Surface calculations in geographic coordinate systems may not create valid output results when the horizontal ground units and vertical elevation units use different measures. This typically occurs when the horizontal (x,y) units are geographic and the vertical (z) units are a linear unit (for example, feet or meters).
The tool was unable to determine an appropriate value for the z-factor because the z-coordinate unit is not defined. By default, a value of 1.0 will be used for the z-factor.
Since the length of a degree of longitude changes with latitude, you will need to specify an appropriate z-factor for that latitude.
If the provided default value of 1.0 is not acceptable, specify an appropriate z-factor for the tool parameter.
If the output spatial reference contains the correct vertical coordinate system, an appropriate z-factor can be calculated.
Alternatively, use a projected coordinate system where the x,y and z-units are the same.
You can use the Define Projection tool or Project Raster tool to apply the proper vertical coordinate system to the input data.