# Changes between Version 1 and Version 2 of Processing/AplUserGuide

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Feb 7, 2012 11:48:58 AM (8 years ago)
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• ## Processing/AplUserGuide

 v1 == Gridding and Mapping the Data == Now that you have an IGM file in the required projection it is time to map the level-1 data to a regular grid. To do this you can use a GIS, some alternative mapping software or the ARSF aplmap package. This will take in the IGM file and level-1 data file and output a map of the level-1 data in the IGM projection. === Using aplmap === The aplmap software is capable of mapping a number of bands of the level-1 data to the projection system of the given IGM file, using the selected interpolation method. === Information on interpolation methods === This section briefly describes the algorithms used for the interpolation of the data within the aplmap software. '''Nearest Neighbour''' Mapping with nearest neighbour searches the neighbourhood around the grid cell and finds the closest point to this from the IGM file. The corresponding value from the level-1 file is then used for this pixel. This option is selected using -interpolation nearest on the command line. '''Inverse Distance Weighted''' The inverse distance weighted algorithm first searches for, at most, the X nearest points within the maximum allowed search distance. The number X is specified from the command line in the -interpolation option. Then when these points have been found it calculates a weight for each point and applies this to the value of the data from the level-1 file. The weight for point i is  calculated as (distance^−2) / (Sum distance^−2) . This option is selected using -interpolation idw maxpoints where maxpoints is the maximum number of points you want to use. '''Bilinear''' For the bilinear interpolation algorithm, 4 neighbouring points are required. These are identified by using the level-3 position as an initial search position and finding the 4 nearest points that form a quadrilateral containing this level-3 position. The level-3 point that is being interpolated is found in terms of a proportional distance, U and V, between these 4 quadrilateral vertex points. The values of U and V are calculated using the following formulae (in level-3 space): P = A + U ∗ (B − A) Q = D + U ∗ (C − D) X = P + V ∗ (Q − P ) The values of U and V, which are within the range 0-1, are then used to weight the level-1 pixel values in the interpolation formula: f(X) = f(A) ∗ (1 − V ) ∗ (1 − U ) + f(B) ∗ (1 − V ) ∗ U + f(D) ∗ (1 − U ) ∗ V + f(C) ∗ U ∗ V where f(x) is the data value of cell x. Bilinear interpolation can be selected using -interpolation bilinear on the command line.