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Magnetic declination

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Maurine Tahar

Hello,

I've deploy the Eco ADCP in two sites and for each of the sites, retrieved the raw csv data and the report that can be automatically created through the app, although I did have to ask Nortek support to send it for the second site as there was a bug with the app at the time.

For the first site, the table at the beginning of the report shows that the ADCP recorded its GPS coordinates, and there was a ~ -20° magnetic declination. The polars shown in the report have a ~|20°| difference in direction with the polars I created directly from the csv data, so it seems the magnetic declination correction was applied to either the report's polars only or the csv data only. I initially assumed the declination was applied on the report's polar.

However, at the second site, no GPS coordinates were retrieved so no magnetic declination correction were added, and the polars from the report and the site match. I retrieved online the magnetic declination for the second site (this site also has a declination of ~ -20°) and, since I assumed for the previous site previously that the magnetic declination was applied on the report and not the csv, I substracted ~ |20°| on the directions of the csv. Unfortunately, once I trace the polars for this second site, the directions don't make sense, the site is located in a kind of channel, but with the -20° angle on the directions, it would mean the current is almost perpendicular to the channel. It would however make more sense to add ~ |20°|.

If I had to add, and not substract, the 20° angle on the csv directions of the second site, it would then mean that for the first site, the declination angle given in the report's table is already taken into account in the csv but not in the report, and that the polars in the report should be substracted approx. 20°. 

Could you please confirm where, between the csv data and the report's polars the magnetic declination is applied when the ADCP could calculate it, or where it should be applied when it couldn't?

 

Thank you

 

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    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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