Control Point Networks
In the process of fieldwork, one of the important decisions for the crew chief is where to establish control points. Modern instruments are electro optical, meaning that they measure positions by sighting features through a telescope. The instrument is on a tripod erected directly over the control point such that the instrument has the same coordinates as the control point, offset only by the height of the scope above the point. It doesn’t take a very long time to set up a modern instrument, but for a variety of practical reasons, we try to avoid an excessive number of control points. On the other hand, not enough or ineffectively placed control points can make it hard or impossible to make the required observations. Collectively, the survey control points comprise our “control point network”.
Control Point Best Practices
Line of sight between successive control points is critical, and the same is true of safe placement. We’d like to put our points in places where they have a good view of the area to be surveyed and are also not likely to be disturbed in the short term. In addition to site-specific factors, practiced surveyors also consider technical aspects of optical measurement. The adjacent picture shows a relatively open site bounded by a steep stone slope. One might think that the top of the slope is a good place to set up because the entire site would be in view. But, taking observations while looking down that slope is unwise because atmospheric refraction would influence the angular readings. This would be especially true on a warm summer day, when heat rising off the stone would exacerbate this natural light-bending phenomenon.