Asked by Tom
on 25 Oct 2018

I am working with a set of vectors for a calculation: the vectors are obtained in spherical polar coordinates and I then need to convert them all so that they are in Cartesian coordinates.

Basically, I have a matrix where I store the positions of a set of N vectors in spherical polar coordinates. So the first row is composed of the theta component of each of the vectors, the second row is the phi component, and the third is the radial coordinate and I would then like to form a new matrix which is the same but now the first row is the x component, the second row is the y component, and the third row is the z component. However, when I try to create this new matrix, MATLAB just says that the dimensionality of the matrices doesn't work, for example:

r(1,1:10)= sin(alpha)*cos(phi);

where alpha and phi have been specified as the row vectors formed from the first and second rows of the original matrix.

The example above is if I were to try to create the first row of the new matrix for 10 vectors in the original spherical polar coordinates. I have looked it up and the advice is that I should basically make MATLAB to do it by looping over the combinations which are needed but I am unsure how to do this, could someone advise?

Answer by Star Strider
on 25 Oct 2018

Accepted Answer

Star Strider
on 25 Oct 2018

My pleasure.

If my Answer helps you solve your problem, please Accept it!

Tom
on 29 Oct 2018

Star Strider
on 29 Oct 2018

If your vectors are in the format that sph2cart expects, you should be able to use it without modification. The arguments can be ‘a scalar, vector, matrix, or multidimensional array.’

I do not understand the problem you are having with it.

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Answer by Tom
on 10 Nov 2018

Edited by madhan ravi
on 10 Nov 2018

I will show you what I am intending, basically I have a piece of code which creates a 3 x N array depending on the values I choose for N_alpha and N_phi, let's say it's a 3 x 4 array, so I have four position vectors with the first row corresponding to the theta component of each vector, the second row is the phi component of the vector and the third row is for the radial component of the vector.

[i, j] = ndgrid(1:N_alpha, 1:N_phi);

alpha = (i-1)*2*pi/N_alpha;

phi = (j)*pi/(N_phi+1);

r = 1;

b = [alpha(:).'; phi(:).'; r * ones(1,numel(alpha))];

I would then like a new array B which is the same but with everything converted to Cartesian coordinates, if I try

B=sph2cart(alpha,phi,r);

it creates a 2 x 2 array which is smaller than the matrix I started with. If I do sph2cart(b) it says that the dimensions don't agree.

Star Strider
on 10 Nov 2018

The output is the same size as the input, in this example 3 (1x4) vectors. If you want longer output vectors, you need to provide longer input vectors.

Example —

[theta,phi] = ndgrid(0:0.15:2*pi);

r = 2*ones(size(theta));

[x,y,z,] = sph2cart(theta,phi,r);

figure

scatter3(x(:), y(:), z(:), '.')

grid on

axis equal

Tom
on 10 Nov 2018

Star Strider
on 10 Nov 2018

‘Basically I have a point which has a column vector in spherical coordinates on a sphere, then I take several such vectors for different points and slot them next to each other to make a matrix.’

Apparently, I still do not understand the problem you are having.

If your data vectors are gridded (have regularly-repeating patterns in the x- and y-coordinates), you can make them into a matrix using the reshape function first, then sph2cart.

If you simply have vectors in spherical coordinates that you want to change to Cartesian coordinates, you can do that easily enough with sph2cart.

You can then plot them using any function you wish. I use scatter3 in my example. Any other 3-D plotting function will work.

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