Dasymetric Mapping (pro rata)

 

Introduction

 

You will use a dasymetric mapping technique in ArcGIS to change the spatial unit for mapping population distribution. The original population map usually uses census enumeration unit (e.g., blocks, block groups, or tracks) as the mapping unit. In many applications, you need to estimate the population (or other census variables) of a different spatial extent, such as watershed boundaries or marketing areas. This requires the use of pro rata dasymetric mapping, i.e., a technique for areal interpolation, to disaggregate and re-aggregate the census data. The assumption of pro rata dasymetric mapping is that population is evenly distribution within the enumeration units. If you know this is not true for your study area, you should consider the intelligent dasymetric mapping technique, which needs ancillary information to characterize the finer spatial distribution of population. The pro rata dasymetric mapping technique has three main steps:

 

1)      Calculate the population densities within enumeration units.

2)      Disaggregate the enumeration units and calculate the population of each new unit based on its area and the population density.

3)      Aggregate these new units to form your target area and calculate the aggregated population.

 

The procedures below show you how to implement the pro rata dasymetric mapping in ArcGIS.

 

Instructions

 

The sample data are available in the I:\Students\Instructors\Geoffrey_Duh\GISMarketing\DasymetricMapping folder. The blockgrp.shp contains the 2000 population of several block groups. The stores.shp contains the locations of two hypothetical stores. Copy these shapefiles to your working folder in C:\Users before you continue.

 

  1. Add blockgrp.shp and stores.shp to ArcMap and view their attribute tables.

 

  1. Calculate the area of enumeration units (block groups) using the Calculate Areas ool in ArcToolbox. Set the output feature class as Blockgrp_Area.shp. When done, complete the table below.

 

Projection of Blockgrp_Area.shp

 

Planar distance units of Blockgrp_Area.shp

 

Unit of F_AREA

 

 

  1. Add a new field called “popudens” with a double data type in the Blockgrp_Area shapefile. Use the attribute calculator to calculate the population density.

 

  1. Use the buffer tool to create 1 mile (radius) buffers for stores.shp. You will estimate the 2000 population for these buffer areas.

 

  1. Use the Union tool to overlay Blockgrp_Area and the buffers of store locations. Save the output as Union_Buffer.shp.

 

  1. Take a look at the attribute of Union_Buffer.shp. The F_AREA field in this shapefile still records the areas of the block groups. You will need to use the Calculate Areas tool again to update the area information.

 

  1. Once you have the area for each polygon in Union_Buffer.shp (you will have a new shapefile containing the area information), use attribute calculator to calculate the population.

 

  1. Now you have the population for the polygons in the new shapefile. The final step is to aggregate the population of the polygons that constitute the store buffer areas. Use the Summary Statistics tool to achieve this. Set the new shapefile as input, POPU00 as the statistics field, SUM as the statistic type, and FID_stores as the case field. The output table will tell you the aggregated population for each store buffer area. You can join the table to the buffer area shapefile to show their population on maps.

 

Please record the population for your buffer areas in the table below.

 

FID_stores

Population