Homework 3

NOTE 1: If you need help, and you think that your question might be of interest to the whole class, send e-mail to 11.188@mit.edu. If you would prefer to ask just the class staff, send e-mail to jf@mit.edu and xujs@mit.edu. Regardless, please don't be shy about asking for help if you are struggling with understanding the assignment or having trouble with a particular aspect of ArcMap. We've heard stories of students struggling for many hours over minor issues that had easy solutions. We'd like to avoid these misadventures, so please contact us sooner rather than later if you get stuck.

NOTE 2: This assignment builds directly on the endpoint of Homework 2. To avoid problems resulting from the use of different endpoints from homework #2 (and to enable everyone to start work on homework #3 even if they have not finished #2), we provide a shapefile, finalresult.shp, in the hm2 sub-directory of the class data locker. This shapefile identifies areas that are acceptable locations based on a set of accessibility, health, land use, and demographic criteria similar to (but not the same as) HW#2. Because it was developed using slightly different cutoffs for the criteria, the shapefile differs somewhat from the actual answer to your HW#2. (So, do not worry if the shapefile looks different from the shapefile you developed to answer HW#2!)

Task 1 [55 points]: Adding Poor Senior Density as a Criterion

In Homework 2, you identified those places that met the accessibility, health, land use, and demographic criteria. You present your first set of results to the non-profit group in charge. Based on the discussion, you are asked to adjust one of the factors. Instead of just restricting the location to block groups with 'high' counts of poor seniors, you agree that it will be desirable to make sure that the center is close to as many poor seniors as possible. Consequently, you decide to adjust your analysis by adding another criteria related to the density of poor seniors.

The Homework 2 screening exercise identified more than one site that met all the initial criteria. Adding in this new 'density of poor seniors' consideration can help you further narrow down the identification of a preferred site. Among those feasible sites already identified, you would prefer those that fall in the higher density-of-poor seniors locations. You decide to focus on some measure of 'density' that captures the right sense of 'proximity to seniors' and can be integrated into your site suitability analysis.You know that the census data provide counts of poor seniors (aged 65+) for each block group so you can use those data (and the area_sqm field) to compute a density-of-poor seniors measure. A thematic map of this measure (by block groups) will show you where the density of poor seniors is high. But that still isn't quite what you want. You would like a measure of how many poor seniors have easy access to the center -- and that measure should include poor seniors from nearby block groups as well as any from the block group in which the center is located. So, you decide to use ArcMap's Spatial Analyst tools to develop a better measure. First, you create a rasterized (grid) version of the block groups (with the block group's poor-senior-citizen density as the cell value). Then you can use the 'neighborhood statistics' tools to create a new grid layer that has, for its cell value, the average of all the cells within some buffer distance (e.g., a circle of 1000 meter radius is what we suggest below).

To follow through on this idea, create a grid coverage that rasterizes the block groups into grid cells that are 100 meters on edge. Be sure to mask off all but the 5-town area when you create the grid coverage. Set the value in each cell to be your estimate of the density of poor seniors in the block group at the center of the grid cell. Create a thematic map showing the density of poor senior citizens across your Cambridge grid cells.

Please use the block group layer M:\data\hm2\eastma2000bg_stateplane.shp for the calculation of the poor senior population density. (Otherwise you would have to project the Library's blockgroup layer to Mass stateplane, and then recompute area, before you could get an accurate measure of area in square meters so that you could compute the density of poor seniors.) .

Hints:

• Compute the poor senior density as the count of poor senior citizens per block group divided by the area_sqm column. Note that area_sqm is in square meters -- not in acres. (Recall that 1 hectare = 10,000 square meters = 2.471 acres, and 1 acre = 43,560 square feet.) You can join this new table to the original block group coverage before rasterizing it so that you can use your newly computed density to set the value of each grid cell.
  
  
• Consider carefully which values from the census you should use for calculating poor seniors per block group. Pay close attention to the universe of the senior poverty measure you used in Homework 2. (Recall your exploration of senior citizen statistics in Part I of homework #2.) Make sure to document your choice in your assignment.

Now, use the Focal Statistics tool within the Spatial Analyst Tools > Neighborhood part of ArcToolbox to generate a new grid layer where the value of each grid cell is the average (mean) of the density of poor seniors in the surrounding grid cells. Define the neighborhood to be a circle (of grid cells) of 1000-meter radius (i.e., 10 cells) centered on each grid cell. Notice how the new layer looks different from the original rasterization of your block group densities. (Do you understand why?)

Now, overlay (visually! - no need to 'intersect') this smoothed density-of-poor seniors layer with finalresult.shp, in the hm2 sub-directory of the class data locker - the shapefile identifying the locations that we will consider to be acceptable based on criteria similar to (but a little different from) those stated in Homework 2.

One way to select a specific location for the poor senior center would be to pick the (1 hectare) site among the acceptable locations from Homework 2 that had the highest density of nearby poor seniors. Add another view to the layout of the thematic map you just created which shows this density of nearby poor seniors. Annotate this layout to clarify which map shows the densities: before and after doing the neighborhood averaging.

Summary of Task 1 Requirements:

Turn in one map layout with two views -- one showing the block group density-of-poor seniors before the neighborhood averaging and one showing the density-of-poor seniors after the neighborhood averaging. Adjust and annotate the map so it is readable and add a few sentences explaining what each map is measuring and comment on any shift in pattern that you observe. As in Homework 2, you will be graded on the quality of both your analysis and your presentation of your results.

