The user defines a number of points in the current data ROI by clicking with the graphical cursor. The program calculates an average intensity in the 3*3 pixel region centred on each user defined coordinate. Triangulation and bi-cubic spline interpolation is used to define a surface throughout the ROI based on the user defined points. The output is in the memory.
For best results the input coordinates should be spaced throughout the ROI and more coordinates need to be defined where the surface should change quickly.
(An alternative and generally superior method of estimating background surfaces is available as part of the FIT sub-menu.)