A line's direction is reckoned from a north-south meridian. Meridian conversion is the process of changing the direction reference from one meridian to another. Because early surveys were compass-based, a traditional conversion is between the magnetic and true meridians. Most contemporary conversions involve true and grid meridians, sometimes geodetic, and let's not forget assumed. When connecting surveys together, particularly in a GIS, it is critical they all be referenced to the same meridian or apparant gaps and overlaps will occur.

 

Most meridians have specific characteristics that must be taken into consideration and some surveying activities require a specific meridian be used. Although simple, meridian conversion can easily be misapplied. The presentation will cover terminology, meridian definition, grid directions (including the Oregon Coordinate Reference System), and requisite information.