Lab 3 Part II: Dasymetric Mapping

 

Introduction

 

In Part II, you will use dasymetric mapping technique in ArcGIS to create a population distribution map. Imagine that you want to create a map of population distribution for the Beaverton Creek subwatershed for your hydrological research. The only population GIS layer is a 1990 census block group map. Such an aggregate census map doesn’t reflect the actual distribution of population in the subwatershed, so you decide to disaggregate the census population using ancillary data. You take a 1992 land-over map derived from Landsat imageries (MRLC) and use dasymetric mapping to disaggregate the population. Please read the paper by Holloway et al. (1997) for more information.

 

Instructions

 

Click here to download the data you will need for this exercise. You need to unzip the file. Follow the instructions below and answer all questions at the end of this exercise.

 

  1. Add the landcover grid and BlockGroups feature classe in the Beaverton Creek feature dataset to ArcMap. Enable Spatial Analyst and add it to the toolbar. Set extent and cell size to the same as layer “landcover” in the option window of Spatial Analyst.
  2. We will use a modified equation based on the equation show in Figure 2 in Holloway et al.’s article to calculate the population for each land-cover cell (pixel).

P = ((RA*PA)*N/E)/AT

Where, P is the population of a cell,

            RA  is the relative density of a cell with land-cover type A,

            PA is the proportion of cells of land-cover type A in the enumeration unit,

            N is the actual population of enumeration unit (i.e., census block group)

            E is the expected population of enumeration unit calculated using the relative densities. E equals the sum of the products of relative density and the proportion of each land-cover type in each enumeration unit.

            AT is the total number of cells in the enumeration unit.

The cell size of the land-cover layer is 133 by 133 feet. The values of RA for different land-cover types are given in the table below.

Land-Cover Code

Descriptions

Relative Density (RA)

1

Low density residential

15

2

High density residential

65

3

Commercial/Industrial

5

4

Agricultural

10

5

Natural

5

6

Water/wetland

0

First, we generate a raster map of N using the convert, feature to raster tool in the Spatial Analyst menu. Set BlockGroups as the input feature,  POP90 as the field, and popu90 as the output grid. Click OK. (Note: Make sure to save all your output files in a designate folder, otherwise, you might not be able to find them later.)

  1. Use Spatial Analyst ’s reclassify tool in create a raster map of RA. In reclassify dialog window, set landcover as input raster, type in the corresponding RA values in the new values column, and set rdensity as the output raster. Click OK.
  2. Next, we need to calculate E. The value of E is decided by the proportions of land-cover types in each census block group. To calculate the proportions, first create a raster map of census block groups’ FIPS codes using features to raster tool. Set BlockGroups as the input features, FIPS as the field, and bgfips as the output raster. Click OK.
  3. Open the Tabulate Area tool in the Zonal toolset in Spatial Analyst Toolbox. Set bgfips as the input raster (zone data), landcover as the input raster (feature class data), and lctab as the output table. Click OK. (Note: You can put the output table in the geodatabase.)
  4. ArcMap automatically adds the lctab to the data frame after the process is finished. Click on the Source tab at the lower left corner of the window to see the table. The lctab table contains the areas of different land-cover types in each census unit. The areas of Low Density Residential are stored in VALUE_1 field, High Density Residential in VALUE_2, Commercial/Industrial in VALUE_3, etc. However, we need the proportion data not the area data.  To calculate proportions, right-click on the table and select open to open it, then, select add field from the options menu and add the following fields as double precision fields: Total, P1, P2, P3, P4, P5, and E.
  5. Right click on the Total heading and select calculate values. Click Yes to dismiss the next popup window. In the window under "total =", enter the following formula:

VALUE_1 + VALUE_2 + VALUE_3 + VALUE_4 + VALUE_5

Then, click OK. Because water/wetland has a relative population density of 0, we don’t count the areas labeled as water/wetland.

  1. To calculate P1, use the formula (P1=) VALUE_1 / Total following the same procedures described above. When done, continue to calculate P2, P3, P4, and P5.
  2. Now, we have all the information needed to calculate E. Use the following formula to calculate E:

P1 * 15 + P2 * 65 + P3 * 5 + P4 * 10 + P5 * 5

  1. Once we have the total area, proportion, and E data, we will use them to create raster maps of these data. Right-click on the BlockGroups polygon layer and select joins and relates, join. Set OBJECTID as the field in BlockGroups that the join will be based on. Select lctab as the table to join to and VALUE_ as the field that the join will be based on in lctab. Click OK. When done, you will see the proportion data attached to the attribute table of BlockGroups.
  2. Follow the procedures describe in 2 to rasterize BlockGroups using lctab.Total, lctab.P1, lctab.P2, lctab.P3, lctab.P4, lctab.P5, and lctab.E and create total, p1, p2, p3, p4, p5, and e raster maps. (Note: Every time you restart ArcMap, be sure to set the analysis extent and cell size of Spatial Analyst again.)
  3. Now, we can calculate the population per cell. Open Raster Calculator and type in the following equation, then, click OK.

cellpopu = [rdensity] * [popu90] * 133.058 * 133.058 / ([e - e] * [total])

  1. Done! Before you begin to answer the questions below, probably it’s a good idea to check if the dasymetric mapping method works. This can be done by summarizing the population of census block groups based on the dasymetric map. Use the select features tool to select the topmost census block group polygon (FIPS = 410670316034) in BkockGroups. Select the Zonal Statistics as Table tool from Spatial Analyst Tools, Zonal toolset in ArcToolbox. Set BlockGroups as the Input zone data, FIPS as zone field, cellpopu as input value raster, and popucheck as the output table. Click OK. Compare the values of SUM_ in popucheck and POP90 in BlockGroups.

 

Questions

 

  1. The 1990 population of the selected census block group is 1547. What is the value you estimated based on the dasymetric map?
  2. What is the unit of the values reported by the Tabulate Area tool in this exercise?
  3. Print a choropleth map showing 1990 census block group population and a dasymetric map showing 1990 population in the Beaverton Creek subwatershed.
  4. The lab data set contains a polygon feature class called BeavetonCreek. Use it to estimate 1990 population of the subwatershed and report your result.
  5. Describe any problem you found that might limit the utility of dasymetric mapping technique.