1. An indoor surveying apparatus comprising: a 2D range finder having a 2D measurement surface, whereby the range finder measures a 2D data set comprised of a plurality of points located at intersection of the measurement surface and surfaces of other objects; a computing device executing an algorithm for automatically projecting 2D data sets measured by the 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 range finder, comprising an image sensor and an objective lens, the 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 range finder, whereby points from a 2D data set measured by the range finder can be projected onto an image captured by the 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 constructed map of indoor environment, images captured by the imaging system, and the calibration coefficients of the imaging system, whereby the images can be used for establishing positions and extents of walls, doors, and windows where the map information is missing.