6. Fewer points are required for the same precisionin locating contours (only 50 to 60 percent of thenumber on a comparable transit-stadia survey arerequired to locate contours with the same degree ofaccuracy).Disadvantages of the plane table method are asfollows:1. The plane table and its plotting and drawingaccessories are more difficult to transport thantransit-stadia equipment.2. Weather not bad enough to rule out transit-stadiawill make plane-table work impossible.3. The use of the plane table is limited to relativelylevel, open country. It is unsuitable for wooded country.4. Control must be plotted in advance for precisework on the plane table.As mentioned above, keeping field notes is notmandated for plane-table topographic work; however,when notes are kept, they should appear as shown infigure 9-8. An explanation of the columns shown inthese notes is as follows:1. OBJ: Self-explanatory.2. ROD INT (S): Rod or stadia interval.3. H-SCALE: Reading from the Beaman archorizontal scale when the stadia interval was taken. (Inthis example, the stadia arc we are using is a horizontalscale subtraction type: vertical scale index = 50.)4. CORRECT H DIST: Corrected horizontaldistance. This distance is computed as explained inchapter 8 of this TRAMAN.5. V-SCALE: Reading from the Beaman arcvertical scale when the middle cross hair was sighted onthe rod and RC (column 7) was recorded. (Vertical scaleindex = 50.)6. PRODUCT *: Product difference; youcompute this by subtracting 50 from the V-scale(column 5) and then multiplying this difference by thestadia interval (column 2). Indicating the correct sign,+ or -, is very important.7. RC: Rod reading when the vertical scale wasread and the center cross hair was sighted on the rod.The RC is always negative because it is considered aforesight.8. DE*: Algebraic sum of columns 6 and 7.9. HI: Height of instrument obtained by addingbacksight reading to existing elevation.10. ELEV: Computed elevation; algebraic sum ofcolumns 8 and 9.11. REMARKS: Self-explanatory.When other types of alidades are used, you mayfind it necessary or advantageous to alter the formatof your field notes. Remember, too, that before youuse any instrument, including the alidade, you shouldalways read and fully understand the operatinginstructions for the instrument.Figure 9-8.-Plane-table notes.9-7
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