Question: The centerline of a pipeline may not be accurately determined via GIS or other method. The locations of structures (e.g., from aerial photography) may also involve inaccuracies. What provisions must be taken to address for inaccuracies in these measurements, in order to accurately determine the relative location of structures with respect to the pipeline?
Answer: The rule does not explicitly address mapping/measurement inaccuracies. Instead, it specifies the use of distances that apply to pipelines, and distances from those pipelines, as they actually exist in the field. The research behind the C-FER equation used to estimate potential impact circles was based on actual measurements of the distances affected by pipeline accidents.
PHMSA recognizes that mapping and measuring technologies involve some level of inaccuracy/tolerance. Operators must take these into account and consider the uncertainties in the distances they measure or infer when evaluating potential impact circles (PICs). Each operator's approach must be technically sound, must account for the uncertainties as they exist in the mapping/measurement methods used by the operator, and must be documented in its IM plan or related procedures. Operators may use a combination of techniques in order to account for these inaccuracies. For instance, aerial photography may be used as an initial screen. Field measurements (such as pipeline locators along with chainage measurements or survey quality range finders) may be used to verify if structures near the edge of the PIC (i.e., within the range of mapping/GIS inaccuracies) are actually inside or outside the PIC. PHMSA will inspect each operator's approach to assure that the operator's process is adequate to identify all covered segments.