LaserTC

Table of Contents

• Overview
• How LaserTC works
• Running LaserTC
• Input Files
• Output Files
• Limitations
• Licensing
• References

Overview

How LaserTC works

The elevation data used to fit the smooth surface are commonly provided in such a way that duplicate or near-duplicate spatial samples may be present on the elevation data file. LaserTC averages the elevation data that occur at the "same" location, thus eliminating duplicate spatial samples. At present, elevation samples are considered to occur at the same location if their coordinates, rounded to integral values, are the same. The rounding is performed on the input elevation data directly. For example, if the elevation data are provided in feet, their coordinates are rounded to the nearest foot prior to eliminating duplicate sample locations. The same is true if the elevation data are provided in meters. This means that the surface fitted to elevation data might be very slightly different if the elevation data are provided in meters rather than feet.

LaserTC is completely command-line driven; it accepts a small number of arguments, separated by spaces, on the command line. When an argument requires one to specify a value, the argument is separated from its value by an equals sign (=), with no intervening spaces. For example, to provide the name of an output file, -of=outputfilename would be used, where "outputfilename" is the name of the desired output file. A summary of the command-line options is presented below.

• -d Run the program in debug mode (prints copious information onto stdout).
• -h Provide a quick help listing.
• -1 The coordinates of each gravity station will not be read from the gravity input file (see below for a description of that file), but instead, the first coordinate triple read from the elevation file provided will be used as the position of the gravity station. This option is provided in order to permit users to easily provide the (relative) coordinates of a gravity station with respect to the digital elevation data provided.

• -:When this option is present, the coordinates on both the gravity station file and any elevation files are assumed to be in the order: Northing, Easting, Elevation (the default is Easting, Northing, Elevation).

• -rmin=value Specify the value of the minimum radius to be used for the calculation (default: 1.0 m).
• -rmax=value Specify the value of the maximum radius to be used for the calculation (default: 50 m).
• -angle=value Specify the value of the angular increment used for the circular portion of the numerical integration. The value supplied must divide evenly into 360; values such as 7 are unacceptable, for example (default: 3 degrees).
• -of=filename Specify the name of the output file to be created (default: LaserTC.out).
• -if=filename Specify the name of the input data file (NO default value).
• -eu=value Specify the value (0 or non-zero) of the units flag; 0 corresponds to meters, non-zero to feet. Both the horizontal (x,y) coordinates and the elevations are affected by this flag. For example, if the flag is set to 0, both (x,y) coordinates as well as the elevations are assumed to be in meters. Note that the units of the coordinates provided on the gravity input file must be the same as the units of the coordinates provided on the elevation data file (default: 1).
• -rho=value Specify the terrain density, in grams/cc, to be used (default: 2.67 grams/cc).
• -sigma=value Provides a value for the tension factor that will be used for the interpolating surface. Values from 0 to 1000 may be used. A value of 0 implies that the surface will be constructed with NO tension applied; in such a case, the surface will approximate a classical spline. As the the tension factor is increased, the interpolating surface exhibits less and less "overshoot" at locations where the elevation changes very rapidly. A tension factor of 1000 will result in a smooth surface that approximates a bilinear surface; little or no overshoot will occur.

  Generally, using a value of 0 for sigma provides very accurate results, and is somewhat faster in execution. Values of sigma > 0 should be used when the terrain surrounding the gravity station is extremely steep, as may happen near cliffs or in mines, where near-vertical rock faces may be present.

• -ss=value When this flag is used, LaserTC will create a grid file for each terrain correction calculation. The parameter value can be either 1 or 2 (other values are simply ignored). The grid file created will be written in one of two formats. If -ss=1 is used, the grid will be in Golden Software's "GS binary" grid format. If one has Golden Software's  SURFER© graphics package installed, one can then quickly view each terrain surface. SURFER© is available from Golden Software of Golden, Colorado, USA. If -ss=2 is used, the grid file created will be written in Geosoft's 16-bit grid file format (Geosoft grid format version 2). In either case, the grid files are named "station.grd", where "station" is the gravity station designator. For each grid file written out, a "posting" file named "station.txt" is also written. The posting file is an ASCII text file that contains the (x,y,z) triples used to construct the terrain surface. The positions and/or values from the posting file may be plotted to determine the extent of terrain coverage for a given station.
  
  Be aware that if you are processing many gravity stations, using the -ss option can result in a large number of grid files being created, as a grid file will be created for each station. Each grid file will be 40860 bytes in length when -ss=1 is used, or 20914 bytes in length when -ss=2 is used. In addition, of course, for each station, a posting file will be created. The length of the posting files depends upon how many elevation samples fall with Rmax of the gravity station.
• -L displays the software license, the version, and the build date, then exits.

Output Files

LaserTC creates three output files. The primary output file (default name: LaserTC.out) contains a summary listing of the gravity stations processed, the calculated terrain corrections, and the estimated uncertainties in the numerical integration. At this time, only those uncertainties associated with the radial portion of the integration are estimated. Future versions will include estimates of the total integration error. A second file, always called LaserTC.log, is created, as well. This file contains a more detailed, step-by-step listing of the calculational steps for each station. Finally, a third file, always called LaserTC.csv, is a comma-separated-value (CSV) file that contains the results of the terrain correction calculations. LaserTC.csv permits easy importation of the results of the calculation into 3rd-party spreadsheets.

Example Output Files

Primary Output File

Innermost terrain correction calculations.  Fri Feb 28 07:19:16 1997
All corrections calculated for a unit density (1.0 g/cc).
Minimum radius for calculation:          1.0 m.
Maximum radius for calculation:         50.0 m.
Angular increment for integration:         5 degrees.
Terrain corrections below are in microGals.

 Station    X-Value    Y-Value  Elev   Obs Grav  Inner TC  No. Eval.   Err Est
Getchell       0.0        0.0    0.0    0.000        22.1      2376     0.005
MallData       0.0        0.0    0.0    0.000        94.6      2376     0.004

Log File

Processing inner-zone elevation data.  Fri Feb 28 07:19:16 1997


Station: Getchell
  Elevation file: mine.dat
  Obs grav:     0.00       Station coordinates:       0.00      0.00      0.00
  Number of elevation data read is:   495.
  Number of elevations retained is:   440.

Station: MallData
  Elevation file: mall.dat
  Obs grav:     0.00       Station coordinates:       0.00      0.00      0.00
  Number of elevation data read is:   100.
  Number of elevations retained is:   100.

Limitations

Licensing