What to Make of All This:
Task 1 asks for one map with two views (of the block group poor senior density before and after neighborhood averaging). Make sure that the neighborhood average view includes some visual representation (i.e. visual overlay) of the HW #2 finalresults.shp sites so the reader can see which high-density cells fall within the HW #2-acceptable locations. Doing this is more a matter of attention to cartography and visual display rather than computing a new combined index.

• A visual overlay of the HW #2-suitable areas and the neighborhood-density-surface is sufficient to let you identify which HW #2-sites are located in places that are high on the neighborhood-density-of-poor seniors surface (hereafter "nd-poor-senior-surface"). By a "visual overlay" we mean placing the outline of the vector layer over the raster layer in an ArcMap window so it is possible to visually inspect the relationship among the layers.

Task 2 [45 points]: Making (and Critiquing) a Suitability Analysis Model

While our formulation of the problem does narrow down the number of possible site locations, many of the criteria were rather arbitrary -- why within 250 meters of a major road instead of 300 or 500 meters? In general, you will want to do some reasoning and make some judgments about how to balance (or adjust!) the various criteria that you have incorporated into your site selection screening process. You will also want to cast your analysis within a framework within which it is as easy as possible to adjust or add criteria, or reweight the analysis to take into account different priorities. Within stock ArcGIS, the best way to do this is to create a ModelBuilder model of your suitability analysis in Homework #2 so that each criterion was implemented by one or more ArcToolbox operations and the ModelBuilder diagram specified how the inputs and outputs of the various operations are connected and, perhaps, "weighted and rated" in order to produce the shapefile of acceptable locations (such as the finalresult.shp layer that we have provided).

For the purposes of the homework exercise, building a ModelBuilder model of the entire suitability analysis is not necessary to get a feel for what it is like to use ModelBuilder. What we want you to do is (1) build a ModelBuilder model to automate the generation of the neighborhood-density-of-poor seniors layer that you built in Part I above and then (2) add to the model a "zonal statistics" operations that determines the average value within each of the acceptable locations in finalresult.shp of the neighborhood-density-of-poor seniors layer. ("ZonalStatistics" is one of a few GIS operators which lets you summarize raster grid cells using vector data for the "cookie cutter".) Turn in a Figure showing your ModelBuilder diagram plus a thematic map shading the acceptable regions in finalresult.shp based on the average density that you computed.

(1) assign a judgmental quality-of-location scale to each of the acceptable locations in finalresult.shp, and then (2)

Write a short report (maximum of 2 double-spaced pages, 11-point font, 1-inch margins) interpreting the results of your suitability analyses (that is, the locations in finalresult.shp and your average density values) explaining any comments you may have about the allowable locations. Should the density criterion for seniors in poverty be relative or absolute? Would you prefer to relax one of the other criteria and shift the site elsewhere? Would you suggest some tradeoffs among the criteria? Do the criteria restrict the sites a lot or a little? Do they appear to capture the individual criteria reasonably?

There are not 'correct' answers to these questions -- and we do not expect you to research the many real-life characteristics of Cambridge that could influence your decision. The intent is for you to back away from ArcMap and GIS and spend some time reflecting on the problem of locating a poor senior center and thinking about how your specific variables, cutoffs, and visualization tools may have biases, artificial limits, or have otherwise colored or overlooked relevant factors. Why do you think you have (or have not) zeroed in on one or more sensible suggestions for where to put the center?

Recognize in Task 2 that we are not trying to get you to pick a 'best' location. Rather we want you to use the data and GIS tools to boil down the many criteria and possible sites to a reasonable and informed discussion of the pluses and minuses of several specific sites and the primary criteria and tradeoffs that are likely to be involved in making the selection. You'll want to be able to refer to your Task-1 map in your discussion and it will be easier to make your points if your maps help the reader visualize the spatial patterns (in terms of buffers, land use density, etc.) that you've identified for the various criteria. There will be times in your professional career when you will be asked to conduct an analysis and recommend a single "best" site based entirely on your own analysis. However, there will likely be many more occassions in which you will work with a group of people (experts or general public) to identify criteria, and to weight and rate them. Doing this fairly, using a transparent process, can be difficult. For example, what if two experts or two citizens disagree on what the weights should be? Part of the point in tackling the same problem with different tools is for you to start to gain an appreciation of how the technical tools can influence the decision - sometimes in subtle ways.

Finally, notice that we've simplified the site selection task to ignore the cost of acquiring the property. Write a few sentences explaining how such cost considerations might steer the site selection toward one or another of the better locations that your analysis identified and what further analysis you might do (using GIS tools and data of the kinds we've been using) to sort through this question. You do not actually have to perform this analysis -- just indicate how you would approach it. You are welcome to reference your answer from HW # 2, but you should amplify on how the additional work in HW # 3 (this assignment) augmented your analysis.

Homework 2 and Homework 3 developed by Kamal Azar, 1999
Modified 1999-2008 by Joe Ferreira, Anne Kinsella Thompson,Thomas H. Grayson , Myounggu Kang, Jinhua Zhao, Michael Flaxman, and Yi Zhu.
Last modified: 4-11-12 by Joe Ferreira.

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