describe transform page to close #3

src/overview.md

@@ -54,3 +54,63 @@ darker, blue or violet colours. The control panel includes an "Explain Layer"
 button which opens a text panel explaining details of the chosen variable.
 
 ## Transform
+
+The [Transform page](https://urbananalyst.city/transform) enables the
+properties of any chosen city to be transformed to reflect equivalent
+properties of some chosen "target" city. This page is best explained by an
+example. Looking at the [Compare page](https://urbananalyst.city/compare) for
+the "bike index" shows that Berlin has relatively poor bicycle infrastructure,
+while Paris is a very good city for cyclists. The
+[Transform page](https://urbananalyst.city/transform) can be used to visualise
+how Berlin could best transform its current bicycle infrastructure to have an
+overall distribution across the whole city equivalent to Paris. Conversely,
+Paris has poorer access to natural spaces than Berlin, and the page could also
+be used to examine how Paris could best transform its access to natural spaces
+so that it functioned more like Berlin.
+
+Urban Analyst values displayed in the [Map page](https://urbananalyst.city/map)
+are aggregated from generally hundreds of thousands of individual calculations
+at every street junction in each city. For the chosen variable, subsets of
+these individual data points are sampled from each city, and the statistical
+distribution for the chosen city is then transformed by changing each point by
+the smallest amount possible so that they reflect the distribution in the
+target city. These values are then aggregated into the polygons defined for the
+city, to produce a visual representation of the least-cost transformation that
+would be necessary for the city to have the same distribution as that of the
+target city. The transformation algorithm is described in detail in the final
+[*Software and Algorithms* chapter](./software.md).
+
+### Extra Layers
+
+The [Transform page](https://urbananalyst.city/transform) includes an
+additional button labelled *Extra Layers*. The transformations described above
+described transforming single layers or variables. The *Extra Layers* panel
+enables transformations not just of single chosen variables, but also of their
+relationships with other variables. Examining the [Compare
+page](https://urbananalyst.city/compare), for example, shows that not only does
+Paris provide poorer access to natural spaces than Berlin, but also that Berlin
+has a better relationship between access to natural spaces and social
+disadvantage. (This can be seen by clicking on the "*Paired*" layer option and
+selecting those two layers.) The *Extra Layers* panel can be used in this case
+to examine not just how Paris might best transform its access to nature to look
+more like Berlin, but also how it might also improve its relationship between
+access to nature and social disadvantage.
+
+By default, values of *Extra Layers* are automatically selected as those which
+have better relationships in the target city. These default values will thus
+change for each choice of target city and focal layer. It may be necessary to
+click on the "Reset" button in the *Extra Layers* panel to update this default
+selection after changing any of these options.
+
+### Output Layer
+
+Finally, the [Transform page](https://urbananalyst.city/transform) also has an
+*Output Layer* option at the bottom of the control panel. This enables results
+of the transformation algorithm to be displayed in one of four ways:
+
+1. *Original* to show original values, prior to transformation;
+2. *Transformed* to show the actual transformed values;
+3. *Absolute* to show the absolute value by which each are in the city would
+   have to be transformed to match the distribution in the target city; and
+4. *Relative*, which displays the absolute transformation values relative to
+   the original, untransformed values.