Priority Order

Grid Priority Order is a fundamental concept in grid merging. It is an ordering of the grids by grid "quality". A GridMerge utility can be used to generate a default Priority Order (Utilities|Generate Priority Order), with the result stored in a text file. The utility sorts the grids by cell size and survey size. Grids with small cell sizes are given a higher priority. Where grids have the same cell size, the larger grids (numRows x numCols) are given a higher priority. 

Users can edit the default control file or create their own. However, care must be taken to maintain the one-to-one relationship between grids in the input directory and file names in the Priority Order File. The priority order is used by Trim Grid Overlaps (Grid Pre-Conditioning|Trim Grid Overlaps), Grid Merge (Grid Merge|Merge Grids) and FeatherMerge (Grid Merge|Feather And Merge Grids). When grid overlaps are trimmed, the grids are trimmed in reverse priority order – low priority grids are trimmed in preference to higher priority grids. When grids are merged, higher priority grids will be patched/feathered on top of lower-priority grids.

Note that some processes can change the number of grids being levelled/merged. If after trimming by Trim Grids To Coastline or Trim Grid Overlaps, there are no non-null values left in a grid, then the grid will not be written to the output directory. Similarly, if a grid is not connected to the control lines or other grids, then Get Absolute Shift/Scale And Apply will not write the grid to the output directory. In these instances the user will need to create a new Priority Order File to maintain the one-to-one correspondence between grids being processed and grid names in the Priority Order File.

The format of the Priority Order File is simply a header line (consisting of the text string "GRID_PRIORITY_ORDER_FILE") followed by the grid names listed in priority order (highest priority first). For 4 grids, the file contents might look something like this:

GRID_PRIORITY_ORDER_FILE

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