Airfield construction is of a special kind; for this reason, it is discussed here under a separate heading.

The route for an airfield is the horizontal location of the runway center line; if there is more than one runway, there is, of course, more than one route. The principal consideration regarding the direction of a runway center line is the average direction of the prevailing wind in the area, since planes must take off into the wind. The azimuth of the center line will be as nearly as possible the same as the average azimuth of the prevailing wind. A study of the meteorological conditions is therefore a part of the reconnaissance survey. Other data gathered on this survey (which may be conducted on foot, by ground surface vehicle, by plane, or by all three) include the land formation, erosional markings, vegetation, configuration of drainage lines, flight hazards, approach zone obstructions, and soil types.

From the reconnaissance data, one or more preliminary center lines are selected for location by preliminary survey. For quick preliminary stakeout, there may be two parties, working away from station 0 + 00 located at the approximate midpoint of the center line. In such cases, stations along the azimuth may be designated as plus and those along the back azimuth as minus.

Level parties follow immediately behind the transit parties, taking profile levels and cross sections extending the width of the strip, plus an overage for shoulders and drainage channels. From the preliminary survey data, a plan and profile are made of each tentative location, and from these, a selection of a final location is made.

Airfield runways, taxiways, hardstands, and aprons are staked out much as a highway is staked out. There are, however, certain special considerations applying to approach zones.

As you know from chapter 3, an approach zone is a trapezoidal area beyond the end zone at each end of a runway. It must be free of obstruction not only on the ground but also off the ground at a specific glide angle. The size of the approach zone depends on the type and stage of development of the field. For permanent naval air stations, the trapezoidal area might be 10,000 feet long with a width of 4,000 feet at the outer end. For purposes of explanation only, we will assume that these are the dimensions of the approach zone for which you are surveying. The glide angle for most types of aircraft is 2 percent, usually given as 50:1, or a rise (or drop) of 1 vertical for 50 horizontal. Figure 10-25 shows, in plan, profile, and isometric, an approach zone and its adjacent transition surfaces and end of runway. You must stake out this approach zone and check it for clearance by the following procedure:

Figure 10-26 shows the approach zone in plan. The dotted line BC lies 750 feet from the center line. The angle at B can be determined by solving the

triangle CBD, tan B = 1,250/10,000, or 0.125000; therefore, angle B measures 7񴭵0创. Determining the distance from the dotted line to the edge of the approach zone at any station is similarly a simple right-triangle solution. Suppose that AB is located at station 0 + 00. Then at station 1 + 00, the distance from the dotted line to the edge of the approach zone is 100 tan 7񴭵0创, or 12.5 feet; therefore, the distance between the center line and the edge of the approach zone at this station is 750 + 12.5, or 762.5 feet.

To check for obstructions, you must setup a transit at the narrow end of the approach zone, set the telescope at a vertical angle equal to the one that the glide plane makes with the horizontal, and take observations over the whole approach zone, as indicated in figure 10-27. Determining the vertical angle is a simple right-triangle solution. If the glide angle is 50:1, then the tangent of the vertical angle is 1/50, or 0.020000, and the angle measures 1񵁢5创. Figure 10-27 shows how the exact vertical location of the glide plane varies with the character of the surface of the end zone.

Under some circumstances it is possible to chain distances over the water; however, it is usually more

to triangulate offshore distances from a convenient line. No matter how you get offshore shore base distances, however, offshore points cannot be marked like ground points with hubs or stakes. Therefore, in the location of offshore points, there must usually be coordination between a survey party on the beach and a party afloat.