How To Build A Horizontal Timber Slat Fence Survey The Heighth Of A Building Without Its Horizontal Distance?

Survey the heighth of a building without its horizontal distance? - how to build a horizontal timber slat fence

For the topography of class, we need a way to is the height of a portion of a building in the center of the tower to see it, (but is) not drag the horizontal distance, because there is a flat wall. Please help this cause is tomorrow and I'm struggling to find something!