Pitney Bowes MapInfo Vertical Mapper - Manual (Page 45)

Chapter 3: Creating Grids Using Interpolation
User Guide
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purposes and the processing time and file size. The default value is calculated by dividing the
diagonal width of the point file by 500, considered an optimum number based on computing power
required to solve this algorithm.
The Triangle Size Limit box enables you to specify a Delaunay triangulation setting to limit the
creation of natural neighbour regions around adjacent points you consider too far apart. Depending
upon the geographic distribution of points in a file, this setting will restrict the generation of
unnecessarily large and/or extremely long, narrow regions. This will limit interpolation between
points across distances that may be inappropriate. For example, a set of field observations may
contain large gaps in the coverage due to sampling problems. If the gaps are quite large, it would be
inappropriate in most cases to interpret new values lying within these areas. Generally, you are
expected to have an understanding of the distribution characteristics of your data in order to enter an
appropriate value.
The Hull Boundary Width box enables you to define a distance value in map units that is added to
the convex hull of the data to define the outermost margin of the interpolated grid.
The Corner Increment box refers to the construction of the four corner segments of the hull of the
data. The value entered represents the incremental arc angle of the segments that make up the
imaginary polygon boundary at each corner. The smaller the value, the greater the number of
segments that will be added and therefore the smoother the corner. The setting is measured in
degrees and is limited to values between one and 30.
The File name box enables you to enter a new file name.
The Extents button displays a summary of the geographic size and the z-value range of the original
point database, the density of the points, and the data value units.
Rectangular Interpolation
Rectangular interpolation is usually applied to data that is regularly and closely spaced, such as
points generated from another gridding application. This technique creates an interpolation surface
that passes through all points without overshooting the maximum values or undershooting the
minimum values.
Rectangular interpolation locates the four nearest data points lying within a circular search zone,
one from each quadrant, and connects them with a double linear rectangular framework (see next
figure). An appropriate value is calculated for each node using the slopes of the connecting sides of
the rectangle. However, in the absence of additional smoothing, linear artifacts are often generated
across the surface when working with an irregular data point distribution.