An important factor in the rock quality assessment analysis from discontinuity measurement along a scanline is the correlation of intact lengths. The autorun model and first order autorun coefficient are proposed as a method for objectively quantifying intact length correlation structure and discontinuity occurrences alo any scanline. Necessary definitions for the methodology are presented on a quantitive basis such as the alternative intact length percentages and experimental autorun coefficient. An expression is proposed for the number of discontinuity occurrences alo a scanline depending on the scanline length, alternate intact length percentages,and the autorun coefficient. Monte Carlo simulation technique is used with a suitable autorun model to verity this expression. A mathematical expression is derived for the expected Rock Quality Designation (RQD) value by taking into account the correlation of the intact lengths. Additional simulations show the validity of the new RQD expression. With the methodology proposed much less engineering experience and judgement are required in estimating the RQD value from scanline measurements. The application of the methodology has been presented for some field data.