This issue is usually a result of the grid resolution and/or gridding methods. Some suggestions:
- The most common cause of a grid not honoring the original data point values is insufficient resolution in the grid file. To resolve the problem, grid the data with a small enough grid line spacing to match the distance between closest data points. The higher the grid resolution, the more accurate it can be with respect to your original data (the less interpolation there is between data points). When you are in the Grid Data dialog, increase the # of Nodes or decrease the grid Spacing.
- Use a different gridding method. The default gridding method, Kriging, creates trends in your data and can extrapolate the Z values in the grid file to extend past the limits of the Z data in your original file. It also creates a best fit grid and does not always exactly honor each data point from the original file. Try another gridding method, such as Natural Neighbor, Radial Basis Function, Triangulation with Linear Interpolation, or Inverse Distance to a Power.
- Add more data points. Sometimes, this problem is because Surfer simply can’t create the contours with the current data points and accurately keep each value. More data points would help. If you do not have more data points, you might be able to add a few “dummy” points to your data file to help constrain the contours to the shape that you want.
Updated February 10, 2017