1. An indoor surveying apparatus comprising: a 2D range finder having a 2D measurement surface, whereby the 2D range finder measures a 2D data set comprised of a plurality of points located at intersection of the 2D measurement surface and surfaces of other objects; a computing device executing an algorithm for automatically projecting 2D data sets measured by the 2D range finder onto a horizontal plane and automatically aligning the projected 2D data sets to construct a map of indoor environment consisting of points of the projected 2D data sets; a calibrated optical imaging system coupled to the 2D range finder, comprising an image sensor and an objective lens, the calibrated optical imaging system having calibration coefficients that depend on a focal length and a distortion of the objective lens, a position and an orientation of the image sensor relative to the objective lens, and a position and an orientation of the objective lens relative to the 2D range finder, whereby points from a 2D data set measured by the 2D range finder can be projected onto an image captured by the calibrated optical imaging system using the calibration coefficients; and a computing device executing an algorithm for establishing positions and extents of walls, doors, and windows and for drawing floor plans using information from the map of indoor environment, images captured by the calibrated optical imaging system, and the calibration coefficients of the calibrated optical imaging system, whereby the images can be used for establishing positions and extents of walls, doors, and windows where the map of indoor environment is missing.