In the previous chapter, you studied the proce-dures used to perform topographic surveying using the transit-tape or transit-stadia methods. As you know, when either of these methods is used, a topographic map is prepared as a separate operation that uses the field notes from the survey to prepare the map. Another method used in topographic surveying and mapping is the plane-table method. This method is preferred by many surveyors since it combines the fieldwork and the office work into one operation that produces a completed, or nearly completed, map in the field. This chapter discusses the basic principles and procedures that you will use when performing plane-able topography.

Another topic discussed in this chapter is map projection. As you will learn, maps can be prepared using various projection methods to portray all or part of the earths surface on the flat plane of a map or chart. As an EA, you will seldom use most of these methods in drawing maps. However, it is important that you understand the principles of map projection so that you will be able to read and interpret accurately the various types of maps that you will use when plotting control points for surveys or when plotting fire missions as a mortar platoon member in a con-struction battalion.

As mentioned above, the plane-table method of topographic surveying and mapping combines field-work (surveying) with office work (drafting) to pro-duce a topographic map. This is so, because when you use plane-table equipment, topographic details are plotted directly on the map in the field. The plane-able method is advantageous in open country and when many irregular lines need to be plotted. It is also advantageous for small-scale mapping. There are, however, some disadvantages. For example, you are required to spend mom time in the field, more equipment (some awkward to handle) must be carried, and you will need mom time to become skilled in using the plane table. Other advantages and disadvantages of the plane-table method are discussed later in this chapter.

A plane-table field party for a large survey should consist of an instrumentman, a note keeper or com-puter, and one or more rodmen. The instrumentman operates the plane table and alidade, makes the observations, and performs the plotting and sketching. The note keeper reduces stadia readings to horizontal and vertical distances and computes the ground eleva-tions for rod observations. He also carries and positions an umbrella to shade the plane table. The rodman carries a stadia board or Philadelphia rod and holds it vertically at detail points and critical terrain features.

We will briefly explain the use of the plane table as follows. Take into the field a sheet of plane-table paper of suitable size and which has the control traverse (fig. 9-1) already plotted to suitable scale. Naturally, you use the same scale as the control traverse to lay off horizontal distances on the map. Attach the paper to the table. Then set up and carefully level the table so that D1 on the paper is directly over D1 on the ground. In this example, D1 is a point of known elevation (532.4 feet). Now the table must be oriented before any detail points can be located. In other words, the table has to be rotated or turned so that the points plotted on the plane-table sheet are in relationship to the corresponding points on the ground. So, with the edge of the alidade blade on D1 and the telescope trained on A, orient the table by rotating it to bring D1A on the paper in line with the

edge of the blade. A more in-depth discussion of orienting the plane table will follow later in this chap-ter. Next, carefully measure the vertical distance be-tween the horizontal line of sight through the telescope and the ground level at D1. Lets say this distance is 4.5 feet. This means that, whenever you sight on a rod, you will line up the horizontal cross hair with the 4.5-foot graduation on the rod.

Figure 9-2 is a sketch of the detail points that we are plotting. Point D1 and point A in this figure correspond to the same points in figure 9-1. Assuming that your alidade is equipped with a Beaman stadia arc (some alidades are not), plot point 1 of figure 9-2 in the following way. With the edge of the alidade blade exactly on D1 on the paper, train the telescope on a rod held on point 1, and line up the horizontal cross hair with the 4.5-foot mark on the rod.

Then, the elevation of point 1 is the elevation of D1 minus the difference in elevation, or

As you know, the difference in elevation was subtracted because the vertical angle was negative. Finally, with the edge of your alidade blade still on D] and your telescope still trained on point 1, you can draw a light line and measure off 622 feet from D1 along the line to locate point 1. At that distance along the line, mark and label the point and write in the elevation. Many topographers use the decimal point in the elevation to mark the point