Error using .* Matrix Dimensions must agree

Hello! I am attempting to find the area of grid cells using two vectors, 1)latitude which is 224 X 1 and 2)longitude which is 240 X 1. The code I am using to do so is

R=6371
x=diff(latitude1)
y=diff(longitude)
Area_grid_points=(((2*pi*R)/360)^2).*cos(latitude1).*x.*y

Unfortunately when I use this code I receive the error "Error using .* Matrix Dimensions must agree." I am assuming that I receive this error because cos(latitude1) produces an array of 224*1 which is different than that produced by diff(longitude1) which is 240 X 1. Can anyone direct me in the right direction to correct this discrepancy? Thanks so much!

Answer 1

[Kelly Kearney]

Well, diff is always going to return a result 1 element shorter than the input, since it's measuring the distance between elements. In your case, you'll have to decide what latitude you want to correspond to each grid cell: top? bottom? middle?

I usually choose the midpoint for things like this:

R=6371
x=diff(latitude1) 
y=diff(longitude) 

latmid = (latitude(1:end-1)+latitude(2:end))./2;

Area_grid_points=(((2*pi*R)/360)^2).*cos(latmid).*x.*y;

[Star Strider]

The gradient function is an alternative to diff, with output sizes matching inputs. Don’t know if it would be appropriate here, though.

[Stephanie Goldstein]

Hi Kelly. Thanks for your response. Unfortunately, I still received the error "Error using .* Matrix dimensions must agree." Do you know how I can make the matrix dimensions equal to one another?

[Kelly Kearney]

I was assuming that lattitude and longitude were the same size, but apparently not. Assuming that both latitude and logitude are row vectors, representing the edges of a rectangular (in lat/lon space) region, perhaps you want:

R = 6371
x = diff(latitude1); 
y = diff(longitude); 

latmid = (latitude(1:end-1)+latitude(2:end))./2;

Area_grid_points = (2*pi*R/360)^2 .* y' * (cos(latmid).* x);

Note that my use of matrix multiplication with a column and row vector is a shortcut for expanding the two vectors into a full grid.

Answer 2

[José-Luis]

If you want element by element multiplication you should use .* instead of *. whether you want this or matrix multiplication, the number of elements and size of your array should be congruent.

Maybe you are looking for something like bsxfun() instead:

 bsxfun(@times, latitude, longitude')

[Stephanie Goldstein]

Hi Jose-Luis. I tried using .* and received an error of "Error: Unexpected MATLAB operator." Here is what I really need help on: how can I make the number of elements and size of my arrays congruent? Also, can you please explain to me what bsxfun(@times, latitude, longitude') does? Thanks so much.

[José-Luis]

Well, I don't know. How do you get the latitude and longitude vectors in the first place.

 doc bsxfun

Answer 3

[Sajjad Ali]

I am also getting error Error using . Matrix dimensions must agree.* in following line of code _[m,n]=size(thick_image(:,:,1));

indTerm=sub2ind([m,n],CentroidTerm(:,1),CentroidTerm(:,2));

Z=zeros(m,n);

Z(indTerm)=1;

ZTerm=Z.*ROI';

[CentroidTermX,CentroidTermY]=find(ZTerm);_ can any one help me to resolve this issue?

[Walter Roberson]

You do not say what size of ROI is

[Jan]

Please create a new thread for a new question and remove this message here. Otherwise it gets unclear to which question an answer belongs and the Accepted status is meaningless. Thanks.