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