Figure 3-1 Ideal Checkpoints
Likewise, a road crossing the planned track at 45º does not give a good track or time information
unless there is a distinctive bridge or intersection to mark a specific point along that road.
Some additional features to remember when selecting checkpoints:
Inferred bridges crossing double-lined rivers are a minimum of 600 feet in length.
Where primary roads cross interstate highways, an overpass or bridge is generally present.
Multiple tower annotations can represent two, three, or even twenty towers, making
accurate position fixing difficult.
3-2 VISUAL FIXING