How to draw a horizontal line when decorating?

The engineering decoration is provided by the civil engineering, and then the decoration unit uses the laser line projector to guide the measurement. The position of the laser is the position of the elastic line, and the two points of the Mo Dou line coincide with the line, and it comes out with a bang!

Decoration of one's own home: when closing the house, the indoor clear height is 2.75-2.8m Turn on the laser line projector to measure the indoor floor height difference (generally not much, otherwise the civil engineering acceptance will not pass). The lowest point takes the height of 1.03- 1.05m as the horizontal line of indoor finished surface1m. The height of the ceiling is also quoted from the online 1m horizontal line.