Some specialized Natgrid control parameters

This module discusses the control parameters for:

: Negative values
: Vertical orientation
: Duplicate input coordinates

Negative values

The control parameter non determines whether negative values are allowed as interpolated values. The default is to allow them. If disallowed, by setting the value of the control parameter non to 1, then any value that would otherwise be negative is set to zero. The plots in Example 5 illustrate this. Compare the plot that allows negative values with the plot that does not allow them.

This feature is useful if you have a function, such a pressure, that should never have a negative value.

Vertical orientation

The control parameter upd determines the ordering of the second dimension of the output array. The default, "upd = 1", implies a south to north ordering; setting upd to 0 reverses the order.

Duplicate input coordinates

The control parameter dup determines whether duplicate input coordinates are allowed. By default they are not allowed, the assumption being that such points might represent a flaw in the input data. If duplicate input coordinates are allowed and different data values are assigned for these points, these values will be interpreted as different data values that are very closely spaced, since Natgrid adds a small random perturbation to each input coordinate so that all input coordinates are unique. See Dave Watson's book, nngridr, for details on this.