"griddata" results in much smaller values than original data

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jie hu
jie hu 2023-4-18
评论： jie hu 2023-4-20
I am using "griddata" function to interpolate original data on query points. However, the interpolation value on the query points are much smaller than the original data. Is there any way to improve the interpolation?
  2 个评论
KSSV
KSSV 2023-4-18
编辑：KSSV 2023-4-18
I suspect your lon, lat values are not exactly corresponding to elev. While running the code did you see the warning?
Warning: Duplicate data points have been detected and removed - corresponding values have been averaged.
I suspect the high values correspond to diplicate lon, lat and have been removed.
When you check
nx = length(unique(lon)) ;
ny = length(unique(lat)) ;
nx*ny is not equal to 579072.
jie hu
jie hu 2023-4-18
Yes, when I run 'IC_plot.m' matlab reports the duplicate points and removed them. averaged value is taken. My idea is to detect the duplicate points and take the biggest value on each point. Is that realizable?

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采纳的回答

Chunru
Chunru 2023-4-18
编辑：Chunru 2023-4-18
Ran in:
It is due to the duplicate points in data. Griddata is averaging over the duplicate points to have a smaller value than peak. If you remove the duplicate points, then the result will be similar.
websave("manila.zip","https://www.mathworks.com/matlabcentral/answers/uploaded_files/1359608/manila.zip")
ans = '/users/mss.system.Ktq2V0/manila.zip'
unzip("manila.zip")
% plot x y z vector
data=importdata('manila.dat');
lon=data(:,2);
lat=data(:,3);
elev=data(:,4);
% Remove duplicate points
% [~, I, ~] = unique([lon lat], 'first', 'rows');
% lon = lon(I);
% lat = lat(I);
% elev = elev(I);
% Use peak for duplicated points
c = unique([lon lat], 'first', 'rows');
n = size(c,1)
n = 48256
lon1 = zeros(n, 1);
lat1 = zeros(n, 1);
elev1 = zeros(n, 1);
for i=1:size(c, 1)
idx = lon==c(i, 1) & lat==c(i,2);
lon1(i) = c(i, 1); lat1(i) = c(i, 2);
elev1(i) = max(elev(idx));
end
max(elev(:))
ans = 20.8693
plot3(lon,lat,elev,'.');
% xlin = linspace(min(lon)+0.2,max(lon)-0.2, 51);
% ylin = linspace(min(lat)+0.2,max(lat)-0.2, 201);
xr = (min(lon1):0.01:max(lon1));
yr = (min(lat1):0.01:max(lat1));
[X, Y]= meshgrid(xr,yr);
Z = griddata(lon1,lat1,elev1,X,Y,'natural');
max(Z(:))
ans = 20.8693
mesh(X,Y,Z)
% jump_grid = 1;
%
% figure
% surf(X(1:jump_grid:end,1:jump_grid:end), ...
% Y(1:jump_grid:end,1:jump_grid:end), ...
% Z(1:jump_grid:end,1:jump_grid:end));
% shading interp;
% whos

更多回答（1 个）

KSSV
KSSV 2023-4-18
data=importdata('manila.dat');
lon=data(:,2);
lat=data(:,3);
elev=data(:,4);
xr = 118.7:0.2:121;
yr = 12.4:0.2:20.7;
[X, Y]= meshgrid(xr,yr);
idx = knnsearch([lon lat],[X(:) Y(:)],'k',10) ;
Z = reshape(max(elev(idx),[],2),size(X)) ;
plot3(lon(1:100:end),lat(1:100:end),elev(1:100:end),'o');
hold on
surf(X,Y,Z)

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