Checking Plot and Developing a Map


Once you have your plot laid out on paper, the next step is to check your work.  As you plot all your points you will eventually return to the point that you started, since when you did your survey you also started and ended in the same spot.  However, it is probable that the plot will not return to the same spot.  In figure 2, the dotted line represents the plotted data as it was collected.  The segment AA’ is the “Closing error” and can be fixed.  This is done by extending the segment AA’ to a point P.  Then a line perpendicular to PA is drawn to a point Q.  Next the segment AP is extended by the same length as AA’.  From this point (a’) another line is extended to Q.  Now along the lines PQ and a’Q scale off the lengths of your segments in proportion to their measured lengths.  As in figure 2, this results in segments dd’, cc’, and bb’.  These segments represent both the length and direction that each point should be adjusted in order to create a higher quality graph.  Of note, this process does not eliminate the errors that were generated during the survey; it just distributes them evenly throughout the plot.

After a corrected plot has been generated, a clean sheet of paper can be overlaid and the points and NS line traced from below.  Onto this piece of paper can be added elevations, as well as any surface data such as building or river placement.  To make a topographical map areas with the same elevation are connected together forming lines of the same height above the datum (normally sea level).  These lines can never cross, intersect or diverge.

Figure 2