rotate velocity data

Posted by Craig Hill at October 21. 2009

When we collected velocity data in a stream, our Vectrino was unknowingly at a slight angle (now determined to be approx. 10degrees...rotated around the x-axis). So, we were actually sampling slightly higher in the water column that we initially thought, and this also means that the recorded Z value included a component of y-velocity and z-velocity. The Y value recorded included a component of z-velocity and y-velocity.

Is there some way to correct for the tilt that the Vectrino Probe was mounted at? I'm not sure what all of the info. in the .hdr file relates to, but is there something in there that records the internal tilt of the Vectrino, or information that would help indicate how to transform and scale the necessary velocity components to correct for the tilt?

Is there a description for .hdr file format anywhere?

Thanks in advance for any help!

-Craig

Re: rotate velocity data

Posted by P.J. Rusello at October 21. 2009

The Vectrino doesn't have any internal attitude sensors so you'll need to estimate the angles needed for the transformation to correct for this tilt. The correction you need to apply is very similar to the transformation between XYZ and ENU coordinates, where heading, pitch and roll are used to rotate velocities into an Earth normal coordinate system.

The most relevant information from the header file (which should be self documenting, there's not a description of the fields outside of the header file since each field is labelled) will be the Transformation matrix. You don't even really need that however since you don't have a need to go to or from beam coordinates. This thread: http://www.nortek-as.com/en/knowledge-center/forum/software/644656788#161256551 is a good place to start for learning about coordinate transforms. In Atle's first post in that thread there is a link to an m-file which be helpful as well. For the Vectrino, you'll need one more row and column on your matrix since it has four beams, but it should be fairly easy to extend the three beam system to your situation.

P.J.

Re: rotate velocity data

Posted by Craig Hill at October 21. 2009

Thanks for the information. I will work forward with the coordinate transforms. I guess from the .hdr file, I was unsure what all of the System(#) values referred to under User Setup and Head Configuration. Everything else is fairly self explanatory.

Thanks,

Craig

Previously Peter J. Rusello wrote:

The Vectrino doesn't have any internal attitude sensors so you'll need to estimate the angles needed for the transformation to correct for this tilt. The correction you need to apply is very similar to the transformation between XYZ and ENU coordinates, where heading, pitch and roll are used to rotate velocities into an Earth normal coordinate system.

The most relevant information from the header file (which should be self documenting, there's not a description of the fields outside of the header file since each field is labelled) will be the Transformation matrix. You don't even really need that however since you don't have a need to go to or from beam coordinates. This thread: http://www.nortek-as.com/en/knowledge-center/forum/software/644656788#161256551 is a good place to start for learning about coordinate transforms. In Atle's first post in that thread there is a link to an m-file which be helpful as well. For the Vectrino, you'll need one more row and column on your matrix since it has four beams, but it should be fairly easy to extend the three beam system to your situation.

P.J.

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rotate velocity data