Grid vs. True North Coordinates

When the curved earth is projected onto a flat piece of paper, two reference frames exist. The true north reference frame uses the latitude-longitude coordinates of the curved earth as the reference axes.  In this coordinate system the longitudes (Y-axis) converge upon a single point, the Earth's rotational pole.  The longitude line is the 0^o azimuth direction or north direction.  In the grid system (state plane or UTM coordinates) the Y axis is the direction of 0^o azimuth and does not converge to a singe point.  In most map projections, these two north reference directions are not the same (see the figure below), and differ by a rotation through an angle called the convergence angle.

Before a directional survey can be plotted or loaded into many workstation applications, the user must ascertain which reference system was used to report the survey and which reference system is used by their application.   Directional surveys can be reported with respect to either grid north or true north coordinates.  The differences in bottom hole location can be significant.  For example, in east Texas and the High Island area offshore the bottom hole locations differ by 220' for a well with a 5000' offset, more than enough to miss a structure or lease line.  

Onshore data is usually reported with respect to true north 0^o azimuth whereas in the Gulf of Mexico, the grid north direction (Y-axis) is usually 0^o azimuth. In addition, different geoscience applications require data with respect to different north references. IESX, Seisworks, Z-map and CPS require grid north coordinates whereas Geographix, Intellex, and most corporate databases require true north coordinates.

John Sharry can help you clean up your data so you know where your wells are.

© 1999 John Sharry
e-mail: jsharry@ix.netcom.com
2/10/99