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Magnetic declination



  • Official comment
    Michael Dunn

    Hi Maurine,

    I can confirm that the processing performed on the webapp is working as intended. But, there a couple things to note:

    First, the declination is always subtracted from the velocity so you must track the sign. For example, East declination is Positive, so –(+) returns a negative, and applying it creates a counter-clockwise rotation to the data.  West declination is Negative, so –(-) returns a positive, and applying it creates a clockwise rotation to the data. It is important to note that if your GPS location is set, all of the reports and charts produced by the web app will already have the magnetic declination subtracted from the velocity so you don't need to perform the operation yourself. 

    Second, you are likely seeing unusual polar values because the GPS data was not available for your second site. When this occurs, you should always figure out where the deployment was and update the map accordingly in the app (click on the map, then click on Change Location; see the below FAQ on how to do this). Once you save this change, then the app will automatically update the magnetic declination and apply it to the data, so there’s no guessing.

    Third, if the frame/bottom mount is magnetic then this could explain the issue with the misalignment between the measured data and expected. Could you please provide pictures and a detailed description?

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    Maurine Tahar

    Thanks for your answer. 

    However, I'm still unsure which dataset is the corrected one for the first site, between the polar shown on the report, and the polar I created from the csv directly downloaded from the app, as they each show a different angle. 

    But if I assume the report polar is the corrected one for the first site, and I apply the same correction logic on the data of my second site, this is where I get a polar showing a direction almost perpendicular to the channel...

    I couldn't retrieve the dataset and report myself from the app for the second site and I had to contact Nortek support to retrieve it and send it to me, so I can't do the GPS adjustement from the app.

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