Many long sea outfalls discharge sewage effluent into the sea surrounding the United Kingdom. To monitor the dispersion and dilution performance of existing or planned outfalls, tracers can be released into the sewage and the resultant plume tracked by boat. In recent years aerial photography has proved to be a useful adjunct to boat surveys as it provides a synoptic view of a plume at one instant, which is something that a boat survey is unable to do as a result of tides, winds and currents.