Here's my methodology for counting the #PeoplesClimate march in London.
My aim was to provide an auditable estimate of the number of people participating in the protest.
This is a personal project and as such the findings are not endorsed by my employer.
I set up a camera in a fixed position on the footbridge by Embankment station, overlooking the route of the march.
I took pairs of photographs, spaced roughly 30 seconds apart.
You can check out the photos in this Flickr album: https://secure.flickr.com/photos/tomwills/sets/72157647811244306/ You'll need to zoom in to see the markings I used to count.
I transferred each pair of photographs to my laptop and used image editing software to take the following measurements:
- timestamp of each photograph
- vertical 'distance travelled', in pixels, for a marker (e.g. distinctive placard)
- width of march, in pixels, at the midpoint between the positions of the marker in the two photographs
- number of faces (including those obscured by placards) within a sample area
- dimensions of the sample area in pixels
I entered this information into a publicly viewable Google Spreadsheet to come up with an estimate of how many people marched past Embankment: https://docs.google.com/spreadsheets/d/1aNZgpZPsoHQFXOlh_XFGopJYM3ArHNRQtNU_EoafA44