Some typical mistakes made in plane-table work are as follows:

DEVELOPMENT OF A TOPOGRAPHIC MAP

In this final section on topography, we will discuss the typical steps leading to the production of a topographic map. In this discussion, you should notice the different operations that are commonly involved and how those operations interplay with one another. In developing a topographic map, you should first gather all available maps, plans, survey data, and utilities data that pertain to the site and study them carefully. Consider the boundaries of the site in relation to the intended use of the topo map. If the map is to be used for design purposes, certain off-site information will be even more important than on-site details; for example, the location and elevations of utilities and nearby streets are vital. The location of drainage divides above the site and details of outfall swales and ditches below the site are necessary for the design of the storm drainage facilities. Topographic details of an off-site strip of land all around the proposed limits of construction are necessary so that grading can be designed to blend with adjacent areas. Decide what datum and bench marks are to be used; consider previous local surveys, U.S. Coast and Geodetic Survey (USC&GS) monuments, sanitary sewer inverts (not rimsthey are frequently adjusted), and assumed datum. Determine whether there is a coordinate system in the area monumented sufficiently for your use; if not, plan to use assumed coordinates. In the latter case, decide on the source of the meridian: adjacent surveys, magnetic, assumed, or shooting the Sun or Polaris (discussed at the EA1 level in Part 2 of this TRAMAN).

Next, perform a reconnaissance survey. Observe the vegetation and decide how many men that you, as party chief, will need to cut brush. Select main control traverse stations at points appropriate for plane-table setups. Decide on the number and location of crossties or secondary traverse lines needed to provide sufficient plane-table stations. Select these points so that plane-table setups will have to be extended only a minimum distance before checking back into control.

The next step is to run the traverse lines; you should check their directions from time to time, where necessary, on long traverses. Checks could be done by astronomical methods (Part 2 of this TRAMAN), by cutoff lines, or by connecting the traverse with established points. Then run the levels, taking elevation on all traverse stations. Close, balance, and coordinate the main traverse. Then adjust the crossties into the main traverse. Balance the levels. Plot the traverse stations by coordinates on the plane-table sheets. Be sure that each sheet overlaps sufficiently. Also, be sure there is sufficient control on each sheet for orientation and for extension of setups (if necessary). Number the traverse stations with the same numbers marked on the guard stakes in the field, and show the elevations.

The plane-table work is the final big step of the fieldwork, but some transit and level work may still need to be done. The location of some details (such as street center lines or buildings) may need to be more precise than the precision obtainable with the plane table; tie in such details to the traverse by transit tape survey. For design purposes, the elevation of some points (such as the inverts of culverts, paved flumes, sewers, and tops of curbs and gutters) may need to be more precise than the precision obtainable with the plane table. Use the level to obtain such elevations. The final step in the production of the topographic map is, of course, tracing the information from the plane-table sheets onto the final drawing.

Random traversing, as previously described, is not the only way of establishing horizontal control. Grids are frequently used. One good way of identifying grid lines is to assign a letter to each line in one set and then run stationing along each line. Another method is described in the paragraphs below.

Referring to figure 9-10, suppose that this site has been chosen (through reconnaissance) for an advanced base with airstrip facilities. As you see in figure 9-10, there is a sheltered water area for a potential harbor; a strip of woodland extending back from the shore; and then a strip of clear, level country where an airstrip could be constructed.

Although topographic data for a map of this area could be obtained by one field party, it would involve extensive time and effort. Therefore, lets assume that three field parties will be used. Two of these parties are 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-table party will work in the clear area and the transit-level parties will operate in the wooded and the water areas. Basic horizontal control for both the plane-table party and the transit-level parties is the main base line, which is run along the edge of the wooded area as shown in figure 9-10. Topographic details in the clearing will be plotted from plane-table stations tied to the main base line. Details in the wooded area and offshore 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 the following manner: transit-level party No. 1 runs the main base line from station 0 + 00, located at random. While running the main base line, hubs are set along

the line at predetermined intervals; in this case, at every 500-foot station. Transit-level party No. 2 runs a lateral base line from 0 + 00 perpendicular to the main 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 main base line. Likewise, from each station on the lateral base line, party No. 2 will run a longitudinal, perpendicular to the lateral base line (and therefore parallel to the main base line). Hubs are driven at the intersection of each lateral and longitudinal (except in the water area). As you can see in figure 9-10, it is these lateral and longitudinal lines that form the grid net work.

From your previous studies you know that points within the grid can be located by coordinates, using the main base line as the X axis and the lateral base line as the Y axis; for example in terms of stations, the X coordinate of point A in figure 9-10 is 15 +00 and the Y coordinate is 10 + 00. For simplicity, these coordinates can be stated in a fractional form as 1500/1000.

With regard to vertical control for a advance base site such as we are discussing, there may be no established bench marks in the immediate area. In this case, a level net may have to be run from an established monument some distance away, perhaps several miles, to establish a bench mark in the area. If this is not possible, then a series of rod readings should be taken over a succession of high and low tides or on the high-water mark wash line along the beach. You may then use the average of these readings as a temporary vertical control datum until a more accurate datum is obtained from tide gauge readings. From a temporary bench mark at or near the beach, a line of levels can be run to station 0 + 00 on the main base line. Temporary elevations of hubs on the main base line and the lateral base line can then be determined. Finally, the transit-level parties will shoot the detail in the vicinity of each of the intersecting grid lines.