A method of monitoring an object in relative motion, the method comprising: capturing one or more primary data sets within a first field of view using a first line array camera having a first one-dimensional array of pixels, each primary data set being a one-dimensional output from the first one-dimensional array of pixels at a point in time; determining that an object in relative motion with the first line array camera is within the first field of view in response to at least one of variations within each primary data set and variations between each primary data set; determining a first point in time when the object has entered the first field of view; characterized in that the method comprises: simultaneously to capturing the one or more primary data sets, capturing one or more corresponding secondary data sets within a second field of view using a second line array camera having a second one-dimensional array of pixels, each secondary data set being a one-dimensional output from the second one-dimensional array of pixels at a point in time, wherein the second line array camera is facing the first line array camera and located at a selected distance away from the first line array camera; determining that an object in relative motion with the second line array camera is within the second field of view in response to at least one of variations within each secondary data set and variations between each secondary data set; determining at least one of a size of the object and a distance between the object and the first line array camera based on the selected distance, and a corresponding set of primary and secondary data sets. capturing one or more primary data sets within a first field of view using a first line array camera having a first one-dimensional array of pixels, each primary data set being a one-dimensional output from the first one-dimensional array of pixels at a point in time; determining that an object in relative motion with the first line array camera is within the first field of view in response to at least one of variations within each primary data set and variations between each primary data set; determining a first point in time when the object has entered the first field of view; characterized in that the method comprises: simultaneously to capturing the one or more primary data sets, capturing one or more corresponding secondary data sets within a second field of view using a second line array camera having a second one-dimensional array of pixels, each secondary data set being a one-dimensional output from the second one-dimensional array of pixels at a point in time, wherein the second line array camera is facing the first line array camera and located at a selected distance away from the first line array camera; determining that an object in relative motion with the second line array camera is within the second field of view in response to at least one of variations within each secondary data set and variations between each secondary data set; determining at least one of a size of the object and a distance between the object and the first line array camera based on the selected distance, and a corresponding set of primary and secondary data sets. simultaneously to capturing the one or more primary data sets, capturing one or more corresponding secondary data sets within a second field of view using a second line array camera having a second one-dimensional array of pixels, each secondary data set being a one-dimensional output from the second one-dimensional array of pixels at a point in time, wherein the second line array camera is facing the first line array camera and located at a selected distance away from the first line array camera; determining that an object in relative motion with the second line array camera is within the second field of view in response to at least one of variations within each secondary data set and variations between each secondary data set; determining at least one of a size of the object and a distance between the object and the first line array camera based on the selected distance, and a corresponding set of primary and secondary data sets.