Symbol rotation vs magnetic north

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05-28-2021 07:55 AM
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ShaunDillon1
Occasional Contributor

I am in New Hampshire which has about 15 degrees West declination. My coordinate system in arcmap is NAD_1983_StatePlane_New_Hampshire_FIPS_2800_Feet.

I am showing points with an arrow which is rotated and should point the way I was looking at that point. These are overlaid a boundary survey and they are often a bit off but I have attributed that to the compass in the tablet used not being very good. But here I have an example where the survey lines are directly north/south and east/west. I have to points at the corner with rotation angle (geographic) of 270 and 180 but they are not pointing 270 and 180:

ShaunDillon1_0-1622213324166.png

If I add the 15 degrees declination it lines up as it should:

ShaunDillon1_1-1622213429147.png

I just don't understand why I should have to add the declination. If anything (in West declination) I thought you added it when you went from map to the field and subtract when going from field to map, so if anything I should be subtracting it, but that will just make it all even worse.

Struggling to wrap my head around this. What am I missing?

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4 Replies
ShaunDillon1
Occasional Contributor

Cant anybody answer this?

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MapleBake
New Contributor

I had the same issue with a magnetic compass overlay on Google Earth.  Did you ever figure out why??

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MapleBake
New Contributor

Maybe rotation is always counter-clockwise from 0 to 360 degrees?

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ShaunDillon1
Occasional Contributor

@MapleBake I think I found the answer a couple of days ago when this issue came up again. Embarrassingly enough I think there was already built a 15 degree declination into the actual symbol! The Triangle used as a pointer was angled at 345.

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