Tips And Tricks

Here are some tips to achieve the best result:

• To avoid removing signal, the along-line low-pass filter cutoff wavelength must be as long as possible, and the across-line high-pass filter cutoff wavelength must be as short as possible. So start with minimumStreakLength = 20,000 (m), and maximumStreakWidth = 3 x line spacing (m). These parameters are always input as metres, even if the grids are geodetic. Also, set the initial minimum and maximum corrections to large negative and positive numbers, respectively.
• Progressively lower the minimumStreakLength value until the lineations start to appear, then back off a bit. Then increase the maximumStreakWidth, if required.
• Once you have the optimum filter lengths, slowly increase the minimumCorrection, and decrease the maximumCorrection, until the levelling artefacts start to appear, then back off a bit.
• Set the medianFilterLength to about half the minimumStreakLength. However, you may need to increase this value to capture short-wavelength, high-amplitude anomalies introducing "leakage" artefacts. Remember, the median filter length needs to be slightly larger than twice the width of the short-wavelength, high amplitude anomalies we are trying to remove with this filter.However, a very long median filter can itself introduce undesirable effects into the filtered grid.
• Sometimes it is not possible to prevent "leakage" artefacts from being introduced. In this case, use polygons to exclude areas from being decorrugated. The polygons are also useful for excluding areas where (real) linear anomalies run parallel, or sub-parallel to the flight lines, and are at risk of being removed. See Tutorial 1: Decorrugating a grid for an example.
• It is perfectly acceptable to put a grid through more than one decorrgation run. For example, if just one area of the grid requires a larger maximumStreakWidth value, then use polygons to only apply the decorrugation in that area. Then decorrugate the grid again using more appropriate filter lengths for the rest of the grid. This strategy will minimise any loss of signal.
• Signal can be easily removed by decorrugation. ALWAYS view the correction grids. These should show long-wavelength linear anomalies in the flight line direction only. Evidence of across line features in the correction grid point to a loss of signal.
• Activate Plug Nulls During Processing if your survey has irregular boundaries and the decorrugation is introducing unwanted edge effects. The nulls will automatically be re-inserted into the output grids. If you have a large grid with irregular boundaries, the plugging of nulls for each iteration will slow down the process considerably. In this case, you may wish to consider plugging the nulls in your grid prior to decorrugation using the Plug Nulls utility. Once the grid is adequately decorrugated, you must use the Restore Nulls utility to re-insert the nulls into the output grids. Alternatively, ignore the edge effects until you have found the optimum decorrugation parameters, then turn Plug Nulls During Processing on (CheckBox in Set Aliases And Parameters) for the last iteration.