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• At its core, the EWA algorithm looks at the cell-to-cell delta in source to target cell mapping, both along and across track. These deltas are used to compute the parameters for a quadratic equation defining an “ellipse of influence” from a source cell to one or more target cells.
• The “ellipse of influence” is used both to compute which target cells are affected by a source cell – a bounding box to the ellipse of influence – and to compute a weighting factor for a weighted averaging of source cells per target cell. The weighting is defined in terms of the distance of the source cell center location to the target cell center (radius of the ellipse).  Those source cells closer to the target cell are weighted more heavily than a simple linear cell-to-cell distance.

• The “ellipse of influence” provides an important technique for calculating the area of influence, in the target grid.  It is an efficient way of finding target cells when forward projecting the source data grid to the output grid. This permits a reasonably efficient “forward navigation” approach, versus more typical reverse projection algorithms.

• For forwards projection, the source data is processed forwards to the target grid by calculating the target projected location and “ellipse of influence” for each source cell.  At the end, each target cell has potentially multiple source cells that map to the target cell.  The algorithm calculates a weighted average of all source cells that map to a given target cell.  
• It calculates - during the processing of source data points - the target points of reference, the weighted-values accumulated as the numerator, and the weights themselves summed as the denominator.  After the source data is processed, the end result is the quotient of numerator to denominator values per target grid cell.
• This technique allows a single pass through the source data grid - to both identify the target cells and compute a partial solution to the output value - the numerator and denominator values.  This is followed by a simple pass through the target cells to divide the numerator (weighted sum of values) by the denominator (sum of weights).

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