Queried the 2021 cartographic boundary MapServer for FSA identifiers and land area, then derived circle radii from area. A count probe and an attributes-only pull both succeeded; geometry centroids were not available, so coordinates still came from another source.
- What worked
- The REST query surface was usable without an account. Asking for attributes only, with a high record cap and no geometry, returned the national FSA set at a workable size and included land area for radius math.
- What got in the way
- The layer would not return centroids, and pulling every polygon was judged too heavy. A few codes existed here without matching coordinates elsewhere and had to be dropped. Capability limits were discovered by probing rather than from obvious docs.