Tips And Tricks

You may have noticed that the Grid Levelling consists of several separate steps: Get Overlaps, Get Relative Shift/Scales, Get Absolute Shift/Scale And Apply and Get Grid Tilts And Apply. While these could be combined into a single process, their separation into individual processes has two big advantages: (a) greater flexibility in tuning the processes, and (b) saving time by not repeating time-consuming tasks unnecessarily. Using GridMerge effectively requires an iterative approach – a sequence of processes are run several times using different parameters in order to achieve the best result. So Get Overlaps, for example, is time consuming, and you don't want to repeat this several times while searching for the optimum parameters for the Absolute Shift/Scale And Apply process. 

Separating  Get Relative Shift/Scales and Get Absolute Shift/Scale And Apply also has a big advantage. The user can use a text editor to adjust the weight associated with any particular grid overlap, and so influence the final levelling.

The aim of Grid Levelling is to obtain a Merge where differences at the grid boundaries are minimised, and are least visible. Only at this stage should the grids be Feathered And Merged.

Here are some random tips to achieve the best result:

    • Trim Grid Edges – Trimming the edges to get rid of edge effects is a good idea. But often grids don't have much overlap. You may have to end up trimming some grids and not others (or even trimming just one edge of a grid using the sub-section tool).
    • Trim Grids to Coastline – This can be very slow depending on how many points are in the coastline polygons. The coastOffset parameter can be used to try speed things up. If some grids are not being properly trimmed, you may have to adjust this parameter. 
    • Trim Grid Overlaps – This is a crucial step in the processing sequence. Too much overlap can be just as bad as too little. The size of the overlap area can affect both the levelling and feathering. The best levelling results are obtained when only the differences along the grid edges are used to do the levelling. However, you do need a reasonable overlap for feathering. So trim the overlaps, but keep a reasonable overlap (if available) to assist feathering – say 3,000m for 100m cell size.
    • After trimming the grid overlaps, some grids may have odd shapes and unwanted outliers that are not required for filling gaps in the coverage, but can disrupt the levelling. Use the Create Jpegs From Grids utility to create jpegs for rapid identification of possible problem grids. Then use the Sub-Section a Grid utility to trim off any unwanted grid sections.
    • Condition Grids – This is not necessary for the merging of a small number of grids. For large merges, it is mandatory to minimise rounding errors.  Be aware that you may have to shift/scale the final merge if you used a base grid that has been centred and scaled.
    • Get Overlaps – If you have control lines, then use them – they give much better control than base grids.
    • Get Absolute Shift/Scale and Apply – If you are having trouble getting the radiometric grids level using both a shift and scale adjustment, then try different values for the scaleWeight parameter. GridMerge is trying to solve an inverse problem for both scales (small numbers) and shifts (large numbers) – so it is necessary to give a greater weight to the scales. 
    • Get Absolute Shift/Scale and Apply – Check the output to see what adjustment has been made to the control lines/base grid. These should be close to 0 for shift, and close to 1 for scale. If significantly different, raise the controlLineWeight/baseGridWeight. This applies to tilting grids as well – tilts for control lines and base grids should be close to 0 (for all 3 tilt plane parameters).
    • Get Absolute Shift/Scale and Apply – Be aware that you can manually edit the Overlap Grids File to increase or decrease the weight for a particular overlap. This can be used to force any inconsistency in the data into areas of the final merge where it is less visible.
    • Get Grid Tilts And Apply – Beware grids that are not well constrained. Study the output from this process carefully. Grids with excessive tilts may have to be declared "shiftOnly".  It is better to use Control Lines when tilting grids, rather than a base grid – they provide a better constraint on the inversion.
    • Merge Grids is fast; Feather And Merge Grids is slow. While searching for the optimal parameters, run Merge Grids using a coarse cell size to speed up the merge. Once you are happy with the result, run Feather And Merge Grids using your desired cell size.
    • Don't rely on feathering to cover up poor levelling! The bigger the final differences are at grid edges, the further they have to be feathered into the surrounding grids. And large feathering distances can introduce unexpected/undesirable artefacts into the feathered grid. Feathering is very slow for large feathering distances, so try to keep the feathering distance small in order to keep processing times reasonable.