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

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nadia nadi
nadia nadi 2017-3-10
编辑： Walter Roberson 2017-4-21
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,

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回答（3 个）

Image Analyst
Image Analyst 2017-3-10
Looks like you forgot to apply your rotation matrix to the "g" coordinates.
Roger Stafford
Roger Stafford 2017-3-13
I think you need
axis equal
so that scaling is the same for both axes.
Roger Stafford
Roger Stafford 2017-4-21
编辑：Walter Roberson 2017-4-21
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.

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