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Obstructions and slopesAll possible problems caused by obstructions and slopes cannot be anticipated but the following hints may help.Use of parallel offsetsTo extend a line beyond an obstruction, a parallel line can be set out by offsets to get around the obstruction thenreturn to the original line. This technique, while useful, demands utmost care in setting-out the offsets as a smallerror can cause a large discrepancy at the line terminal.In the example, C and D are offset from A and B at right angles to AB. CD is extended to E and F beyond theobstruction. Finally, G and H are offset from E and F to bring the line back to the original direction.Note: The potential error is magnified because AB is short relative to the length of the obstruction. To mitigate theerror, another offset line should be set out on the opposite side of the obstruction as a check.Use of dogleg traverseAn alternative method is to use a simple dogleg traverse, noting the same need for accuracy and checking: set up instrument on B, sight on A, then turn the instrument (180° − α) onto an intermediate point C.Measure distance BC at C, set up instrument and sight on B, then turn an angle (180°+2α) and set out distance CD equal to BCto establish point D at D, sight on C, turn angle (180° − α) and set out point E (and hence extend the original line).Use of baselineInaccessible points can be set out from a known baseline by triangulation, for example across a river. In theexample, two stations A and B provide the baseline:Option A: via intermediate point set out a peg C close to where the setting-out point (X) is to be located with the instrument at A, observe the angle CBA with the instrument at B, observe the angle ABC with the instrument at C, observe the angle BCA sum the angle to ensure they add up to 180º calculate bearing and distance of X from C and set out as normalOption B: direct setting-outa) calculate bearing A to setting-out point C, B to setting-out point C, and also from A to Bb) at A, observe B and set bearing A to B on the instrument, turn instrument to bearing A to Cc) with the instrument locked, set out a marker at a short distance beyond point Cd) repeat step c) setting-out a point a short distance in front of point Ce) at B, repeat steps b) to d) substituting the bearings B to A and B to Cf) stretch string lines between the markers and where the lines cross marks point Cg) measure angle at C between B and A to ensure the triangle sums to 180ºh) check observations and adjust as necessary.Levelling on slopeOn a slope, keep the distance between back-sights and fore-sights reasonably equal and short to avoid introducingcumulative error. If levelling up the slope, ensure the line of collimation is not less than 0.5 m above ground level atthe proposed peg location.CIRIA C70951