Ring Analysis - what geographic level is aggregated?

SherrieTaylor

I am conducting a ring analysis of 25 miles of a point.  The aggregations are way off, by 33%, compared to the Census Bureau employment numbers at the same radii.  I assume it has to do with the geographic aggregation level, but I can't find the documentation that states how a ring analysis is aggregated.  Can anyone help?  Is it by tract or block group????
Thanks much!
Sherrie

Jason_RobinsonRobinson

I am conducting a ring analysis of 25 miles of a point.  The aggregations are way off, by 33%, compared to the Census Bureau employment numbers at the same radii.  I assume it has to do with the geographic aggregation level, but I can't find the documentation that states how a ring analysis is aggregated.  Can anyone help?  Is it by tract or block group????
Thanks much!
Sherrie

Sherry,

As you already suspected the difference is likely due to differences in spatial apportionment methods.  What BAO uses is for most intents and purposes uses the Hybrid method of data apportionment that is detailed in the online help for Business Analyst 10 Desktop (where one has the ability to select three preset types as well as the ability to modify each).  Here is a link: http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/Data_tab/000z000000v7000000/

You can add or subtract levels by clicking the plus or minus buttons. Click on any calculation method to change the data calculation method to BlockApportionment (block apportionment) or CentroidsInPolygon (cascading centroid). Click on a data layer to change the data layer. For example, you can change BlockGroups to ZIP Codes. The counties layer is the largest boundary option for data retrieval.

The numbers in the distance Advanced Tolerance Settings dialog box image above correspond with the image below.

20 miles - Block Group - block apportionment is indicated in the yellow area.

100 miles - Block Group Centroids is indicated in the red area.

200 miles - Census Tract Centroid is indicated in the dark blue area.

400 miles - ZIP Code Centroids is indicated in the light blue area.

More than 400 (or larger as indicated in the Advanced Tolerance Setting dialog box image above) - County Centroids is indicated in the off white area.

The distance threshold is calculated using the maximum value of a trade area's height and width. Unlike the example above, many trade areas are not perfect circles but are irregular shapes. The example below shows how a maximum value is calculated for an irregularly shaped trade area.

To determine the distance threshold, a polygon is drawn around the outermost points of a trade area. The greater value between width and height of the polygon is used as the distance threshold figure. In the example below, 80 is the greatest value. The 80 mile distance falls between the 20 and 100 mile thresholds meaning block group centroids are retrieved from the trade area. Each centroid contains demographic information used to retrieve data.

Regards,

Jason R.