update overview for #2

src/overview.md

@@ -3,15 +3,54 @@
 This chapter provides an overview of the three main pages of the [Urban Analyst
 platform](https://urbananalyst.city):
 
-- [Maps](https://urbananalyst.city/maps) showing interactive maps for each
-  city;
-- [Stats](https://urbananalyst.city/stats) comparing aggregate statistics for
+- [Compare](https://urbananalyst.city/stats) comparing aggregate statistics for
   all cities; and
-- [Transformations](https://urbananalyst.city/transform) for transforming any
-  chosen city to become more like other cities.
+- [Map](https://urbananalyst.city/maps) showing interactive maps for each
+  city;
+- [Transform](https://urbananalyst.city/transform) for transforming any chosen
+  city to become more like other cities.
+
+## Compare
+
+The [Compare page](https://urbananalyst.city/compare) is the first and main
+page of Urban Analyst, enabling the properties of all UA cities to be compared.
+A pull-down panel enables each variable or "layer" to be selected. The page
+then displays a graphical representation of values of the chosen layer for all
+cities. As in all UA pages, lower values are generally better than higher
+values. The control panel includes an "Explain Layer" button which opens a text
+panel explaining details of the chosen variable.
+
+### Single and Paired Variables
+
+The control panel of the [compare page](https://urbananalyst.city/compare)
+includes an option to select "paired" variables. The resultant graphs then
+display the strength of relationship between any chosen *pair* of variables.
+For example, choosing social index and the nature index will display the
+strength of relationship between access to natural spaces and social
+disadvantage. Both of these variables are measured such that low values are
+better than high values. A positive relationship between the two would then
+mean that lower social disadvantage is coupled with better access to natural
+spaces, while high social disadvantage is coupled with worse access to natural
+spaces. Conversely, a negative relationship would indicate that higher social
+disadvantage was coupled with better access to natural resources. Or, in the
+words brought up by clicking the "Explain Layer" button, "Low values indicate
+that good access to natural spaces is coupled with disadvantageous social
+conditions."
+
+## Map
 
-## Maps
+The [Map page](https://urbananalyst.city/map) shows interactive maps for each
+city, with values for all UA variables displayed in small polygons. These
+polygons are defined by city-specific assessments of spatial disadvantage.
+Berlin, for example, regularly measures a compound index of social disadvantage
+aggregated into XX polygons. The map for Berlin uses these polygons provided by
+the city to aggregate all measured variables. The variables are described in a
+[subsequent chapter](./variables.md).
 
-## Stats
+As in all UA measurements, lower values of all variables are generally better
+than higher values. Colour scales on all maps thus generally display lower
+values in brighter, yellow colours, while higher values are displayed in
+darker, blue or violet colours. The control panel includes an "Explain Layer"
+button which opens a text panel explaining details of the chosen variable.
 
-## Transformations
+## Transform