Comparing Maps
When you click on the Compare tab in DRA 2020, you now get a suite of tools for comparing your map other maps.¹
At the top, you will see the same bird’s-eye view below that you see in Analyze view (formerly Analytics view) that evaluates six essential characteristics of your map (see Ratings Diagram).
By default, your map is compared to the five Notable Maps. You can, however, pick 1–5 different maps to compare your map to, by clicking on the Pick Map(s) button on the right.
You can search for maps to compare to just like in the main list of maps (see Filtering & Sorting Maps).
Heads Up!
Before describing the rest of Compare page, it’s important to emphasize two things.
First, unlike the the Analyze page where the ratings use whatever datasets you have chosen for the map, on the Compare page all ratings for all maps use the default datasets so the maps are compared on an apples-to-apples basis. Hence, if you’ve changed the datasets for a map, the ratings for it on the Compare page might be a little different.
Second, map picker only shows Published maps and your maps (My Maps) for:
- The same state
- The same type of plan — congressional, state senate, or state house
- The same number of districts +/— 2 districts — this flexibility allows you to analyze changes in maps when the number of apportioned districts has changed; that are
- Complete, contiguous, and free of holes — we don’t also enforce ‘roughly’ equal populations
With that background, the rest of the Compare page is divide into the sections described below.
Ratings to Selected Maps
If you’re comparing your map to two or more other maps — either because you’re comparing to the Notable Maps or a set of maps that you’ve manually selected — you get a series of pairwise ratings diagrams that show your map’s ratings filled in green with a black border overlaid on top of the ratings for a comparison map filled in orange.
Note: To save space, the labels for the Ratings dimensions are elided. You can see the full name of each dimension by hovering over the values represented by circles.
You can press the camera icon to download the image.
You also get a line graph that shows the ratings for all the selected maps at the same time. The current map is shown with the thicker black line while the comparison maps are shown with thinner colored lines.
You can hover over each dimension to see the ratings for each map.
If you’re comparing your map to just one other map, the comparison map is simply overlaid onto the ratings diagram at the top.
District Across Maps
If you’re comparing your map to just one other map, you get a table that shows the “diff” between the (old) comparison map and your (new) current map.²
The lefthand side of the table shows basic information about each district (see District Statistics for details). The righthand side of the table shows how the districts in the comparison map flow into the districts in the current map. The rows are sorted in descending order of size.
Update 12/1/21 — We’ve added an Add Points button above the table. When you click on it, a picker will present a list of Custom Overlays that contain points (in layers imported from GeoJSON) or landmarks (in maps). The names for points/landmarks in the overlay that you choose will be associated with the current districts in the Points of Interest column based on their coordinates. The points can represent anything, but an obvious scenario is the coordinates for incumbents’ home addresses.
Update 12/14/21 — We’ve added a note below the table to quantify how much the cores of the comparison districts would be preserved or fragmented by the current districts. For example “The districts in the comparison map average 1.32 effective splits and 1.31 bits of uncertainty in the current map. On a scale of [0–100] these district changes rate 17 (very fragmented).” The number of effective splits and uncertainty of membership are described in COI Splitting. Uncertainty is normalized using district split in half — the 50/50 row in the examples at the end — as the worst splitting which translated to a zero.
You can press the Download button to download the table to a CSV file that you can import into a spreadsheet and format to your liking.
Ratings to All Similar Maps
In the final section, you will see a box-plot that compares your map to all similar maps, comparing each dimension independently. The ratings for your map are highlighted as black dots.
In addition to showing the distribution of similar maps along each dimension, the box-plot specifically highlights the 2nd and 3rd quartiles of maps on either side of the median. When you hover your cursor over any individual dimension, you will see figures related to these data.
Again, you can press the camera icon to download the image.
Footnotes
- This suite of tools supersedes the original functionality for comparing maps that was embedded within Analyze view (see Comparing Maps).
- See diff in Wikipedia. Many thanks to Steven Elliot for suggesting this.
