Six of the twelve gridding methods in Surfer support anisotropy. Anisotropy allows you to specify the ratio and angle for the preferred orientation in the data. This isn’t the orientation of the data sampling, but of a direction of flow in the data itself.
Usually, points closer to the grid node are given more weight than points farther from the grid node. If the points in one direction have more similarity than points in another direction, it is advantageous to give points in a specific direction more weight in determining the value of a grid node. If this is the case, then you should use anisotropy. The relative weighting is defined by the anisotropy ratio. By default, the anisotropy ratio is 1.0 (isotropic or no anisotropy). The Minimum Curvature gridding method allows you to set the anisotropy ratio, but it must be either in the horizontal or vertical directions, so there is not an Angle option. Note that anisotropy works in both directions from the grid node; there is not a way to specify anisotropy in only one direction.
Anisotropy is commonly used in two situations:
- To introduce or emphasize a bias or trend direction when calculating the grid. For example, if the local trend direction for carbonate mounds is NW-SE, then you can apply a 135° anisotropy angle, specify a ratio, and then points further away in the direction of anisotropy will have the same weight as closer data points perpendicular to the anisotropy direction.
- To equalize the effects when the X range differs greatly from the Y range in the data. If there is an extremely large difference between the X and Y data ranges, the default map created from this data at a 1:1 scale could be very small in one direction. This is usually the case with profile data (e.g. drill hole data), seismic data, or any data taken along lines where the data points are close together along the lines, but the lines themselves are farther apart. You can rescale the map to increase the size of the smaller side, but then the contours would appear “stretched”. You may want to grid the data using anisotropy to compensate for the difference in ranges. The anisotropy ratio may be set to the ratio of Y/X or X/Y, depending on the data and the anisotropy direction that you specify.
Updated November 7, 2018