Gridding Method
On the Gridding Method tab of the Quick Contours form, select a gridding method and specify the maximum number of points and a search radius.
Three contouring algorithms are provided:
- The Inverse Distance method
- The Inverse Distance + Minimum Curvature method
- Minimum Curvature method
In most circumstances, applying one of these should give a good representation of the surface that best fits the data.
Max number of points (Inverse distance only)
Enter the maximum number of points that will be used in the contouring function when it calculates a grid point estimate.
The default value is eight. This means that the algorithm will only use the eight points nearest to the centre of each grid cell when estimating values. If there are less than eight points in search area then all points will be used.
Increasing the number of points will smooth the contoured surface particularly if the raw data points are densely spaced. It will also slow down the contouring process. Reducing the maximum number of points can result in poorly estimated grid points, particularly if values in the search differ markedly. There is seldom any advantage in using more than 25 points to estimate a cell value.
Search radius
This defines the radius of the circular search area around each grid cell. The grid points are estimated using the values that fall within this area.
If the search area is too small, there may be grid points with no value. If the search area is too large, the contour grid will contain cells with estimates based on one point at the limit of the search. This can result in contours being extended into areas that should not have contour values.
As a general guide set the search radius about one and a half times the cell size. If for example, the cell size was 30m in width and 40m in height a good search radius would be 50 meters.
Tension
The Minimum Curvature method attempts to fit curves with the least acute bends between points and produces smooth contours. You can control how closely the curve fits the original data using the Tension slider. The higher the tension value, the closer the shape of the smoothed string will be to the original data.
