens., 14(3): 565-582.

    Otto and Chau 1989: Otto G.P. and Chau T.K.W, Region-growing algorithm for matching of terrain images Image & Vision Computing 7(2):83-94.

    Tsai 1987: Tsai R. Y, A versatile Camera Calibration Technique for High-Accuracy 3D Machine Vision Metrology Using Off-the-Shelf TV Cameras and Lenses, IEEE Journal of Robotics and Automation 3(4): 323-344

Acknowlegments

Many people have contributed to testing the ITG system in its pre-commercial phase. The principal contributors are:

    Barry Parsons (Earth Sciences, Oxford University, UK)
    Philip England (Earth Sciences, Oxford University, UK)
    Jenefer Brett (formerly at Earth Sciences, Oxford University, UK)
    Tim Day (formerly at University College London)
    Ian Dowman (Geomatic Engineering, University College London
    Mia Denos (Tumbling Dice Ltd, Newcastle, Tyne and Wear UK)
    Mike Cook (formerly at Laser-Scan Ltd, Cambridge, UK)
    Mike Jackson (formerly at Laser-Scan Ltd, Cambridge, UK)
    Bill Savage (Laser-Scan Ltd, Cambridge, UK)
    Phil Murfitt (formerly at Laser-Scan Ltd, Cambridge, UK)
    Chris Higgins (formerly at Laser-Scan Ltd, Cambridge, UK)
    Dave Gugan (Genisis, Manchester, UK)
    Tony Chau (formerley at University College London)
    Paul Otto (formerly director, Canon Research Europe)
    Rob Davies (Terradat Ltd, Cardiff, Uk)
    Neil Thacker (University of Machester, UK)


Content (c) 2007 Tumbling Dice Ltd. ITG is a Tumbling Dice Ltd product.