Changes

====Regional trend assist====

====Regional trend assist====

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.

[[file:using-and-improving-surface-models-built-by-computer_fig4.png|300px|thumb|{{figure number|4}}Cross sections through the same data. (a) The surface model does not honor the data. (b) Surface is shifted to data by modeling the error between the data and the original surface and then adding the original and error models.]]

[[file:using-and-improving-surface-models-built-by-computer_fig4.png|300px|thumb|{{figure number|4}}Cross sections through the same data. (a) The surface model does not honor the data. (b) Surface is shifted to data by modeling the error between the data and the original surface and then adding the original and error models.]]