Many problems with exporting grids can stem from incorrect grid geometry, specifically grid rotation, which is particularly important for users who want to export their grids to Landmark or Zmap format. Grids can be rotated deliberately during the gridding process, or as a part of a CRS transformation.
Resampling is the best method for correcting proprietary grid rotation. First, open Mapping and display the rotated grid on a Mapsheet. It is important to create a new Mapsheet that is close to the grid boundaries; under Mapsheet/New/Pick on Map and use the left mouse button (LMB) to click and drag a square that closely hugs your data. Open Surface Modeling and click …/Grid/Processes/Resample and under the Output Geometry tab use the Mapsheet option to select the file you just made and set the rotation angle to zero; run the task. A good way to QC your result is to display your grid values in Mapping – you should be able to see a difference in the rotation angle of the grid values (fig. 1).
Fig. 1: The difference in grid values between an unrotated grid (left) versus a rotated grid (right).
Rotation can occur during transformation to a different Coordinate Reference System (CRS). Correcting this type of rotation is quite simple, but involves a few more steps as it’s not known the approximate angle at which the grid was rotated. Open Surface Modeling/Grid/Create Grid. Use your transformed grid as the input and name the output file. In the Output Geometry tab, choose Grid and choose the original grid before the transform (fig. 2). In doing this, you are applying the old grid geometry to your new grid in the required CRS. Using the old grid geometry, you are un-rotating the grid while maintaining the CRS information that you need for your project.
Fig. 2: The proper way to apply Output geometry from a grid with a different CRS than the output.
These are the two most common problems when grid rotation becomes an issue in your project. The option to resample is useful when you know the rotation angle and can correct it using the Resample module. Fixing the grid geometry in a CRS problem requires a few more steps, but it is a quick way to fix rotation problems after a transform. If you have more questions about grid rotation, or any other Petrosys functions, contact our support line at support@petrosys.com.au.