Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . point b and resecting on B, a line drawn along the edge ofthe rule will intersect the line ^r in the undetermined points:.A more dif^cult case of making a location by the two-poititproblem is the following: Two points A and B (Fig. 63), notconveniently accessible, being located on the paper at a and b,it is desired to put the plane-table in position at a third point,C. A fou-ti point, D, is selected, such that the intersectionfrom C and D upon A and B make
Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . point b and resecting on B, a line drawn along the edge ofthe rule will intersect the line ^r in the undetermined points:.A more dif^cult case of making a location by the two-poititproblem is the following: Two points A and B (Fig. 63), notconveniently accessible, being located on the paper at a and b,it is desired to put the plane-table in position at a third point,C. A fou-ti point, D, is selected, such that the intersectionfrom C and D upon A and B make sufificiently large angles forgood determinations. Put the table approximately in positionat D, by estimation or compass, and draw the lines Aa, Bb,intersecting in d\ through d draw a line directed to C. Thenmove to and set up at C, and assuming the point c on theline dC, at an estimated distance from d, and putting the 194 PLANE-TABLE TRIANGULATION. table in a position parallel to that which is occupied at D, by-means of the line cd draw the lines from c to A, and from c toB. These will intersect the lines dA, dB at points a and b\. Fig. 63.—Two-point Problem. which form with c and d a quadrilateral similar to the trueone, but erroneous in size and position. The angles which the lines ab and ah make with eachother is the error in position. By constructing now throughc a line cd making the same angle with cd as that which abmakes with ab\ and directing this line cd to D, the table willbe brought into position, and the true point, c, can be found bythe intersections of aA and bB. Instead of transferring theangle of error by construction, it may be convenient to pro-ceed as follows, observing that the angle which the line abmakes with ab is the error in the position of the table. Asthe table now stands, db is parallel \\\\.\\AB, but it is desiredto turn it so that ab shall be parallel to the same. If, there-fore, the alidade be placed on ab and a mark set up in thatdirection,
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