Buffers That Disagree With the Report
An analyst must identify parcels within 500 feet of a proposed pipeline. The parcels and pipeline are stored in a State Plane coordinate system in US survey feet. The analyst runs a Buffer tool on the pipeline with a distance of 500, then selects parcels that intersect the buffer, and reports 1,180 parcels. A colleague reruns the same logic but projects both layers to a Web Mercator coordinate system first, buffers 500, and gets 1,240 parcels. The analyst is asked to explain the discrepancy and produce a defensible number for a regulatory filing.
1. Explain the source of the 60-parcel discrepancy and state which result is defensible.
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The discrepancy is a projection artifact. Web Mercator is a conformal projection designed for web display; it grossly distorts distance and area away from the equator, and the distortion grows with latitude. Buffering 500 units in Web Mercator does not produce a 500-foot ground distance, so the second buffer is effectively a different size on the ground than the first. The difference in parcel counts is the expected consequence of buffering in a distorted space.
State Plane in US survey feet is a projected system designed for local distance accuracy in the region. Buffering 500 feet there is the defensible choice for a distance-based regulatory question, provided the analyst confirms the data's native units match the buffer units (US survey feet, not international feet) and that the coordinate system is appropriate for the area. The 1,180 figure is the one to defend; the 1,240 figure reflects a projection mistake, not a better analysis. Document the coordinate system and units in the filing so the method is reproducible.
2. What verification steps would you require before the 1,180 figure goes into the filing?
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Three checks. First, unit verification: confirm the layer's coordinate system reports US survey feet and that the buffer distance of 500 is interpreted in those units - a mismatch between survey feet and international feet produces a small but real error that matters at regulatory tolerance. Second, method verification: confirm the selection is 'intersect' against the buffer polygon and that the buffer was created with the intended end-cap and dissolve settings, since a dissolved buffer versus per-segment buffers can change which parcels are captured at corners and gaps. Third, spot-check: independently measure the ground distance from the pipeline to a handful of parcels near the 500-foot boundary and confirm they fall on the correct side. Also confirm the parcel layer has no geometry errors or slivers that would inflate the count. Only after these checks should the number be finalized.