census data created in countries across the world. on this map of the dominican republic, irregular red lines surround subnational administrative units for which census data is available. one problem for balk's team is how to reconcile these data with precise grid-format satellite data shown here in blue. to obtain boundaries of urban areas a third source of information was used -- nighttime images of the earth from space. balk: noaa produces a nighttime lights data set that was measured in 1994 and 1995 and it's a globally consistent data set. that means they use the same measures everywhere. it measures brightness of places. we combined the lights data with the population data as well as a few other data sets, to make sure that we can confirm each light is actually a population place. narrator: using a combination of various population and geographic data sets balk is able to determine how many people live in low-elevation coastal zones. balk: this is southern vietnam. so, we see the white areas are administrative boundaries. the red areas indicate the nighttime lights -- the urban areas. and this blue data set is the elevation zone that we'