This technique is sometimes used when the surface is tilted, projection up and down dip is desired, and algorithms that project slope do not produce acceptable results ([[:file:using-and-improving-surface-models-built-by-computer_fig3.png|Figure 3]]). The general steps are (1) build a first- or second-order trend surface through the data, (2) back interpolate from the trend surface a ''z'' value at each data location, (3) subtract the back interpolated values from the original data creating difference values, (4) build a surface model of the difference, and (5) add the difference surface to the trend surface. | This technique is sometimes used when the surface is tilted, projection up and down dip is desired, and algorithms that project slope do not produce acceptable results ([[:file:using-and-improving-surface-models-built-by-computer_fig3.png|Figure 3]]). The general steps are (1) build a first- or second-order trend surface through the data, (2) back interpolate from the trend surface a ''z'' value at each data location, (3) subtract the back interpolated values from the original data creating difference values, (4) build a surface model of the difference, and (5) add the difference surface to the trend surface. |