Changes between Version 1 and Version 2 of Processing/AplUserGuide


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Timestamp:
Feb 7, 2012 11:48:58 AM (7 years ago)
Author:
mark1
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  • Processing/AplUserGuide

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     129== Gridding and Mapping the Data ==
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     131Now 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.
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     133=== Using aplmap ===
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     135The 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.
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     137=== Information on interpolation methods ===
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     139This section briefly describes the algorithms used for the interpolation of the data within the aplmap software.
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     141'''Nearest Neighbour'''
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     143Mapping 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.
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     145'''Inverse Distance Weighted'''
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     147The 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.
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     149'''Bilinear'''
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     151For 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):
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     153 P = A + U ∗ (B − A)
     154 Q = D + U ∗ (C − D)
     155 X = P + V ∗ (Q − P )
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     157The 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:
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     159f(X) = f(A) ∗ (1 − V ) ∗ (1 − U ) + f(B) ∗ (1 − V ) ∗ U + f(D) ∗ (1 − U ) ∗ V + f(C) ∗ U ∗ V
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     161where f(x) is the data value of cell x.
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     163Bilinear interpolation can be selected using -interpolation bilinear on the command line.
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