"Patching" Onto A Large Merged Grid

As indicated earlier (Using GridMerge With Large Grids), 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. This raises the issue of how to level and merge grids where one of the grids is large.

For example, after levelling/merging all the magnetic survey grids in Australia into a single merge, the 80 m cell size merged grid is around 16 Gb in size. As new surveys become available, users may wish to add them to the current merge without starting the levelling/merge from scratch. That is, users may wish to just "patch" a new survey (NEW_GRID) on top of the existing merge (MERGE_GRID) – see Figure below. 

GridMerge facilitates this "patching" using a 3-stage strategy as follows: (a) get a small subsection of the merge that extends beyond the area covered by the new grid; (b) level and merge the new grid to this sub-section; and then (c) merge the updated sub-section back into the original merge. This strategy exploits the fact that there is no limit to the size of grids that can be merged using Merge Grids (or on the lowest-priority grid in a Feather And Merge Grids). In detail, the steps to follow are as follows:

• Use the Sub-Section Grid tool to sub-section an area of the MERGE_GRID that surrounds the NEW_GRID – let us call this grid the MERGE_SUBSECTION. The MERGE_SUBSECTION must extend beyond the NEW_GRID by distances greater than the feathering distance. So, if you are using a feathering distance of 5 km with Feather And Merge Grids, then ensure that the MERGE_SUBSECTION extends beyond the NEW_GRID by say 6 km, or more.
• Place the MERGE_SUBSECTION and the NEW_GRID into a new directory and level NEW_GRID to MERGE_SUBSECTION using the processing sequence starting with "Trim Grid Overlaps" and finishing with "Feather And Merge Grids" – see Processing Sequence – Magnetics or Processing Sequence – Radiometrics. However, don't condition the grids (MERGE_SUBSECTION must remain at the same level as MERGE_GRID). Also, specify MERGE_SUBSECTION as the base grid for the levelling, and as the lowest-priority grid for trimming overlaps and the feather/merge. NEW_GRID is thus merged on top of MERGE_SUBSECTION. Let us call the output from "Feather And Merge Grids" the UPDATED_SUBSECTION.
• Now, put the UPDATED_SUBSECTION into a directory with MERGE_GRID and merge the two after first specifying the MERGE_GRID as the lowest priority grid for the merge. That is, the UPDATED_SUBSECTION is to be merged on top of MERGE_GRID. There is no need for feathering, as UPDATED_SUBSECTION has not been modified beyond the feathering distance from NEW_GRID, and will merge seamlessly with MERGE_GRID.