Location Offset Correction

Use the Offset Correction option (UXLAG GX) to apply an offset correction, based on direction of travel, to locations or positions in a database.

Location Offset Correction dialog options

Offset distance

The distance to shift the locations. This usually corresponds to the distance between where the measurement is made (e.g. the coil centre of an instrument), and the location at which the position is measured (e.g. the position of the GPS unit or other location device).

  • Set this value to 0.0 to restore the X and Y locations to their initial values. (The backup channels must already exist.)

    • Script Parameter: UXLAG.DIST

    Default offset direction

    The direction in which to shift the data, relative to the direction of travel. If the coil is in front of the GPS unit, select "Shift forward". This value is overridden if there exists a "Lines" group and a valid offset value is found in the "LagDir" channel, corresponding to the particular line being processed.

    Script Parameter: UXLAG.DIR: 0: Forward, 1: Backward, 2: Right, 3: Left

    Smoothing interval for heading

    If specified, the input locations are thinned to be no more than this distance apart, then smoothly re-interpolated at five times the average point separation to produce a curve from which the heading at any point is determined. If left blank, then this interval is calculated to be the greater of the input offset distance, or two times the average point separation.

    Script Parameter: UXLAG.SMOOTH

    Lines to correct

    Select which lines to process.

    Script Parameter: UXLAG.LINES: S: Selected lines, D: Display line, A: All lines.

    Raw X backup channel

    Raw Y backup channel

    The names of the raw X and Y data channels (see notes below).

  • It you want to perform multiple corrections, change the name of the backup channels for each correction.

    • Script Parameter: UXLAG.X, UXLAG.Y

    Application Notes

    There is often a fixed separation between the center of measurement, e.g. the center of a coil, and the location of the device used to record position. This offset is a function of the orientation of the instrument, which normally remains fixed relative to the line heading, for instance an instrument cart pushed in front, or pulled behind an operator.

    This tool takes as input a fixed offset distance, and an offset correction that is to be applied relative to the current line heading. The heading is determined from a smoothed version of the line path; the smoothing is based both on the offset required, and the spacing between sample locations. The calculated offset is then added to the original location, in the direction specified by the offset direction.

    The algorithm is as follows:

    1. Determine average distance between each point = D. 

    2. Smoothing interval = MAX(2*D, Offset distance) = I (or input value). 

    3. Thin input points to be at least the smoothing interval I apart from each other. 

    4. Smoothly re-interpolate the thinned points at five times the original average distance D. 

    5. For each input point, calculate the bearing using the nearest points on the smoothed curve. 

    Automatic Backup of Original Locations

    Using the current coordinate channels,this tool preserves original locations, and automatically uses them should the correction need to be repeated (for example, if the offset direction or distance need to be altered). The methodology is as follows:

    1. The first time this tool is run, the current coordinate channel (original locations) are first copied into the backup channels; by default, these are named "X_Raw" and "Y_Raw". These channels are protected and should not be altered by the user.
    2. The tool then uses the locations from the "X_Raw" and "Y_Raw" channels in processing to determine the offset corrected locations. The current coordinate channel will contain the corrected values.
  • To restore the current coordinate channels to the original values, you can enter "0" for the offset distance. The tool will then copy the "Raw" data back into the current coordinate channels.
  • If you wish to perform multiple corrections (for example, Shift Left 2 and Back 5): run the tool twice, each time changing the name of the backup channels. This will preserve your original locations (in the first backup pair) and starting point for each additional change. Also, for data that requires multiple offsets, consider using the Sensor Offset tool.

    • Setting Offset Directions for Individual Lines

    The offset direction may not necessarily be consistent for all lines. For instance, the operator might push the cart along one line, and then pull it back the next. Use a LineList Table to override the default offset direction, by first running Line List Table and editing the database/table as required. Location Offset Correction will then use OffDir channel from the Line List Table for the required offset direction for individual lines (see Line List Table).