Tri Parameters
Use Tri Parameters to control the placement of grid generation control points and the fill algorithm used on masked areas.
This option can also be accessed by selecting the Tri Parameters button 
from the Grid Calc Advanced toolbar.
Instructions
On the Grid Calc menu, point to Data, and then click Tri Parameters to display the Tri Parameters panel.
Do not apply grid fill
Select this option to calculate grid nodes outside the data area where the grid is masked using straight triangulation between data points and the outer construction points.
Apply grid fill to masked areas
Select this option to allow the modeller to extrapolate values beyond the data area using an inverse distance method.
When trending is used, both methods will carry the residuals over the entire grid area where they are added to the trend, yielding the desired effect of extending the trend to the edge of the grid. Grid fill with trending may not produce significantly different results than no grid fill with trending. With grids that cover a much larger area than the data, grid filling can be slow and the benefits negligible.
Ledge around dataset for construction points
This defines the distance outside the grid area for the location of the construction points used in triangulation. The grid area is defined by four coordinates. This ledge, outside the grid area for the location of construction points, is measured in grid cell increments, for example, the number 1 in the ledge input box represents one times the grid cell size (1 x 25 if the grid cell size was 25 metres).
The ledge influence on a model is dependent upon the proximity of data points to the grid area boundary. If the data points to be modelled are well inside the grid boundary, the construction points usually have little effect on the grid edge. However, if the data points approach the grid area boundary, construction points may have a significant effect on the grid boundary. In this case, it may be necessary to move the construction points further from the grid boundary, using the ledge option.
Click OK.
