how to find the end of major and minor axis of an ellipsoid

Dear,

I have this information of an ellipse

a=29.2139; b=17.7345; xC= 43.9666; yC=42.9211; angle=29.7207;

can any one tell me why the end of the major and minor axis are not in the right place. I tried to rotate them using matrix H, but still it gives me wrong places. can any one help please.

Many thanks,

Nadia,

[Walter Roberson]

Please show your code.

Answer 1

[Image Analyst]

Looks like you forgot to apply your rotation matrix to the "g" coordinates.

[nadia nadi]

hello, thanks for replying. I applied but still gives me the wrong positions.

[Image Analyst]

See attached demo.

Answer 2

[Roger Stafford]

I think you need

axis equal

so that scaling is the same for both axes.

[nadia nadi]

Sorry I did it but it does not work. Best,

Answer 3

[Roger Stafford]

I will make some assumptions about the ellipse you describe:

1. The ‘a’ and ‘b’ values are the lengths of the ellipse’s semi-major and semi-minor axes, respectively.
2. xC and yC are the cartesian coordinates of the ellipse’s center.
3. The ‘angle’ is the angle in degrees, measured counterclockwise, from the x-axis to the ellipse major axis.

The ellipse axes’ four ends will then be:

Major axis, upper right: x1 = xC+a*cosd(angle)
                         y1 = yC+a*sind(angle)
            lower left:  x2 = xC-a*cosd(angle)
                         y2 = yC-a*sind(angle)

Minor axis, upper left:  x3 = xC-b*sind(angle)
                         y3 = yC+b*cosd(angle)
            lower right: x4 = xC+b*sind(angle)
                         y4 = yC-b*cosd(angle)

You can make a plot of the ellipse as:

   t = linspace(-180,180,1000);
   x = xC+a*cosd(t)*cosd(angle)-b*sind(t)*sind(angle);
   y = yC+a*cosd(t)*sind(angle)+b*sind(t)*cosd(angle);
   plot(x,y,’y-‘,[x1,x2],[y1,y2],’r*’,[x3,x4],[y3,y4],’g*’)
   axis equal

Note: It is not a good practice to use 'angle' for the name of your variable, since that is a reserved name for one of matlab's functions.