This is an ambitious figure in which three main types of data are being represented. Firstly, the size of a set of material flows is shown in this figure. Secondly, a comparison is made between 1971 and 1997 figures. And thirdly, the direction of the flows are indicated. Combining all these pieces of information into one diagram is useful and an impressive feat. However, what is not clear, or at least not to the untrained eye, is what the outline on the map represents. Is it the Hong Kong skyline? Or is it a map of (part of) Hong Kong?

From the paper:

Diagrammatic representation of important material flows into and through the SAR [Hong Kong Special Administrative Region]. All units are in tonnes day. Arrows are intended to give some indication of the direction of flow of materials, and 1971 data are shown in red.