Data Preparation

GridMerge operates on ER Mapper (.ers) grids only. In addition, the following restrictions apply:

    • The grids (and control lines, if used) must all have the same coordinate system. The grid's coordinate systems should be of the same type (geodetic or projected coordinates), and share the same projection and datum. GridMerge does not check the grid coordinate systems. Output grid merges will have the same coordinate system as the input grids. 
    • Rotated grids are not allowed. GridMerge assumes that all grids have Rotation=0.
    • Registration offsets are not allowed. The X and Y registration offsets should be zero (RegistrationCellX=0 and RegistrationCellY=0). 

The GridMerge utility Check Grid Parameters (Utilities|Check Grid Parameters) can be used to assist in ensuring that the above criteria are met. It generates a report summarising the parameters for all grids in the input directory. The onus is then on the user to check that: the projection and datum are the same for all grids; there are no rotated grids; and that all grids have zero X and Y origin offsets.

Grid Artefacts

One of the primary causes of poor levelling/merging of grids is the presence of gridding or micro-levelling artefacts along grid edges:

    • Some gridding algorithms extrapolate cell values near grid edges. These values are not well constrained by the flight line data and can cause problems with both levelling and feathering. 
    • A second source of error is micro-levelling (decorrugation). Directional filtering can struggle to remove elongate artefacts from grids where those artefacts are along the grid edges. Consequently, micro-levelling can occasionally leave the edges of grids that are parallel to the line direction either too high or too low. This can seriously corrupt the grid leveling/merging.

Wherever possible, grid artefacts should be identified and removed prior to grid levelling/merging. The View Grid utility  (Utilities|View Grid) can be used for imaging and viewing grids. The Trim Grid Edges tool (Grid Pre-Conditioning|Trim Grid Edges) can be used to trim (set to null) a fixed number of cells of grid edges. The Sub-Section a Grid utility (Utilities|Sub-Section a Grid) can be used to null selected portions of a grid.

Directory Structure

The very first thing a user is required to do after launching GridMerge is to specify the "Working Directory". This cannot be set via a control file. The grids to be levelled are then assumed to reside in a sub-directory of the Working Directory. The Working Directory will also contain files created by GridMerge such as Program Control Files, Priority Order Files, Boundary View Information Files, and so on. Any directories created by GridMerge will be created in the Working Directory. GridMerge adjusts grids by reading the grids from an input directory, modifying them, and writing them to an output directory. The grid names always stay the same, as indicated in the diagram below.