Declination Diagram - add or subtract declination?
Hey everyone. I'm kind of confused about the declination diagram. I know how to add a declination diagram in ArcGIS and how to find the declination degree. I'm making a map for a location in South Carolina. In the location I'm at the declination is 5 degrees. I just don't understand the adding and subtracting part. In the location I'm in do I add the GM-angle or subtract it? Thanks!
Mapping Center Answer:
Attached is an isogonic map of the conterminous United States showing lines of constant angular distance between true north and magnetic north (this is from Chapter 12, "Direction Finding and Compasses", in Map Use, Sixth Edition). You will see that South Carolina is roughly located near the 5 degrees west line.
Since the change is in the westerly direction, you would subtract this from true north. That is to say, when you have a west declination you subtract the declination angle from the true azimuth to get the magnetic azimuth. This is not, however, related to the grid north angle – that is the angle of north in relation to the grid coordinate system on the map. Not having the angle in your question, I can't comment on whether it should be added or subtracted.
You can read more about this subject in our "Adding a Declination Diagram in ArcMap" blog entry here on Mapping Center.
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