Vertical velocity
by
Atle Lohrmann
—
last modified
May 12, 2005 12:31 PM
Hi there
I looked at the data very quickly and I can confirm your observation about large vertical velocities being reported in the corrected and uncorrected velocity data. Here is what's happening:
a) When collecting data, your vessel mounted system is tilted about 10 degrees. For this reason, a relatively large Vz component creeps into the data.
b) When we correct the data with the GPS/heading, we process the Veast and Vnorth components but nothing is done wih the Vz term, which remains large.
This has no bearing on the horizontal velocity, which is processed correctly and has been in use for many years. In other words, there is no need for conern unless you specifically want to use the vertical velocity data.
Best regards,
Atle Lohrmann
I looked at the data very quickly and I can confirm your observation about large vertical velocities being reported in the corrected and uncorrected velocity data. Here is what's happening:
a) When collecting data, your vessel mounted system is tilted about 10 degrees. For this reason, a relatively large Vz component creeps into the data.
b) When we correct the data with the GPS/heading, we process the Veast and Vnorth components but nothing is done wih the Vz term, which remains large.
This has no bearing on the horizontal velocity, which is processed correctly and has been in use for many years. In other words, there is no need for conern unless you specifically want to use the vertical velocity data.
Best regards,
Atle Lohrmann
| Quote |
Good morning A company has carried out a series of NDP measurements for us in June 2002. I am sending you a few of the resulting *.dat files. What we have noticed in the measurement files is a very high value for the Z-component of the velocity ( the vertical velocity VU)- this is at times order of magnitude dm/s. The equipment used for the measurements was a 1.5 Mhz VM Nortek NDP . 1/Have you any idea what may have caused this deviations ? 2/For future measurements : how can we prevent this error from occuring ? 3/does the fact that the Z values seem unreliable also means that the X and Y values can not be trusted ? 4/ In case of the fact that X and Y should be erronous,is there a possibility to correct this data ? |
Current state:
Being created
The correction should be done in the SurveyP software and we will try to take care of this in the not-so-distant-future 
" />
'> Menanwhile, you can make a very quick (approximate) correction like this:
Vz' = Vz - Ux*sin (pitch), where Ux is the boat velocity in the direction of the boat.
A more complete correction could be done in matlab by using XYZ to ENU transformation script you can find here:
http://www.nortek-as.com/cgi-bin....5;t=14;
Best regards,
Atle Lohrmann
" />'> Menanwhile, you can make a very quick (approximate) correction like this:Vz' = Vz - Ux*sin (pitch), where Ux is the boat velocity in the direction of the boat.
A more complete correction could be done in matlab by using XYZ to ENU transformation script you can find here:
http://www.nortek-as.com/cgi-bin....5;t=14;
Best regards,
Atle Lohrmann
| Quote |
| Is it possible to correct the Vz values for the tilt? and if yes, does a procedure/routine exist for carrying out this correction ? |
Current state:
Being created
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