Hello,
I'm having trouble understanding a large change in the coverage area generated by the Generate Travel Areas analysis tool (under Use Proximity) in Map Viewer.
We're generating overlapping individual polygons for 23 facilities within a county border because we want those individual drive time regions, but then we dissolve down to a single polygon and clip to the county border to calculate the area in square miles (done in ArcGIS Pro).
For several years the area of that polygon has been about 540 square miles. Then all of a sudden in February 2025 it dropped to 511 and was the same in July. As of this month it's 499 square miles.
I'm wondering if anyone can help me make sense of why there would be drops like these when you're using "time unspecified" as your arrival time. Certainly there's room for error when you start dissolving and otherwise working with the polygons, but you can see the shrinkage when you have the original versions of the newer feature layers overlapping the older feature layers in Map Viewer. The shrinkage is spread across the county, not just in any one particular area.
Both Map Viewer and Map Viewer Classic generate the same results. The results have been reproduced by a co-worker.
The drop in area translates to a fairly large drop in % of population within a 10-minute drive of a facility, and I'm trying to find a better explanation than "because the software says so" for the client.
Settings Being Used in MapViewer:
- Input layer: 23 points representing facilities within a county border
- Optional Barrier Layers: None
- Travel Mode: Driving time
- Cutoff: 10 minutes
- Travel direction: Toward Input Locations
- Arrival Time: Time Unspecified
- Overlap policy: Overlap
Any insights appreciated!
