Changes between Version 1 and Version 2 of Processing/AplUserGuide

Timestamp:

Feb 7, 2012, 11:48:58 AM (14 years ago)

Author:

mark1

Comment:

--

Processing/AplUserGuide

@@ -127 +127 @@
 
 
+== 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.
 
 
 
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