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@@ -29,19 +29,39 @@ Note that vector features without category in given <b>layer</b> are
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<i>skipped</i>.
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<p>If the user has a mask set, then interpolation is only done
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-for those cells that fall within the mask. However, all
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-vector points in the current region are used even
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-if they fall outside the mask. Vector points outside the current
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-region are not used in the interpolation. A larger region may
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-be set and a mask used to limit interpolation to a smaller area
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-if it is desired to use vector points from outside the region in the
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-interpolation. The <b>-n</b> flag may also be used to
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-achieve a similar result.
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-
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-<p>If more than <b>npoints</b> fall into one target raster cell,
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-the mean of all the site values will determine the cell value (unless
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-the <b>-n</b> flag is specified, in which case only the <b>npoints</b>
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-closest to the centre of the cell will be interpolated).
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+for those cells that fall within the mask. The module has two separate
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+modes of operation for selecting the vector points that are used in the
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+interpolation:<dl>
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+<dt>Simple, non-indexed mode (activated by <b>-n</b> flag)</dt>
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+<dd>When the <b>-n</b> flag is specified, all vector points in the
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+input vector map are searched through in order to find the
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+<b>npoints</b> closest points to the centre of each cell in the output
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+raster map. This mode of operation can be slow in the case of a very
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+large number of vector points.</dd>
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+<dt>Default, indexed mode</dt>
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+<dd>By default (i.e. if <b>-n</b> flag is <i>not</i> specified), prior to
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+the interpolation, input vector points are indexed according to which
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+output raster cell they fall into. This means that only cells nearby
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+the one being interpolated need to be searched to find the
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+<b>npoints</b> closest input points, and the module can run many times
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+faster on dense input maps. It should be noted that:
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+<ul>
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+<li>Only vector points that lie within the current region are used in
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+the interpolation. If there are points outside the current region,
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+this may have an effect on the interpolated value of cells near the
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+edges of the region, and this effect will be more pronounced the fewer
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+points there are. Use the <b>-n</b> flag if you wish to also include
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+points outside the region in the interpolation.</li>
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+<li>If more than <b>npoints</b> points fall within a given cell then,
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+rather than interpolating, these points are aggregated by taking the
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+mean. This avoids the situation where some vector points can be
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+discarded and not used in the interpolation, for very dense input
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+maps. Again, use the <b>-n</b> flag if you wish to use only the
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+<b>npoints</b> closest points to the cell centre under all
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+circumstances.</li>
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+</ul>
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+</dd>
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+</dl>
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<p>
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The <b>power</b> parameter defines an exponential distance weight.
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Paul Kelly