COMPOUND AND REVERSE CURVES
Although equations are available which solve compound curves and reverse curves , they are difficult to remember so it is best to treat the problem as two simple curves with a common tangent point t.
In the case of the compound curve, the total tangent lengths T1I and T2I are found as follows:
R1 tan1/2 = T1t1 = t1t and R2 tan2/2 = T2t2 = t2t, as t1t2 = t1t t2t then triangle t1It2 may be solved for lengths t1I and t2I which, if added to the known lengths T1t1 and T2t2 respectively, give the total tangent lengths.
In setting out this curve, the first curve R1 is set out in the usual way to point t. The theodolite is moved to t and backsighted to T1, with the horizontal circle reading (180◦−1/2). Set the instrument to read zero and it will then be pointing to t2. Thus the instrument is now oriented and reading zero, prior to setting out curve R2.
In the case of the reverse curve, both arcs can be set out from the common point t