resample_mean

 PURPOSE:  resamples grid using mean of all source cells with target cell
           of defined size at location x-y.
 -------------------------------------------------------------------
 USAGE: value = resample_mean(scs, tcs, tx, ty, xll, yll, grid)
 where: 
        [scs] is the source cell size of the input grid in map units
        [tcs] is the target cell size in map units, that is the size of
              the block to calculate mean for
        [tx],[ty] are the the target cell x and y coordinates (center of  
                  the resampled cell)
        [xll,yll] are the coordinates of the lower left corner (center of
                  cell) of the source grid
        [grid] is the source with the cells to be resampled
 -------------------------------------------------------------------------
 OUTPUTS:
        [value] is a scalar holding the mean of all cells that fall
        into the range of the target cellsize (counting center of cells) 
        
 -------------------------------------------------------------------
 Notes: if the distance of any corner of the resampled cell is more than 0.5 the
 cell size over the source grid edges, NaN is returned. 
 This means the original size of the larger grid is kept, filled with NaNs
 which can be cut later using crop_nan()

 SEE ALSO: resample_max, resample_min, neighbourhood_statistics, block_statistics

 Felix Hebeler, Geography Dept., University Zurich, March 2006.

0001 function value = resample_mean(scs, tcs, tx, ty, xll, yll, grid)
0002 % PURPOSE:  resamples grid using mean of all source cells with target cell
0003 %           of defined size at location x-y.
0004 % -------------------------------------------------------------------
0005 % USAGE: value = resample_mean(scs, tcs, tx, ty, xll, yll, grid)
0006 % where:
0007 %        [scs] is the source cell size of the input grid in map units
0008 %        [tcs] is the target cell size in map units, that is the size of
0009 %              the block to calculate mean for
0010 %        [tx],[ty] are the the target cell x and y coordinates (center of
0011 %                  the resampled cell)
0012 %        [xll,yll] are the coordinates of the lower left corner (center of
0013 %                  cell) of the source grid
0014 %        [grid] is the source with the cells to be resampled
0015 % -------------------------------------------------------------------------
0016 % OUTPUTS:
0017 %        [value] is a scalar holding the mean of all cells that fall
0018 %        into the range of the target cellsize (counting center of cells)
0019 %
0020 % -------------------------------------------------------------------
0021 % Notes: if the distance of any corner of the resampled cell is more than 0.5 the
0022 % cell size over the source grid edges, NaN is returned.
0023 % This means the original size of the larger grid is kept, filled with NaNs
0024 % which can be cut later using crop_nan()
0025 %
0026 % SEE ALSO: resample_max, resample_min, neighbourhood_statistics, block_statistics
0027 %
0028 % Felix Hebeler, Geography Dept., University Zurich, March 2006.
0029 
0030 %% get distance of sample cell llc from llc of source grid
0031 xllc = tx-xll;
0032 yllc = ty-yll;
0033 xurc = tx+-xll;
0034 yurc = ty-yll;
0035 
0036 %% convert differences to grid indices
0037 x_start = round(xllc/scs)+1;
0038 x_end = round(xurc/scs)+1;
0039 y_start = size(grid,1)-round(yllc/scs);
0040 y_end = size(grid,1)-round(yurc/scs);
0041 
0042 %% do something about grid edges
0043 % if the distance of any corner of the resampled cell is more than 0.5 the
0044 % cell size over the source grid edges, NaN is returned.
0045 % This means the original size of the larger grid is kept, filled with NaNs
0046 % which can be cut later using crop_nan()
0047 
0048 if x_start < 1
0049     if abs(xllc/scs)<0.5*tcs
0050         value=nan;
0051         disp(' x_start < 1')
0052         return
0053     else
0054         x_start=1;
0055     end
0056 elseif x_end>size(grid,2)
0057     if abs(xurc/scs)>size(grid,2)+1+(0.5*tcs)
0058         value = nan;
0059         disp(' x_end>size(grid,2)')
0060         return
0061     else
0062         x_end=size(grid,2);
0063     end
0064 end
0065 if y_start < 1
0066     if abs(yllc/scs)<0.5*tcs
0067         value=nan;
0068         disp(' y_start < 1')
0069         return
0070     else
0071         y_start=1;
0072     end
0073 elseif y_end>size(grid,1)
0074     if abs(yurc/scs)>size(grid,1)+0.5*tcs
0075         value = nan;
0076         disp(' y_end>size(grid,1)')
0077         return
0078     else
0079         y_end=size(grid,1);
0080     end
0081 end
0082 
0083 %% calc mean and return
0084 value=mean(nanmean(grid(y_start:y_end,x_start:x_end)));
0085