Adjustment of observations

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Adjustment of observations is a model for the solution of an overdetermined system of equations based on the principle of least squares. It is used extensively in the disciplines of surveying, geodesy, photogrammetry (the field of geomatics, collectively).

Formulation[edit]

There are three forms of adjustment of observations: parametric, conditional, and combined. In parametric adjustment, one can find an observation equation h(X)=Y relating observations Y explicitly in terms of parameters X (leading to the A-model below). In conditional adjustment, there exists a condition equation g(Y)=0 involving only observations Y (leading to the B-model below), with no parameters X at all. Finally, in a combined adjustment, both parameters X and observations Y are involved implicitly in a mixed-model equation f(X,Y)=0. Clearly, parametric and conditional adjustments correspond to the more general combined case when f(X,Y)=h(X)-Y and f(X,Y)=g(Y), respectively. Yet the special cases warrant simpler solutions, as detailed below. Often in the literature, Y may be denoted L.

Solution[edit]

The equalities above only hold for the estimated parameters \hat{X} and observations \hat{Y}, thus f\left(\hat{X},\hat{Y}\right)=0. In contrast, measured observations \tilde{Y} and approximate parameters \tilde{X} produce a nonzero misclosure:

\tilde{w} = f\left(\tilde{X},\tilde{Y}\right).

One can proceed to Taylor series expansion of the equations, which results in the Jacobians or design matrices: the first one,

A=\partial{f}/\partial{X};

and the second one,

B=\partial{f}/\partial{Y}.

The linearized model then reads:

\tilde{w} + A \hat{x} + B \hat{y} = 0,

where \hat{x}=\hat{X}-\tilde{X} are estimated parameter corrections to the a priori values, and \hat{y}=\hat{Y}-\tilde{Y} are post-fit observation residuals. In the parametric adjustment, the second design matrix is an identity, B=-I, and the misclosure vector can be interpreted as the pre-fit residuals, \tilde{y}=\tilde{w}=h(\tilde{X})-\tilde{Y}, so the system simplifies to:

A \hat{x} = \hat{y} - \tilde{y},

which is in the form of ordinary least squares. For the more general cases, Lagrange multipliers are introduced to relate the two Jacobian matrices and transform the constrained least squares problem into an unconstrained one (albeit a larger one). In any case, their manipulation leads to the \hat{X} and \hat{Y} vectors as well as the respective parameters and observations a posteriori covariance matrices.

Computation[edit]

Given the matrices and vectors above, their solution is found via standard least-squares methods; e.g., forming the normal matrix and applying Cholesky decomposition, applying the QR factorization directly to the Jacobian matrix, iterative methods for very large systems, etc.

Extensions[edit]

If rank deficiency is encountered, it can often be rectified by the inclusion of additional equations imposing constraints on the parameters and/or observations.

Worked-out examples[edit]

Applications[edit]

Related concepts[edit]

Bibliography[edit]

Lecture notes and Technical reports:

Books:

  • Edward M. Mikhail, Friedrich E. Ackermann, "Observations and least squares", University Press of America, 1982
  • Wolf, Paul R. (1995). "Survey Measurement Adjustments by Least Squares". The Surveying Handbook: 383–413. doi:10.1007/978-1-4615-2067-2_16.  edit
  • Paul Wolf and Bon DeWitt, "Elements of Photogrammetry with Applications in GIS", McGraw-Hill, 2000
  • Karl-Rudolf Koch, "Parameter Estimation and Hypothesis Testing in Linear Models", 2a ed., Springer, 2000
  • P.J.G. Teunissen, "Adjustment theory, an introduction", Delft Academic Press, 2000
  • Edward M. Mikhail, James S. Bethel, J. Chris McGlone, "Introduction to Modern Photogrammetry", Wiley, 2001
  • Harvey, Bruce R., "Practical least squares and statistics for surveyors", Monograph 13, Third Edition, School of Surveying and Spatial Information Systems, University of New South Wales, 2006
  • Huaan Fan, "Theory of Errors and Least Squares Adjustment", Royal Institute of Technology (KTH), Division of Geodesy and Geoinformatics, Stockholm, Sweden, 2010
  • Gielsdorf, F.; Hillmann, T. (2011). "Mathematics and Statistics". "Springer Handbook of Geographic Information". p. 7. doi:10.1007/978-3-540-72680-7_2. ISBN 978-3-540-72678-4.  edit
  • Charles D. Ghilani, "Adjustment Computations: Spatial Data Analysis", John Wiley & Sons, 2011
  • Charles D. Ghilani and Paul R. Wolf, "Elementary Surveying: An Introduction to Geomatics", 13th Edition, Prentice Hall, 2011
  • Erik Grafarend and Joseph Awange, "Applications of Linear and Nonlinear Models: Fixed Effects, Random Effects, and Total Least Squares", Springer, 2012