In Accessible Areas and Lengths

 

In the case of two survey points, which can be seen from one another but are not able to be directly measured, one can follow this procedure.  In the case of a pond or other such obstacle which can be circumnavigated, perpendicular segments are made to the line of sight until a parallel line can be measured.  In figure 3 the view from A to B is not obstructed, but there is a pond in the way so the distance cannot be measured.  In this case two segments perpendicular to AB were made until the segment EF, which is parallel to AB was able to be created.  EF was subsequently measured and added to the lengths of AC and DB.

In the case of  two survey points which can be seen, but the distance between them can not be measured, and the intervening obstacle not be navigated around, a little math is needed.  Figure 4 shows an example of how the measurement of AB can be completed.  For this example the points G and C are chosen along the segment AB.  Then two lines are extended perpendicular from these points to points E and F.  Point H is found by forming a line of points E and F and then extending this to intersect line AB.  Next segments BH,GC,CA,EG, and FC are measured.  The distance from A to be can be found using the following formula:

Figure 3

Figure 4