how to calculate direction in rotated coordinate system

I have a direction defined by azimuth and evation (e.g. Azi, Elev).

I would like to calculate values of these Azi i Elev after coordinate system

rotation: first around Z axis (angle e.g.: Zrot) and then around rotated (in first rotation

around Z axis) Y axis (angle e.g.: Yrot).

So: I rotate coordinate system around Z axis, then around "rotated" Y axis and I would like to

calculate Azi and Elev in rotated coordinate system.

How can be it done?

[darova]

DO you have a picture of the result?

Answer 1

[David Goodmanson]

Hi UWM,

Assuming the correct rotation has already been made, then calling the resulting vector u, and making sure it is normalized by using u/norm(u), suppose the three components of u are given in the order (north,east, up) in an appropriate coordinate system. Assuming elevation is measured up from the horizon and azimuth is measured clockwise from north (as with a compass rose), u has the form

cos(el)*cos(az)
cos(el)*sin(az)
sin(el)

then you can divide the second expression by the first to arrive at

el = asin(u(3))
az = atan2(u(2)/u(1)

There are a lot of variations on this theme depending on how the vector components and the angles are defined, but the basic idea will be the same.

[UWM]

Hi David,

thank you for help. I am not understood everything well so

below is my code. But it looks like something's wrong - e.g.

rotation around Y axis is done not after rotated axis (in first rotation

around Z axis) but around 'old' coordinate system axis.

Maybe you can help me.

clc
clear
% Direction
% geodetic elevation and azimuth (Esvo - elevation is the elevation in deg from the x-y plane (-90 90), Asvo azimuth is the clockwise angle in the x-y plane measured in deg from the positive x-axis (0 360).
Esvo = 50;
Asvo = 40;
% elevation and azimuth in MatLab
Esv = Esvo;
if Asvo < 180; Asv=-Asvo;
else Asv = 360-Asvo;
end
% coordinate system rotation angles (Aro - around Z axis, Ero - around Y axis)
% Aro possible range (0 360); Ero possible range (0 90)
Aro = 5
Ero = 1:90
Ar = -Aro;
Er = 90-Ero;
r=1;
dtr = pi/180;
dts = 180/pi;
Es=Esv*dtr;  % deg to rad
As=Asv*dtr;  % deg to rad
% Direction position in MatLab
[x0,y0,z0] = sph2cart(As,Es,r);
poz0 = [x0 y0 z0];
for j=1:length(Aro)
    for i=1:length(Ero)
        
        % coordinate system rotation
        ROTZ = [ cosd(Ar(j)) -sind(Ar(j)) 0;
            sind(Ar(j))  cosd(Ar(j)) 0;
            0       0       1];
        
        ROTY = [ cosd(Er(i)) 0  sind(Er(i));
            0     1        0;
            -sind(Er(i)) 0  cosd(Er(i))];
        
        % Direction position after rotation (in Matlab)
        poz1 = poz0*ROTZ*ROTY;
        x1 = poz1(1);
        y1 = poz1(2);
        z1 = poz1(3);
        
        % Direction azimuth and elevation after rotation (in Matlab)
        [az,el,r] = cart2sph(x1,y1,z1);
        
        % rad to deg
        AZYa=az*dts;
        ELVa=el*dts;
        
        % Direction azimuth and elevation after rotation (in geodetic system)
        if AZYa <0; AZY=abs(AZYa);
        else AZY=360-AZYa;
        end
        ELV = ELVa;
        
        Results(1,i,j) = AZY;
        Results(2,i,j) = ELV;
    end
end

[David Goodmanson]

Hi UWM,

Although there is more than one convention for how to do this, I am guessing that you are doing an active rotation, i.e. there is an overall fixed coordinate system S and you are rotating a vector object within that system. After a series of rotations, the final result is expressed as coordinates in S.

You are expressing the vector as a row vector and multiplying by rotation matrices on the right. If you have not done so already, it's a good idea to run some test cases on rotz,roty and also rotx at, say, 45 degrees to verify that they are doing what you think.

Now you wish to first rotate about the z axis, which rotates the object and also rotates the y and x axes to xnew and ynew. That's followed by a rotation about ynew.

Abbreviating rotz as Rz etc, a rotation of v about the z axis followed by a rotation about the old y axis would have been

vrot = v*Rz*Ry           [1]

the matrix Rynew, a rotation about ynew, is related to the original Ry by

Rynew = Rz^(-1)*Ry*Rz

and so

vrot = v*Rz*Rz^(-1)*Ry*Rz = v*Ry*Rz

This uses rotations involving the original coordinate system, only with the order reversed compared to [1]. Hope this helps.

[UWM]

Thanks, now it works :)

Answer 2

[Matt J]

Using AxelRot (Download), and assuming all angles are given in radians,

[x,y,z] = sph2cart(Azi_Old,Elev_Old,1);

P=AxelRot( [x,y,z;0,1,0].' , Zrot*180/pi, [0,0,1], []);

Q=AxelRot( P(:,1), Yrot*180/pi, P(:,2), []);

[Azi_New,Elev_New]=cart2sph( Q(1),Q(2),Q(3) );

[UWM]

Thank you for help, I checek also your approach and it also works very well:)

[UWM]

Hi Matt,

I have still some problem with this issue. It seems that in both approaches after first rotation (around Z axis) we get new position in Old Coordinate System (POZo turns into POZn) and this new position (POZn) is then rotated around Yn axis.

But my goal is a bit different: I would like rotate XoYoZo system around Zo axis, get POZo in New Coordinate System (POZo in XnYnZn) and finally rotate XnYnZn system around Yn axis and obtain POZo after this rotation.

Maybe you can help me

Best regards