The population of an entire city has been estimated from space to speed up medical and disaster relief efforts, British researchers say.

The London School of Hygiene and Tropical Medicine, working with the charity Doctors Without Borders, has led a project to estimate the population of Am Timan in Chad using satellite images, NewScientist.com reported Thursday.

The entire population is to receive meningitis vaccinations and the charity wants to know how many doses to order.

"Population numbers are crucial to everything we do," Ruby Siddiqui of the global charity said. "We need to plan the size, scale and mode of interventions and without population numbers, we can't do this."

Currently groups rely on the quadrat method to estimate population size, visiting a sample of individual households to find out how many people live in each type of dwelling and using that to estimate the population of the entire town or refugee camp. But the method is slow, involves dozens of samplers, and can be hazardous or impossible to carry out in conflict zones such as Syria.

In the satellite project, surveyors visited sample households in Am Timan to estimate how many people lived in each kind of dwelling, but then calculated a quick city-wide estimate by counting dwellings in satellite images.

The quadrat method, which required sampling visits to 1,160 dwellings, gave a population of 49,722, while the satellite technique, which required sampling visits to only 348 dwellings, gave estimates of 46,625.

"These results are very good, and there's no doubt they'd be good enough for what [Doctors Without Borders] wants to do," researcher Chris Grundy of the London School of Hygiene and Tropical Medicine said.