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Why is the service area polygon different when adding extra breaks?
When performing a service area analysis, default breaks can be set in Analysis Settings to generate service area for one or multiple breaks. Service area polygons are generated for the breaks if the Generate Polygons option is checked. The service area polygon can be trimmed by checking the Trim Polygons option under the Polygon Generation tab.
In some cases, when Trim Polygons is checked and a trim distance is set, the service area polygon with one break (for example, a 5-minute break) displays differently compared to the polygon for the 5-minute break in a service area with multiple breaks (for example, 5, 6, and 7 minute breaks).
When trimming service area polygons, the trim value only applies to the outermost polygon. Therefore, when there is only one break value, the trim is applied to the one break. When there are multiple breaks, the trim is not applied to the 5-minute break. As a result, the 5-minute polygon generated with only one break appears to be more detoured than the polygon generated with multiple breaks.
For more information, refer to the following ArcGIS Web Help documentation, Service area analysis.
Use one of the following workarounds to obtain service area polygons with multiple breaks that are similar to service area polygons generated with one break.
Uncheck the Trim Polygons option so that the service area polygons generated are more generalized and similar when using more than one break. There is a slight difference in the result, as disabling the trimming operation affects the polygon generation routine used to carve out regions encompassing areas in-between the break values. To uncheck the Trim Polygons option, follow the instructions below:
Enable the trimming operation and generate a polygon for each single break. After the polygons are created, overlay the individual service area polygons.