GRIDS

Geophysical Grid Management — User Manual

Overview

GRIDS is a graphical tool for preparing and manipulating geophysical grid data. It covers the stages from raw scattered point data through to a clean, corrected grid ready for further analysis: clipping data to an area of interest, kriging irregularly-spaced points onto a regular grid, removing polynomial trends, and performing grid arithmetic (addition, subtraction, scaling).

The sidebar organises all commands into two sections: Data (commands that work with raw XYZ point data, load boundary overlays, or post-process grids after loading) and Utilities (commands that perform grid arithmetic or file-format conversions). The result of each grid operation becomes the active grid, displayed in the Grid Preview panel on the right.

Interface

Toolbar buttons

ButtonAction
Load Grid Open a grid file (binary .bin or .alpc, or ASCII .txt / .dat / .asc / .xyz) and make it the active grid. The preview updates immediately.
Save Grid Write the current active grid to a file. A file-save dialog lets you choose the path and format (binary or ASCII).
Save Plot Save the current preview image as PNG or JPG at a chosen pixel size (current size, preset, or custom).
Clear Log Erase the output log panel.

Typical workflow

  1. Select physical features files, if wanted.
  2. choose data — clip raw XYZ measurements to the area of interest and optionally filter outliers.
  3. grid data — krig the clipped points onto a regular grid (result stored in memory).
  4. remove a trend — subtract a polynomial regional surface from the active grid.
  5. Apply any further grid arithmetic under Utilities — add, subtract, or multiply as needed.
  6. Use Save Grid (toolbar) to write the final grid to disk.

physical features

Loads one or more files describing geographical boundary features — such as coastlines, rivers, glaciers, or other vector overlays — and displays them as line overlays on top of data maps in the preview panel. Once loaded, the outlines persist and are drawn automatically on every subsequent spatial map.

When to use: When interpreting results in a geographic context, for example to see how a gravity or magnetic anomaly relates to a coastline or drainage network.

OptionDescription
Outline data files One or more ASCII files of geographic boundary coordinates. Use Add file(s)… to browse; select one or more entries and click Remove selected to delete them. Each file may contain multiple polyline segments separated by a pen-up sentinel (a line reading 9999 9999).
Show on maps When ticked, the loaded outlines are drawn on all subsequent spatial-domain map previews. Untick to hide them without discarding the data.
View outlines Display the currently loaded outline data immediately in the preview panel, even before running any other command.
The outline files are read once and kept in memory for the session. Running physical features again replaces the stored outlines with the new file list. Lines beginning with # and blank lines in the outline files are ignored.

choose data

Clips a set of irregularly-spaced XYZ points to a rectangular bounding box and an optional Z value range, producing a filtered subset of the original data.

When to use: Before kriging, to restrict the dataset to the area of interest, remove outlying points spatially, or discard anomalous Z values.

OptionDescription
Input XYZ ASCII file Source file of whitespace-delimited X Y Z triplets (one per line). Comment lines beginning with # are ignored. Up to 20 000 points are read.
Output XYZ ASCII file Destination file for the filtered point set. Optional — if left blank, the filtered points are kept in memory and displayed without saving to disk.
Bounding limits — X min, Y min, X max, Y max The spatial rectangle to retain. Points whose XY coordinates fall within the rectangle are kept. Auto-filled from the data extents when an input file is selected.
Outlier removal — Lowest Z value, Highest Z value Points with Z below the minimum or above the maximum are removed. Auto-filled with the actual Z range when a file is selected. Set to extreme values to disable Z filtering.
Remove duplicate XY points If ticked, points sharing the same XY coordinates (to 6 decimal places) are reduced to one.
Show selected area Re-plots the data points in the preview panel and overlays four thin lines marking the chosen bounding rectangle. Use this to verify the limits before running.
When an input file is selected, the preview automatically shows a scatter plot of all points coloured by Z value, and the bounding limits and Z range fields are auto-filled from the data. If no output file is specified, the filtered XYZ data is held in memory ready for immediate use with grid data.

grid data

Grids irregularly-spaced XYZ point data onto a regular 2-D grid using ordinary kriging. The result becomes the active in-memory grid.

When to use: To convert scattered field measurements into a regular grid suitable for further processing or display.

