A spider map as used by Transport for London is a schematic diagram of bus services serving a particular locality.
Generally mounted on the vertical surfaces of bus shelters it enables potential travellers to select the correct stop to board a bus, and the correct one to alight at.
Designed to be self explanatory in the way the famous London Underground tube map is, they are a valuable addition to the explanatory interface of an extensive and sometimes perplexing transport system.
At the centre of the map is a rectangular area with a yellow background which shows the local street layout and bus stops labelled with letters (A to Z, and if necessary AA to ZZ) of all the bus-stops in the local area. Beyond this is a schematic bus map for an area about 2 kilometres (1.2 mi) radius (pale yellow background), which shows all bus stops in their relative positions. Further out of the map shows the remainder of the route against a white background, but without showing all bus stops. Bus routes themselves are show as distinctive coloured lines, and are clearly numbered.
In addition there is a table of routes (by route number, destination and local bus stop) leaving from that node, for either daytime or sometimes nighttime routes.
It is important to remember that these maps are highly specific to a local area, and it is not sensible to talk about a spider map for London. About 1500 are needed to describe the London Transport area.
The term spider map seems to have come from potential users, and TfL calls them officially "bus route diagrams", but has begun to use the word "spider".