A web-search quickly shows up the common information regarding the size, weight and passenger capacities of the buses. The most useful site has been Iain's Bus Stop which gives a history of all the London Buses. It has a fairly detailed section on RM's. Specifically from the history of the RM I've realised that all of the buses I've been looking at on the 14 and 22 routes from Putney are the RML variant at 30' long.
For many years my commute into South Kensington has been on the London General No. 14 bus, that runs between Putney Heath and Tottenham Court Rd. This route is one of the few that is still plied by Routemasters and at the present moment no conversion date has been set, despite numerous other routes having changed over the past year. What this means is that until I actually purchase a bus of my own I have some reference examples which I can study...
A notebook and a keen eye have been the main tools of the survey. Most of the measurements have been guesses or measured with items of know length such as my boot or the notebook itself. On the odd occasion I have been found brandishing a tape-measure to get decent measurements of the windows. Coupled with engineering drawings from the publications detailed below and a handful of reference photo's taken on a visit to the London Transport Museum I probably have most of the passenger space mapped out to within 2" of accuracy, more than enough for working concept sketches.
The majority of literature on Routemasters is primarily concerned with what colour they were painted and who was running them on what routes. Whilst this is all very interesting, all I care about is how it is built. There are however two books which have proved very useful:
Through a combination of the reference material listed above and dimension taken either straight off of a bus that I've travelled on, or extrapolated from various photo's and diagrams in publications I have been able to draw up a fairly detailed model of the structure of a Routemaster. It's probably accurate to within an inch, which is plenty detailed enough for detailed concept sketches.
Once I've actually got a physical bus in my posession it will be possible to take much more accurate measurements. One question that needs to be answered is what do we use as a datum point? Do we measure from the front, from the back or from somewhere in the middel? Is the datum point internal or external?
Since a large number of components in the bus share common measurements it makes sense to list these in a large spreadsheet and derive all of the components for both the bus framework and the interior from that table.