TLS Data Processing Modules. Armin Gruen & Zhang Li Institute of Geodesy & Photogrammetry Swiss Federal Institute of Technology Zurich, Switzerland E-Mail: agruen,[email protected]
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Georeferencing by Aerial-Triangulation GPS/INS/Camera Translational Displacement Correction GPS/INS displacement vector correction using the aircraft attitude data (Tanzvector data, RMS of directional error is 0.3 deg) INS/Camera displacement vector correction using the output of INS
rotation matrix from camera frame to INS body. 0, 0, 0 is the boresight misalignment between axes of INS and Camera.
r0 + rresGeoreferencing by Aerial-TriangulationModel 1: Direct Georeferencing Model (DGR)
X0-2,Y0-2,Z0-2 are unknowns to model the residual errors after the GPS/INS/Camera displacement correction.
rotation matrix from INS body to ground coordinate frame, INS, INS, INS is the original altitude which from INS sensor. unknowns 1, 1, 1 is used to model INS draft errors.
Colinearity equations: (X,Y,Z) (x,y)
Vectors of unknowns
Coefficient martices for unknown vectors
r0 + rres
Georeferencing by Aerial-TriangulationModel 2: Piecewise Polynomial Model (PPM)
coordinates of perspective center denoted in the ground coordinate system, XSi0,YSi0,ZSi0;XSi1,YSi1,ZSi ;XSi2,YSi2,ZSi2 are 0,1 and 2-order unknown polynomials coefficients of ith segment to model the perspective center.
rotation matrix from ground frame to camera frame
There are 2 kinds of constraints that may be applied to each parameters at the segment boundaries. The zero order continuity constraints ensure that the value of the function computed from the polynomial in each 2 neighboring segments is equal at their boundaries, i.e.
The first order continuity constraint required that the slope, or first order derivative, of the functions in 2 adjacent segments be forced to have the same value at their boundary, i.e.
r0 + rres
Georeferencing by Aerial-TriangulationModel 3: Cubic Spline Interpolation Model (CSI)The Cubic Spline Interpolation model computes the orientation parameters of reference image lines at regular intervals, then calculates the parameters of intermediate image lines by cubic polynomial interpolation. These reference image lines are so-called "orientation fixes"
Feature point extraction. The user can select a point in one image , software can automatically derive the nearest feature points in a image window sized 4141 pixels using Forstner interest operator.
Imagepyramid generation and pixel level accuracy conjugate points generation.
Subpixel accuracy matching by Least Squares Matching. 6 geometric parameters are used in the adjustment.
Using the above semi-automatic approach, several hundards of tie points (standard deviation of 0.2-0.3 pixel) are extracted in an interactive way.
Forward Nadir Backward
Forward Nadir Backward
By modeling the remaining errors after GPS-INS, INS-Camera displacement vector correction as a constant offset value, about 0.10, 0.07m and 0.08m absolute accuracy in planar and heightwas achieved using DGR model.
Using PPM and CSI model, better results, about 0.08, 0.05m and 0.07m absolute accuracy in planar and heightwas achieved, especially the 20 sections PPM model, about 1 pixel accuracy was achieved.
Better accuracy results can be achieved by using more sections in PPM model and more orientation fixes in CSI model.
Image MatchingImage Pyramid Generation Enhancement of the texture in original image (level 0) using the Wallis filter. Image pyramid generation by reduction factor 2, including the original image, the pyramid level is fixed to 4.
forward nadir backward
lower level feature
upper level feature
Ortho-Image GenerationDirect method
Nearest neighbour, Bilinear & Cubic Resampling
Iterative Search Algorithm
Ortho-Image GenerationIndirect method This is our ortho-image generation method of choice
Scan line number
An experimental software package has been developed to allow the processing of the TLS raw images including georeferencing, image matching, DEMs generation, ortho-image generation and stereo measurement.
From our experimental results, the geometric accuracy of TLS system is about 1-3 pixels and not obtained the sub-pixel accuracy, these may attribute to the non-precise/fully sensor modeling.
Untill now only one strip of GSI testing area has been evaluated in our experiments, the TLS images of different image scale, different terrain type and multi-strip need to be evaluated. Further more, the stability of TLS system calibration parameters need to be evaluated.