How to blank a grid value with a boundary file?

Question

Leon 2021-4-21

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/809315-how-to-blank-a-grid-value-with-a-boundary-file

评论： Leon 2021-4-21

I have a grid like the below:

[X, Y] = ndgrid([-180:0.25:179.75],[-90:0.25:89.75]);
Z = griddata(...); %on the same grid as X and Y

I also have a boundary file like the below (a lot more complicated in reality):

-5
-30
-30
-5
-5

How do I use the boundary file as above to only get the griddata (X1, Y1 and Z1) within the polygon and plot them onto a map as a contour like the below?

[C1, h1] = contourf (X1, Y1, Z1);

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Answer 1

Clayton Gotberg 2021-4-21

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/809315-how-to-blank-a-grid-value-with-a-boundary-file#answer_681325

在 MATLAB Online 中打开

Use the inpolygon function to determine which points are inside the boundary you define, then replace those points which are outside with NaN.

[X, Y] = ndgrid([-180:0.25:179.75],[-90:0.25:89.75]);
Z = griddata(...); %on the same grid as X and Y
    
boundaries = readmatrix('boundary_file') % replace with actual boundary file
% may need to change file-reading function
boundary_x = boundaries(:,1);% x values of boundary file
boundary_y = boundaries(:,2);% y values of boundary file
in_bounds = inpolygon(X,Y,boundary_x,boundary_y);
X(~in_bounds) = NaN;
Y(~in_bounds) = NaN;
Z(~in_bounds) = NaN;