## calculate the angle between a line (between two points) and the perpendicular line

### Amneh Alshawabka (view profile)

on 20 Mar 2019
Latest activity Edited by Jan

on 20 Mar 2019

### Jan (view profile)

if point1 is a(x1,y1) and point 2 is b(x2,y2). I want to draw a vertical from point b in order to calculate the angle between the line ab and the verticle line.
I wrote this code but I think i can't get the verticle line correctly as I got the same value of the angle and i'm sure the angle value is not the same.
for i=1:size(a,1)
angle = atand(b(1,i) - a(1,i))-(b(2,i) - a(2,i)))
end
any one can help me please?

Geoff Hayes

### Geoff Hayes (view profile)

on 20 Mar 2019
Amneh - I don't understand why you need a for loop especially as you overwrite the angle on each iteration of the loop. Where have you defined the vertical line? Could you use a third point (for this line) as c = (x2, 2*fabs(y2)) (arbitrarily choosing a position on the y-axis) and then use the usual equation for finding the angles of a triangle (your three points)?
Geoff Hayes

### Geoff Hayes (view profile)

on 20 Mar 2019
I'm using a loop because points a and b are moving and changing their positions. I want to caluctae the angle for every fram. I assumed that the vericle line is between (point a (x1,y1) and Point (x1,y2))
Geoff Hayes

### Geoff Hayes (view profile)

on 20 Mar 2019
I assumed that the vericle line is between (point a (x1,y1) and Point (x1,y2)) Can you provide a picture? I don't understand how the vertical line is between (x1,y1) and (x1,y2) (or do you mean (x2,y2) for the second one?). Also I thought I want to draw a vertical from point b which suggests that the vertical line is drawn from point b and is not in between the two points. Please clarify.

### Jan (view profile)

on 20 Mar 2019

Maybe you mean:
angle = atand(b(1,:) - a(1,:)) - (b(2,:) - a(2,:)))
% ^???
without a loop. But here the number of parentheses does not match. I'm not sure which problem you want to solve.

### Geoff Hayes (view profile)

on 20 Mar 2019
Edited by Geoff Hayes

### Geoff Hayes (view profile)

on 20 Mar 2019

Amneh - if a is (x1,y1) and b is (x2,y2) then let c be (x2,y1). With these three points, you should have a right-angled triangle. We can determine the opposite and adjancent as
opposite = abs(a(1) - c(1));
and so the angle is
(I think!)

### Amneh Alshawabka (view profile)

on 20 Mar 2019

Thanks alot for your help . I'm a new Matlab user.
Actually i'm not sure how to creat a vertical line either from point a or b!! I want to caculate the angle between line ab and the verticale line from b to the ground as shown in the picture below.

Jan

### Jan (view profile)

on 20 Mar 2019
Please post comments in the section for comments. The best location for the picture is the original question, where readers expect the full information (I've inserted it there).