Contouring a mesh and assigning magnitude arrows in Matlab

I want to assign vector to a contourf graph, in order to show the direction and magnitude of wind. For this I am using contourf(A) and quiver(x,y), where as A is a matrix 151x401 and x,y are matrices with the same sizes (151x401) with magnitude and direction respectively.

When I am using large maps i get the position of the arrows but they are to densily placed and that makes the graph look bad.

The final graph has the arrows as desired, but they are to many of them and too close, I would like them to be more scarce and distributed with more gap between them, so as to be able to increase their length and at the same time have the components of the contour map visible.

Can anyone help , any pointers would be helpful

Answer 1

[Sara]

use

quiver(x(1:dx:end),y(1:dy:end),u(1:dy:end,1:dx:end),v(1:dy:end,1:dx:end))

where dx and dy are how many elements to skip. In this way, the arrows are spaced out.

[George]

Is the sequence I am using correct?

contourf(A)
hold
quiver(x(1:20:401),y(1:20:401))

It does not let me to over plot the arrows

thank you

[Sara]

Try using

hold on

Works for me...

[George]

you can see the arrows of the quiver?

I cant :(

[Sara]

How did you get the pic on the right then? Attach your script and your data, since it works for the random numbers I'm using, it may be a detail that differs.

[George]

contourf(mesh)
hold on
quiver(x(1:20:401),y(1:20:401))

These are the files, and the commands i use, if you open after the contour the property editor and isolate the quiver you will notice it has only placed 5 arrows at the beginning of the axis

thanks

[George]

the pic on the right i got it with

contourf(mesh)
hold on
quiver(x,y)

[Sara]

Ok, found the problem. You need to specify also the x and y coordinates of your points :). Try this and then replace xx and yy with your coordinates:

clc
clearvars
close all

load('matlab.mat');

[nx,ny] = size(A);
xx = 1:1:ny;
yy = 1:1:nx;
contourf(xx,yy,A)
hold on
delta=10;
quiver(xx(1:delta:end),yy(1:delta:end),x(1:delta:end,1:delta:end),y(1:delta:end,1:delta:end),0.5)

[George]

wow, thank helped me a lot, indeed now the is near to what i am after

i got frustrated cause I followed the quiver of Matlab

may i ask how you detected the issue ?

thank you :)

[Sara]

Tried your code with a delta of 2 instead of 20 and saw that the arrows where showing up in the bottom left corner of the image, i.e. matlab was simply using 1:1:nelem as coordinates not knowing any better.

Note, I added the 0.5 at the end of the quiver command so arrows don't touch. Please, modify that too so that the image will look like what you want.

[George]

Thank you very much for the help

Sorry if i ask how you detected the mistake, but i wanted to know so I can learn :)

the 1:1 nelem never crossed my mind !!

thank you very much