Coordinate

Dear Sir, Is there any benefit concerning collection of data based on beam, XYZ or ENU. In other words, when do we use each of them? Also I want to know the rule of pitch, roll and heading on the transformation matrix. How we can make sure that the achieved data is exactly align a line perpendicular to the bottom surface. Regards, Mehrdad

mehrdad,

Coordinate transforms are discussed multiple times on the forums. I recommend searching for further discussions on this, but the short answer is there is no difference between the three coordinate systems and a user can transform between the three without losing any information in the process. Beam coordinates are primarily useful for higher level turbulence calculations and for dealing with phase wrapping issues.

Details of the transformation matrix and how pitch, roll, and heading affect it can be found in numerous references, I particularly like the appendix of: Lohrmann, A., B. Hackett, and L.P. Røed, 1990: High Resolution Measurements of Turbulence, Velocity and Stress Using a Pulse-to-Pulse Coherent Sonar. J. Atmos. Oceanic Technol., 7, 19–37.

You can find the specifications of the tilt sensors used in Nortek's profilers here: http://www.nortek-as.com/lib/brochures/AquadoppProfiler%2006%20b.pdf

Other than utilizing divers to install a deployment frame or in a laboratory setting, you will only be able to determine alignment with an earth coordinate system from the tilt sensor data. If you bottom surface is irregular, sloping or changing with time, you would need an additional sensor such as a dedicated bathymetry mapper to judge the rotation of your instrument's coordinate system relative to the boundary.

Dear Peter, Thanks for your response. Actually I have the Vector one not the Profiler. Would you please give information related to that? I also searched the forum, but I'm kind of lost about this procedure. From the Header file I see that the transformation matrix is as follows. Would you please let me know how I can reach to this matrix? Is this the inverse of G matrix? I mean, if I want to change the coordinate from beam to XYZ I just need to multiple it by this matrix. Is this right? Also it seems that for profiler, there is another scaling factor, but not for the Vector! 2.6465 -1.3435 -1.2891 -0.0334 2.4282 -2.4011 0.3257 0.3601 0.3455 Regards, Mehrdad

Dear Peter, Thanks for your response. Actually I have the Vector one not the Profiler. Would you please give information related to that? I also searched the forum, but I'm kind of lost about this procedure. From the Header file I see that the transformation matrix is as follows. Would you please let me know how I can reach to this matrix? Is this the inverse of G matrix? I mean, if I want to change the coordinate from beam to XYZ I just need to multiple it by this matrix. Is this right? Also it seems that for profiler, there is another scaling factor, but not for the Vector! 2.6465 -1.3435 -1.2891 -0.0334 2.4282 -2.4011 0.3257 0.3601 0.3455 Regards, Mehrdad

Hi mehrdad,

The Vector and Aquadopp profiler share the same internal electronics so the tilt sensor specifications will be the same. But you can find information on the Vector here: http://www.nortek-as.com/lib/brochures/Vector%2006%20b.pdf

Searching the forums is relatively easy, use the field at the top of the column of posts with the text "search board" in it. Typing in "coordinate transform" yields this: http://www.nortek-as.com/en/knowledge-center/forum/search?q=coordinate+transform

There are numerous posts related to this issue. Not that regardless of the instrument, the basic math used in a coordinate transform is the same. You can also take a look at this file: http://www.nortek-as.com/lib/forum-attachments/coordinate-transformation/view

It will provide you with the basics of coordinate transforms between the various systems.

 

P.J.

Regarding the scaling factor, in the past the transformation matrix was output as an integer and had to be scaled by 4096 to be the appropriate value for actually performing transforms. Now, most instruments will report the transformation matrix pre-scaled so you don't need to divide by 4096 anymore. It's easy to tell if you need to or not, if the number is a floating point value, you do not need to scale the transformation matrix.