Continuation filter (GridFFT)

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The continuation filter can calculate an upward or a downward continuation. The filter includes optional damping for controlling the stability of downward continuation. See [INTREPID spectral domain operations reference (R14):Continuation filters (reference)] for further details.

Continuation filter--parameters

See Filter parameter tables for an explanation of parameters tables.

¹ In task files: Positive - upward continuation, Negative - downward continuation Interactive: See [Continuation filter--interactive]

² Downward continuation only If you do not specify Median frequency of rolloff, INTREPID calculates a suitable value. This is based on the rate at which the continuation is increasing high frequencies. See also [Suggested values for Degree of Rolloff]

Interactive

Continuation filter--interactive

Specify the filter as shown in Specifying a filter (interactive). The properties area is illustrated here.

If your input grid is geodetic, GridFFT displays both a metres and degrees text box for you to specify the Level of continuation. Whatever you enter in one of the text boxes, INTREPID automatically converts and displays the equivalent in the other text box, using Robin's formula. This conversion between metres and degrees is an approximation. INTREPID bases it on the mid-latitude of the grid.

Task files

Continuation filter--task files

Note: The sign of the Level of continuation parameter (Distance =) determines the direction. If it is positive, the continuation is upward; if negative, the continuation is downward.

Here is an example Begin - End block:

Continuation2D Begin
  Distance = -50.0 # +ve upward, -ve downward
  UseDamping = yes
  DampingDegree = 10
  DampingFrequency = 10.0
Continuation2D End
___

Units for Level of Continuation

Specify the Level of continuation using the same distance units as the spatial version of the grid. For example, if the grid is in metres, then specify Level of continuation in metres. For geodetic grids, specify it in degrees.

(Interactive only) Remember that you can set GridFFT to display the correct units for the grid. See Frequency, wavelength and distance unit multiples for details. This may help avoid confusion.

Level of Continuation--examples

Enter the relative distance for continuation (the distance by which you want to change the effective height).

For example, you may have a survey flown at 90 m mean terrain clearance and recorded in metres. If you want to change its effective height to 60 m, specify a downward continuation of 30.

Specify the continuation in the same units as the grid. If you fail to do this, you will get incorrect results. The following examples describe grids with the same cell size, but measured in different units. If we specify a downward continuation of 30:

• For a projected grid in metres with 100 m cell size, the continuation distance will be 30 m.
• For a projected grid in feet with 300 ft cell size, the continuation distance would be 30 ft or 9.4 m.
• For a geodetic grid with 0.001° cell size, the continuation distance would be 30° or 3000 km!

Suggested values for Degree of Rolloff