Declination adjustment

[lkilcher]

Does the sign of the declination adjustment change if an ADP or ADV is up vs. down-facing?

What about the order of the rotations? Does it matter?

[mmartini-usgs]

Yes, the order matters. I would make the declination adjustment to the heading value. Then apply the rotations.

I have had considerable trouble with ADV rotations for cabled Vectors. Are you having issues with this? I have a very good test data set.

[mmartini-usgs]

Here is a little more from my notes: Nortek documentation indicates that +z is always into the probe. However, this can vary depending on how the firmware interprets the up/down bit and other unknown factors. To know for sure the direction of +z, one can examine the last row of the transformation matrix provided in the .hdr or .whr files. This is the matrix T in Nortek’s transformation code for MATLAB. If the third row of T is negative, +z is out of the probe. Therefore: If the Nortek supplied transformation matrix row 3 is negative, do nothing, +z is out of the probe. To rotate it back into the probe, rotate about the x axes by applying the transformation matrix:

     | 1 0 0 |
 A = | 0 -1 0 |
     | 0 0 -1 |

[lkilcher]

I'm certainly interested in your test data set. What does it look like?

I think what you're describing is documented in the refs/transform.m script (which I got from the old Nortek forum website). Part of my concern here has to do with the question: how does the orientation matrix from an IMU (when the ADV has one) compare with the rotation matrix described in that script? Previously I'd thought I was tackling this correctly, but now I'm realizing there might be a bug here. So, this requires a bit more attention.

[lkilcher]

Also note that this is all very closely related to #44.

[lkilcher]

Also, regarding your comment:

I have had considerable trouble with ADV rotations for cabled Vectors. Are you having issues with this?

I think this is one thing that DOLfYN does fairly well. The tricky part is knowing what the rotation matrix between the body-frame and head-frame is. If you get that right, and put it into dat.props['body2head_rotmat'], and also define dat.props['body2head_vec'] correctly, then using dolfyn.adv.motion.CorrectMotion should do the rest.

[mmartini-usgs]

Hi Levi,

Hmm. Attached is what I worked out for that experiment. Nortek is using “mixed” conventions, as far as I can tell, and that really tripped things up. I wrote the tome to keep track of what I was doing and why in my code. It was not a simple problem and Nortek was not particularly helpful.

This is the data –

https://stellwagen.er.usgs.gov/mvco_14.html

https://stellwagen.er.usgs.gov/mvco_15.html

I can send you the raw stuff to work with if you like.

This is what the platforms looked like:

https://soundwaves.usgs.gov/2014/10/fieldwork2.html

https://soundwaves.usgs.gov/2017/02/research4.html

Our collaborators at WHOI threw up their hands and basically rotate their data to match the principle axis of the current and call it a day. OK if you are looking for turbulence, lousy if you want to actually measure the current with the ADV, so I tried to solve the problem.

Best regards,

Marinna

From: Levi Kilcher notifications@github.com Sent: Tuesday, March 26, 2019 2:18 PM To: lkilcher/dolfyn dolfyn@noreply.github.com Cc: Marinna Martini mmartini@usgs.gov; Comment < comment@noreply.github.com> Subject: [EXTERNAL] Re: [lkilcher/dolfyn] Declination adjustment (#43)

Also, regarding your comment:

I have had considerable trouble with ADV rotations for cabled Vectors. Are you having issues with this?

I think this is one thing that DOLfYN does fairly well. The tricky part is knowing what the rotation matrix between the body-frame and head-frame is. If you get that right, and put it into dat.props['body2head_rotmat'], and also define dat.props['body2head_vec'] correctly, then using dolfyn.adv.motion.CorrectMotion should do the rest.

— You are receiving this because you commented. Reply to this email directly, view it on GitHub https://github.com/lkilcher/dolfyn/issues/43#issuecomment-476782775, or mute the thread https://github.com/notifications/unsubscribe-auth/AWH-kproiRYSMznkuDpcUlypOh57OPH8ks5vamRFgaJpZM4cJSpj .[image: Image removed by sender.]

[lkilcher]

This was resolved in #50. Note that this added a new method, dat.set_declination(angle), which should be used to set the declination.