"griddata" results in much smaller values than original data

Question

jie hu 2023-4-18

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1948918-griddata-results-in-much-smaller-values-than-original-data

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
显示无隐藏无

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 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?

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Chunru 2023-4-18

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1948918-griddata-results-in-much-smaller-values-than-original-data#answer_1217923

编辑：Chunru 2023-4-18

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

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

jie hu 2023-4-20

thanks very much for your help. It works!

请先登录，再进行评论。

Answer 2

KSSV 2023-4-18

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1948918-griddata-results-in-much-smaller-values-than-original-data#answer_1217933

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)