Using GridMerge With Large Grids

GridMerge is designed to produce large grid merges. The Magnetic Map of Australia, for example, is around 15 Gb in size. GridMerge uses its own memory management procedures to produce, process and display these large grids. However, not all GridMerge processes use memory management, and there are practical limits on the size of the individual grids that can be levelled.

The processes under the Grid Pre-Conditioning and Grid Levelling menus work with up to two grids in memory at a time without memory management. So the size of individual grids is limited by the amount of computer memory available. 

Grids of any size can be viewed in the Main Window display panels and the View Grid utility. GridMerge will automatically sub-sample the displayed grids to fit available memory. The maximum number of grid samples for display, before sub-sampling occurs, is set in the GridMerge configuration file (install_directory/config/gridmerge.cfg) using the MAX_PLOT_SAMPLES parameter. The smaller this number, the greater the sub-sampling, and the faster a grid will be displayed. The default is 1.0 x 108 samples, which equates to a grid of about 25 Mb. If you need to see greater detail in a displayed grid, then increase this value. But obviously this will increase the time taken to display the grid.

All of the tools under the Utilities menu, other than Create Boundary View, either use memory management or don't require it, and can handle large grids of any size. Likewise, Merge Grids can merge grids of any size into larger compilations. These processes invoke memory management once a grid exceeds a certain size. This size is set using the  MAX_GRIDSIZE_IN_MEMORY parameter in the  GridMerge configuration file. The default is 200 Mb. Above this limit, input/output will be a little slower.  

The Feather And Merge Grids process works slightly differently. For Feather And Merge Grids (as for Merge Grids), the first grid to be interpolated onto the merged grid is the lowest-priority grid. Subsequent grids in the feather/merge (or merge) appear on top of the lower-priority grids. For Feather Merge Grids, the first grid (i.e. the lowest priority grid) can be of any size. Subsequent grids in the reverse-priority order must be able to fit into memory. This is important where users want to "patch" one, or more, new surveys on top of an existing large merge, rather than re-level the entire grid collection. How to do this is explained in "Patching" Onto A Large Merged Grid.