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FAQ: In ArcGIS Spatial Analyst 8.x and ArcInfo Workstation GRID, why do NODATA patches appear near barrier lines with the IDW interpolator?

Question

In ArcGIS Spatial Analyst 8.x and ArcInfo Workstation GRID, why do NODATA patches appear near barrier lines with the IDW interpolator?

Answer

The NODATA cells are a result of the IDW algorithm and the method used to select points for interpolation.

Each input point uses a radius of influence to select the points for interpolation. This radius is the distance in map units that is used to limit a search for the nearest input sample points. If the number of input points required cannot be satisfied within the specified radius, a smaller number of points is used. In the case where no radius is specified, the entire dataset is used.

Taking a selected input point as center, all possible points within the specified radius are considered. If a line segment is between any such point in the radius of influence and the center point intersects a barrier (arc), the point will be removed from the selected point list and the resulting cell is assigned NODATA.

To visualize this, generate a test output using a very small radius. The output should contain NODATA areas wherever the criteria stated previously is satisfied.