1. A method for determining bridge boundaries in images, the method comprising: determining, by one or more computing devices, a wide bridge mask and a narrow bridge mask based at least in part on a road mask and a water mask, wherein each of the one or more computing devices comprises one or more processors, and wherein determining, by the one or more computing devices, the wide bridge mask and the narrow bridge mask based at least in part on the road mask and the water mask comprises: determining, by one or more computing devices, a first intersection of a wide road mask and the water mask to obtain the wide bridge mask; and determining, by the one or more computing devices, a second intersection of a narrow road mask and an eroded version of the water mask to obtain the narrow bridge mask; selecting, by the one or more computing devices, a portion of an image depicting a bridge based at least in part on the wide bridge mask and the narrow bridge mask; identifying, by the one or more computing devices, a plurality of line features included in the portion of the image depicting the bridge; and determining, by the one or more computing devices, a bridge boundary in the portion of the image based at least in part on the plurality of line features; wherein the water mask indicates whether a body of water exists at each of plurality of locations respectively corresponding to a plurality of pixels included in the image depicting the bridge; wherein the eroded version of the water mask comprises the water mask eroded by an erosion distance; wherein the road mask comprises a vector of polylines indicating the location of one or more roads in the image; wherein the wide road mask comprises the road mask rasterized at a first width value; wherein the narrow road mask comprises the road mask rasterized at a second width value; and wherein the second value is less than the first value.