Problem doing plots using contour with NaN elements in a matrix

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Ricardo Duarte 2021-2-15

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编辑： dpb 2021-2-17

Dear all,

I have 3 matrices: lon, lat, depth.

Than I want to remove all the depth values that are bigger than 0. To do so I use:

depth(depth>1)=NaN.

By doing this I remove the land part, which is ok.

The problem then is that I want to create a contour arround the sea level, which means at the depth =0. To do so I'm doing:

[C,H]=contour(lon,lat,depth',[0 0]);

As a result I obtain an empty C matrix and I don't understand why!!!

Hope you can help me!

Thanks in advance.

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dpb 2021-2-16

编辑：dpb 2021-2-16

Find the contour first, THEN triangulate. If you have to remove the positions that are >0 then, so be it, but still the countour locations will have been determined and returned in the countour object.

Again, the countour is a boundary; you've got to have something that can be interpolated across the zero plane for countour() to be able to figure out where that is. When you have something that is like [-1.234 NaN] in one direction; what's it supposed to do? There simply is no way to infer where zero is between those two values.

As is, you're not removing all land, anyways, by using the >1 level; you probably will be better served to leave completely alone and find the zero contour object, then remove everything above 0.

I'm not sure what you're doing with the contour object, anyhows???

I've never messed with the guts of contour much other than recollect digging into the convoluted storage pattern of the returned contour object array for somebody here once upon a time to help them retrieve information from it, but having done is all that I do remember.

Ricardo Duarte 2021-2-16

Ok, I will try to explain the complete problem.

I have the lon, lat, depth. With that I want to calculate the transmission loss (just at the sea) of a certain region represented by my lon, lat, depth matrices. To do so I need to define the sea level (depth=0) and remove land in order obtain my X and Y vectors and after performe the delauny triangulation. The problem here is that ,if I do not remove land before the contour, when I obtain the C matrix returned by contour, it will triangulate also in the land part and later (in the remaining part of my code) the TL will be calculated also in the land part, which is not correct.

So the code I have is:

[C,H]=contour(lon,lat,depth', [0 0]); I will use C to assing X and Y (at this stage I want to have just sea information)

X=C(1,:); Y=C(2,:);

Then I use the delauny doing this:

tri=delaunay(X,Y); (if I have land, the triangulation will be performed also in the land, which is what I don't want)

and then triplot(tri,X,Y);

And then the rest of the program...