Pitney Bowes MapInfo Vertical Mapper - Manual (Page 36)

Inverse Distance Weighting Interpolation
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Vertical Mapper 3.7
Exploring the Inverse Distance Weighted Interpolation Dialog Box
The Inverse Distance Weighted Interpolation dialog box enables you to set a number of user-defined
parameters in the algorithm.
The Cell Size box enables you to set the cell size in real units. The grid dimensions (in cell units)
vary inversely with cell size: the smaller the cell, the larger the grid file. The value you choose should
be a compromise between the degree of resolution required for analysis and visualization purposes
and the processing time and file size. The default value is calculated by dividing the diagonal extent
of the point file by 300, which is considered an optimum number based on the computing power
required to solve this slightly more complex algorithm.
The Search Radius box enables you to define the maximum size, in map units, of a circular zone
centred on each grid node within which point values from the original data are averaged and
weighted according to their distance from the node. You can also define the minimum and maximum
number of data points averaged within each zone. The default setting is calculated as four times
average point density.
The Display Radius box enables you to define the size, in map units, of a circular zone centred on
each grid node within which an original data point must lie in order for that grid cell to be assigned a
value and be displayed. This setting is used to control the extent of grid creation in areas of the map
that do not contain data, such as the outer margins of the map area. The default setting that appears
is the same as the search radius, which is appropriate for most data. However, if the original data
points are characterized by very patchy distribution with occasional large gaps containing no sample
sites, a smaller display radius should be considered in order to avoid interpolation of grid values
within areas that do not contain data.
The Exponent box enables you to define the exponential rate of decay of influence by neighbouring
points that lie farther from the grid node. Increasing the exponent will decrease the relative influence
of more distant neighbours. Values range between one and 10 with a value of two being appropriate
for most data.