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VALUES OF PLANE-TABLE METHOD Advantages of the plane-table method of topographic surveying are as follows:1. The map is made directly in the field, thus combining the data collection and drafting into a single operation. The area under survey is visible as a whole, which tends to minimum the overlooking of important data. Errors in measurement maybe easily checked by taking check observations on a prominent point whose position has been plotted on the map. If the edge of the blade does not contact the proper point or points, an error is indicated. An error thus located can be easily corrected on the spot.2. Since all computation and plotting is performed in the field, the keeping of field notes is not a mandatory requirement in plane-table topography; the decision is left up to your supervisor; however, plane-table field notes are useful as a training device. You should keep this in mind when, later in your career, you are training junior EAs in plane-table work.3. The graphic solutions of the plane table are much quicker than the same solutions by methods requiring angular measurements, linear measurements, and computations. Thus a great deal more area can be covered in much less time. 4. When the country is open and level, the plane-table topographer has a wider choice in the selection of detail points. He need not be hampered by backsight-foresight requirements. He can locate inaccessible points easily by graphic triangulation or quickly determine the location of a point with reference to one, two, or three points of known location.5. Irregular lines, such as streams, banks, and contours, can be sketched.
Figure 9-7.-Progression. 6. Fewer points are required for the same precision in locating contours (only 50 to 60 percent of the number on a comparable transit-stadia survey are required to locate contours with the same degree of accuracy). Disadvantages of the plane table method are as follows:1. The plane table and its plotting and drawing accessories are more difficult to transport than transit-stadia equipment.2. Weather not bad enough to rule out transit-stadia will make plane-table work impossible.3. The use of the plane table is limited to relatively level, open country. It is unsuitable for wooded country.4. Control must be plotted in advance for precise work on the plane table.As mentioned above, keeping field notes is not mandated for plane-table topographic work; however, when notes are kept, they should appear as shown in figure 9-8. An explanation of the columns shown in these notes is as follows: 1. OBJ: Self-explanatory. 2. ROD INT (S): Rod or stadia interval. 3. H-SCALE: Reading from the Beaman arc horizontal scale when the stadia interval was taken. (In this example, the stadia arc we are using is a horizontal scale subtraction type: vertical scale index = 50.) 4. CORRECT H DIST: Corrected horizontal distance. This distance is computed as explained in chapter 8 of this TRAMAN.5. V-SCALE: Reading from the Beaman arc vertical scale when the middle cross hair was sighted on the rod and RC (column 7) was recorded. (Vertical scale index = 50.)6. PRODUCT : Product difference; you compute this by subtracting 50 from the V-scale (column 5) and then multiplying this difference by the stadia interval (column 2). Indicating the correct sign, + or -, is very important.7. RC: Rod reading when the vertical scale was read and the center cross hair was sighted on the rod. The RC is always negative because it is considered a foresight.8. DE: Algebraic sum of columns 6 and 7. 9. HI: Height of instrument obtained by adding backsight reading to existing elevation.10. ELEV: Computed elevation; algebraic sum of columns 8 and 9.11. REMARKS: Self-explanatory. When other types of alidades are used, you may find it necessary or advantageous to alter the format of your field notes. Remember, too, that before you use any instrument, including the alidade, you should always read and fully understand the operating instructions for the instrument.
Figure 9-8.-Plane-table notes. |
 
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