OptionDescription
Input XYZ ASCII file Source file of X Y Z triplets. Auto-filled with the output file from a preceding choose data run when available.
Grid dimensions (NX × NY) Number of columns and rows in the output grid. Tick Suitable for FFT to snap both dimensions to the same power of 2 (2, 4, 8, … 4096), which is required for efficient FFT-based post-processing in STRIKE.
Grid limits — X min, Y min, X max, Y max Spatial extent of the output grid. Auto-filled from the data extents when an input file is selected.
Smoothing factor (0 = plane, 1 = exact) Controls the covariance model. 0 produces a best-fit plane (maximum smoothing); 1 honours every data point exactly (interpolation). Values between 0 and 1 give intermediate smoothing.
Nearest neighbours Number of neighbouring data points used to estimate each grid node. Larger values give smoother results but take longer to compute. Range: 1–16.
The output grid is stored in memory only. Use Save Grid (toolbar) to write it to disk. Computation time scales with NX × NY × Nearest neighbours. For large grids (e.g. 128 × 128) with many neighbours, expect a wait of several seconds to minutes. Progress is reported in the Output Log during kriging.

remove a trend

Removes a 2-D polynomial trend surface from the active grid by least-squares fitting, leaving only the residual (detrended) values.

When to use: To remove a broad regional background signal (e.g. a continent-scale gravity gradient or a tilted baseline) before inspecting shorter-wavelength features.

OptionDescription
1st order (linear plane) Fits and removes Z = ax + by + c — a tilted plane. Removes a linear gradient in any direction.
2nd order (quadratic surface) Fits and removes Z = ax² + by² + cxy + dx + ey + f — a bowl or saddle shape. Removes gentle regional curvature.
3rd order (cubic surface) Fits and removes a 10-term cubic polynomial. Removes more complex regional warping.
Requires an active in-memory grid. Load a grid via Load Grid or run grid data first. Higher-order removal extracts more of the regional signal but also risks removing real shorter-wavelength features if the grid is small. The residual (data minus fitted surface) becomes the new active grid.

add a grid

Adds every value in a grid file element-wise to the corresponding value in the active grid. The result replaces the active grid.

When to use: To combine two grids — for example, to restore a previously removed trend, or to merge two gridded datasets covering the same area.

OptionDescription
Grid to add (from file) The grid file to read and add to the active grid. The file grid must have exactly the same dimensions (NX × NY) as the active grid.
Both grids must have identical shapes. The operation fails with an error message if the dimensions differ. Grid spatial coordinates are taken from the active grid; the file grid's coordinates are not checked.

subtract a grid

Subtracts every value in a grid file element-wise from the corresponding value in the active grid. The result replaces the active grid.

When to use: To remove a background field, to compute the difference between two surveys, or to subtract a modelled surface from observed data.

OptionDescription
Grid to subtract (from file) The grid file to read and subtract from the active grid. Must have the same dimensions as the active grid.
Both grids must have identical shapes. The operation fails with an error message if the dimensions differ.

multiply data

Multiplies every value in the active grid by a scalar factor. The result replaces the active grid.

When to use: To change units (e.g. convert mGal to µm/s²), to negate a grid, or to scale data before combining grids.

OptionDescription
Scaling number The scalar to multiply every grid value by. Use −1 to negate the grid, 0 to zero it, or any other value to scale it.

reduce 6 to 3

Extracts one value column from a 6-column geophysical data file and writes a compact 3-column (X Y value) output file. The input format is the standard 6-column layout used by Bouguer.py: X, Y, Elevation, Station Number, Free-air anomaly, Bouguer anomaly.

When to use: When a downstream program (e.g. for kriging or fractal analysis) expects a plain 3-column XYZ file, or when you want to isolate a single geophysical field from a multi-column dataset.

OptionDescription
Input 6 column file Source ASCII file with at least 6 whitespace-separated columns per data line. Lines with fewer than 6 columns are silently skipped. The radio buttons below are disabled until a valid file is selected.
Output 3 column file Destination file that will receive the extracted 3-column data (X, Y, selected value), one point per line.
Elevation Use column 3 — the terrain elevation at each measurement point.
Free Air Use column 5 — the free-air gravity anomaly.
Bouguer Value Use column 6 — the Bouguer gravity anomaly.
The Elevation / Free Air / Bouguer Value radio buttons are initially disabled. They are enabled automatically once an input file with at least 6 columns has been selected. The output file contains three columns per line: X, Y, and the selected value, separated by spaces.