#### Rules for Construction

A valid starting point and vector for will follow two rules:

First, a circle of the given diameter and starting point should not intersect the scan. Figure 2 shows a violation of this rule.

Figure 2: Invalid starting point due to intersection with the scan. | A - Scan B - Start Point C - Down Vector

Second, the circle projected from the start point along the down vector must intersect the scan. Figure 3 shows a violation of this rule.

Figure 3: Invalid starting point due to the circle missing the scan. | A - Scan B - Start Point C - Down Vector