Referring to figure 9-10, suppose that this site hasbeen chosen (through reconnaissance) for an ad-vanced base with airstrip facilities. As you see infigure 9-10, there is a sheltered water area for a poten-tial harbor; a strip of woodland extending back fromthe shore; and then a strip of clear, level country wherean airstrip could be constructed.Although topographic data for a map of this areacould be obtained by one field party, it would involveextensive time and effort. Therefore, let’s assume thatthree field parties will be used. Two of these partiesare transit-level parties since they will use either tran-sits or levels as appropriate to the work performed.The third party is a plane-table party. The plane-tableparty will work in the clear area and the transit-levelparties will operate in the wooded and the water areas.Basic horizontal control for both the plane-tableparty and the transit-level parties is the main baseline, which is run along the edge of the wooded areaas shown in figure 9-10. Topographic details in theclearing will be plotted from plane-table stations tiedto the main base line. Details in the wooded area andoffshore will be plotted from stations on a grid net-work that is tied to the main base line.The grid network can be established in thefollowing manner: transit-level party No. 1 runs themain base line from station 0 + 00, located at random.IWhile running the main base line, hubs are set alongFigure 9-10.—Advanced base site.the line at predetermined intervals; in this case, atevery 500-foot station. Transit-level party No. 2 runsa lateral base line from 0 + 00 perpendicular to themain base line and sets hubs at every 500-foot station.From every 500-foot station on the main base line,party No. 1 will run a lateral, perpendicular to the mainbase line. Likewise, from each station on the lateralbase line, party No. 2 will run a longitudinal,perpendicular to the lateral base line (and thereforeparallel to the main base line). Hubs are driven at theintersection of each lateral and longitudinal (except inthe water area). As you can see in figure 9-10, it isthese lateral and longitudinal lines that form the gridnet work.From your previous studies you know that pointswithin the grid can be located by coordinates, usingthe main base line as the X axis and the lateral baseline as the Y axis; for example in terms of stations, theX coordinate of point A in figure 9-10is 15 +00 andthe Y coordinate is 10 + 00. For simplicity, thesecoordinates can be stated in a fractional form as1500/1000.With regard to vertical control for a advance basesite such as we are discussing, there may be noestablished bench marks in the immediate area. In thiscase, a level net may have to be run from anestablished monument some distance away, perhapsseveral miles, to establish a bench mark in the area. Ifthis is not possible, then a series of rod readings shouldbe taken over a succession of high and low tides or onthe high-water mark wash line along the beach. Youmay then use the average of these readings as atemporary vertical control datum until a more accuratedatum is obtained from tide gauge readings. From atemporary bench mark at or near the beach, a line oflevels can be run to station 0 + 00 on the main baseline. Temporary elevations of hubs on the main baseline and the lateral base line can then be determined.Finally, the transit-level parties will shoot thedetail in the vicinity of each of the intersecting gridlines.MAP PROJECTIONNow let’s discuss map and chart projection. Thisdiscussion includes the characteristics anddevelopment of various types of projections.A paper cylinder (without ends) and a paper conecan be cut along the side and flattened out withoutdistortion. For this reason, the two most common basicprojection methods are the Mercator, in which the9-10
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