Surfer uses the X, Y and Z values proportionally when calculating the slope values. Therefore, the X, Y and Z values must be in the same units, and the units must be linear (e.g. feet or meters) for the slope calculation to be correct.  

If the X and Y units in the grid file are in lat/lon units, then the slope calculated for each node will be almost vertical (90°) because the X and Y extents (the lat/lon ranges) are so small compared to the difference in Z values. When your XY units are latitude and longitude, convert the coordinate system of your data or grid file to a system with linear units (e.g. UTM with meters) prior to calculating the slope. For instructions on converting the coordinate system of raw data or grid files, please see these related articles:

• How can I convert the coordinate system of a grid file (e.g. form Lat/Long into feet or meters) using Surfer?
• Convert the Coordinate System of Data, Image, Vector, and Grid Files in Surfer

Updated September 4, 2020