Changes

* ''i'' = survey point number (i = 0 at surface)

* ''i'' = survey point number (i = 0 at surface)

The intervals can be defined in several ways depending on the accuracy and simplicity of calculation required. The ''tangential'' or ''terminal angle method'' (Figure 4a) assumes a constant deviation for the entire interval from one survey point to the next. Thus, the measured depths (MD_''i'', MD_''i''–1) for each interval coincide with the depth at the survey points, and the angle used would be for the lower survey point. Although easy to calculate, this method is likely to be substantially in error and is generally not recommended<ref name=pt06r20>Craig, J. T. Jr., Randall, B. V., 1976, Directional survey calculation: Petroleum Engineer, March, p. 38–54.</ref><ref name=pt06r57>Inglis, T. A., 1987, Directional Drilling, Petroleum Engineering and Development Studies, Volume 2: London, Graham and Trorman, chap. 9, p. 155–171.</ref>. It is mentioned here for historical reasons, as it has been widely used. Alternatively, the ''angle averaging method'' (Figure 4b) uses the average for the two survey points at either end of the segment.

The intervals can be defined in several ways depending on the accuracy and simplicity of calculation required. The ''tangential'' or ''terminal angle method'' (Figure 4a) assumes a constant deviation for the entire interval from one survey point to the next. Thus, the measured depths (MD_''i'', MD_''i''–1) for each interval coincide with the depth at the survey points, and the angle used would be for the lower survey point. Although easy to calculate, this method is likely to be substantially in error and is generally not recommended.<ref name=pt06r20>Craig, J. T. Jr., Randall, B. V., 1976, Directional survey calculation: Petroleum Engineer, March, p. 38–54.</ref><ref name=pt06r57>Inglis, T. A., 1987, Directional Drilling, Petroleum Engineering and Development Studies, Volume 2: London, Graham and Trorman, chap. 9, p. 155–171.</ref> It is mentioned here for historical reasons, as it has been widely used. Alternatively, the ''angle averaging method'' (Figure 4b) uses the average for the two survey points at either end of the segment.

[[file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig4.png|thumb|{{figure number|4}}Linear approximations of a curved well course by the (a) tangential method<ref name=pt06r20 /><ref name=pt06r23>Dailey, P. 1977, A guide to accurate wellbore survey calculations: Drilling-DCW, May, p. 52–59 and 118–119.</ref>, (b) angle averaging method<ref name=pt06r20 />, and (c) balanced tangential method<ref name=pt06r20 />.]]

[[file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig4.png|thumb|{{figure number|4}}Linear approximations of a curved well course by the (a) tangential method,<ref name=pt06r20 /><ref name=pt06r23>Dailey, P. 1977, A guide to accurate wellbore survey calculations: Drilling-DCW, May, p. 52–59 and 118–119.</ref> (b) angle averaging method,<ref name=pt06r20 /> and (c) balanced tangential method.<ref name=pt06r20 />]]

A better approximation, the ''balanced tangential method'' (Figure 4c), is derived by placing the interval depths (MD_i, MD_''i''–1) half way between the individual survey points, thus assuming that the deviation is constant in an interval around the measured point.

A better approximation, the ''balanced tangential method'' (Figure 4c), is derived by placing the interval depths (MD_i, MD_''i''–1) half way between the individual survey points, thus assuming that the deviation is constant in an interval around the measured point.