Most well proposals are generated from rectangular coordinates derived from the UTM or local grid system. The surface location to target direction will therefore be referenced to Grid North. Since wells must be surveyed with sensors that reference direction to either Magnetic or True North, it will be necessary to convert between these references.
Long.
Lat.
92° 301 w 30° 401 N 91° 201 28° 40' 91° 201 25° 40'
Magnetic declination correction converts azimuth values between the Magnetic North and True North systems. The magnetic declination correction is the angle between the horizontal component of the earth's magnetic field lines and the lines of longitude. When Magnetic North lies to the west of True North, the magnetic declination is said to be westerly, and if to the east, easterly.
Values of magnetic declination change with time and location. Magnetic Declination models are updated every year. Their values and rates of change can be obtained from Computer programs like GEOMAG or "world magnetic variation charts" or "isogonic charts" which are issued by all major hydrographic institutes in the world once every five years (1980, 1985, '90, etc.). Computer programs like GEOMAG use current magnetic models and calculate up-to-date local declination figures. The most accurate method to determine local declination is to measure the magnetic field with a magnetic transit.
When magnetic results are recorded, the declination and the date must be included. Local values of magnetic declination should be stated in the well program to plus or minus 0.1O.
Grid Correction Angle. A grid correction converts azimuth readings between the True North systems and the specified grid system. The angle of correction is the angle between the meridians of longitude and the Northings of the grid system at a specified point. The magnitude of the correction angle depends upon its location within the grid and its latitude. The closer the point is to the grid central meridian and to the equator, the smaller the correction.
The computation of the grid correction angle or angle of convergence will require special mathematical techniques depending on the type of projection of the curved earth's surface on to the flat grid. The directional software packages will at minimum handle UTM and Lambert conformal conic convergence. The chosen sign convention displays Grid North as "x" number of degrees east or west of True North. For example, when you convert the geographic coordinates latitude N 30° 00' 00" and longitude W 95° 00' 00" to UTM coordinates (using the Hayford Inter-national - 1924 Ellipsoid), the computer will display the following results:
UTM Coordinates: Hemisphere = North Zone = 15
Northing = 3320517.348
Easting = 307077.096
This listing indicates a grid convergence of 1o 00' 00". Grid convergence as calculated by the directional software package is the angular difference in degrees between True North and UTM Grid North. UTM Grid North is said to be "X" number of degrees either east or west of True North. When working with the UTM system, the calculated direction between two UTM coordinates is referenced to Grid North. To convert this UTM Grid North direction to a True North direction, you must apply the grid convergence to the calculated UTM Grid North direction. This sign convention is not necessarily the same for all contractors and should be clearly communicated and understood before drilling begins.
Once accurate magnetic declination and grid convergence angles are acquired, all that is needed to change reference systems is to add or subtract these angles from one another. While this seems a simple task, misunderstandings surrounding the relationship between these references can cause a target to be missed. To avoid this confusion, declination/grid conversion polar diagrams should be drawn on all maps and clearly defined on all survey printouts. With this in mind, the following procedure is suggested:
True North azimuth will equal 90°; Magnetic azimuth will equal 90° plus/minus declination; Grid azimuth will equal 90° plus/minus grid convergence. With these three references it is a simple matter to determine whether declination and/or convergence need to be added or subtracted to switch from one system to the other.
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