find nearest value on mesh grid under certain contour

I have a surf matrix (colormap gray), a contour matrix (red). The magenta points are local minmum in red contours. I want to find the closest z value on the neigbhourhood contours. Is there any way to achieve this?

[Chunru]

How do you represent the contour as matrix?

[Katherine Zheng]

I run below code with the attached MAT-file. The B2 matrix (contains the contour) is modified from the output of watershed function.

figure     
% surf
surf(Xi2,Yi2,Zi2-max(Zi2(:)); 

Unrecognized function or variable 'Xi2'.

axis tight; title('m-min y-max')
colormap(gray); colorbar; daspect([1 1 5]);
hold on
% red contour
surf(Xi2,Yi2,B2,'EdgeColor','r')
% peak and valley points             
scatter3(Xi2(regmin),Yi2(regmin),Zi2(regmin),'mo','filled','sizedata',40)
scatter3(Xi2(regmax),Yi2(regmax),Zi2(regmax),'yo','filled','sizedata',40)

[KSSV]

How about using knnsearch

[Katherine Zheng]

@KSSV I am not sure whether knnsearch can specify the value associate with the points. It seems only consider distance only?

[KSSV]

You will get the index of the nearest point. If you know the index, you can get the associated value.

[Katherine Zheng]

The value is my priority concern then the distance.

[Bruno Luong]

For each magenta point do you want to find the z value on the contour that enclosed the point? If the pointt is the locall minima, meaning that do you want to find the lowest Z along the contour?

Answer 1

[Chunru]

load(websave("matlab.mat", "https://www.mathworks.com/matlabcentral/answers/uploaded_files/1116160/matlab.mat"))
% whos
surf(Xi2,Yi2,Zi2-max(Zi2(:))); 
axis tight; title('m-min y-max')
colormap(gray); colorbar; %daspect([1 1 5]);
hold on
% red contour
surf(Xi2,Yi2,B2,'EdgeColor','r')
% peak and valley points             
scatter3(Xi2(regmin),Yi2(regmin),Zi2(regmin),'mo','filled','sizedata',40)
scatter3(Xi2(regmax),Yi2(regmax),Zi2(regmax),'yo','filled','sizedata',40)

idxB2 = ~isnan(B2);  % B2 has values of 0 and nans
% The values along the countour
Xc = Xi2(idxB2);
Yc = Yi2(idxB2);
Zc = Zi2(idxB2);
% These are values along the countour
[Xc Yc Zc]

ans = 727×3

1.3480 1.4440 0.0439 1.3480 1.4920 0.0357 1.3480 1.5160 0.0349 1.3480 1.6240 0.0122 1.3480 1.6480 0.0194 1.3480 1.6720 0.0230 1.3480 1.7560 0.0306 1.3480 1.7920 0.0192 1.3480 1.8160 0.0011 1.3480 1.8520 -0.0082