User's Guide

AUTOFIT

Automatically edits a set of tie points

Function:

Edits a set of tie points from a tie point location (TPL) file by rejecting points with high residuals until the Root Mean Squared Error (RMSE) between successive iterations is less than a tolerance value or a minimum RMSE is met. Points are not deleted, but are marked as rejected in the output TPL file. The RMSE value is calculated between the search and reference tie points.

Parameters:

INTL
Input tie point location file. The TPL file containing the tie point coordinate pairs to be used in calculating polynomials which define the coordinate transformation.

OUTTL
Output tie point location file. All tie point records (up to MAX_TIE_POINTS) contained in the input file are included; however, the active-flag field of the tie point record is updated to indicate which points have been rejected.

OUTPLOT(--)
Output plot file. Contains the tie point locations, and the x and y residuals for each active point in OUTTL. This file can be used as input to CORRPLOT to produce a plot of the residuals. By default, a plot file is not created. A ";plt" will be appended to the name.

PDEGREE(2)
Polynomial degree. The polynomial degree to be used in calculating the coordinate transformations. The supported degrees are one and two. For polynomials of degree one, at least four points are required. For polynomials of degree two, at least seven points are required.

TOLVAL(0.005)
Tolerance value. Tolerance (in pixels) between successive iterations. This value is used as a stopping criteria for the program. The default value should be appropriate for most cases; however, slight modifications may be required in some cases. If TOLVAL is too small, this criteria may never be met. STOPRMSE is provided for this reason.

STOPRMSE(0.0)
Stopping RMSE value. The RMSE must be below this value before calculations stop. STOPRMSE is used in addition to TOLVAL. If either criteria is met the program stops.

PRINT(TERM)
Output destination. The destination of the output. The output contains the RMSE, the point which is marked as rejected, and the RMSE difference between successive iterations. 


  = TERM:      Terminal.  Output is sent to the user's
               terminal.
  = LP:        Line printer.  Output is sent to the
               printer defined by $PRINTER.
  = Filename:  User-supplied filename.  Output is sent
               to a disk file named "filename;prt".

Example:

  1. LAS> autofit intl=tiept.loc outtl=tiept.out pdegree=1

    Point pairs from the tie point location file TIEPT.LOC;TPL are used to derive a pair of first-degree polymomial equations that predict input coordinates from output coordinates. All tie points are written to TIEPT.OUT;TPL. However, only those points that lower the RMSE for each iteration until successive iterations have RMSE values less than TOLVAL are left active.

Description/Algorithm:

AUTOFIT uses a tie point location file containing tie point pairs which define a mapping from the output (REFERENCE) coordinates to the input (SEARCH) coordinates (an INVERSE transformation) to derive a pair of bivariate polynomials. The polynomials are calculated using the method of least squares. These polynomials are used to calculate the RMSE. PDEGREE and the number of tie point pairs used control the fitting.

For each iteration, active points are left out one at a time while the RMSE is calculated on the resulting regression fit. The point which, when left out, lowered the RMSE the most is then marked as rejected in OUTTL. The resulting RMSE for the current iteration is displayed. This continues until successive RMSE values between iterations are less than TOLVAL, the RMSE for the current iteration is less than STOPRMSE, or the number of active tie points is too few for the specified PDEGREE.

Nonfatal Error Message:

  1. [autofit-maxpoints] Not all possible points used

    The maximum number of tie points (MAX_TIE_POINTS) have been selected. OUTTL will only contain this maximum number of points. If more points are needed, see the system manager.

Fatal Error Messages:

  1. [autofit-alloc] Unable to allocate dynamic memory

    An error occurred allocating memory during processing. Re-run the function, and if the error persists, contact the system manager.

  2. [autofit-fatal] Fatal error encountered

    The error message displayed immediately preceding this message is the specific error encountered. Processing is terminated.

  3. [autofit-minpts] Not enough points for polynomial degree

    There are not enough points to edit for PDEGREE. Either lower PDEGREE, or provide more points and re-run. If more points are needed, a new tie point file may have to be created.

  4. [autofit-print] Error opening print file

    There was an error opening the PRINT file. Make sure the destination directory allows write permissions.

  5. [autofit-wrapback] Could not edit to TOLVAL

    Removing more points did not lower the RMSE. Raise TOLVAL or specify STOPRMSE and re-run.

User Note:

  1. Related functions are CORRPLOT, EDITCORR and POLYFIT.