[<< wikibooks] Trigonometry/Angles of Elevation and Depression
Suppose you are an observer at 
  
    
      
        O
      
    
    {\displaystyle O}
   and there is an object 
  
    
      
        Q
      
    
    {\displaystyle Q}
   , not in the same horizontal plane. Let 
  
    
      
        O
        P
      
    
    {\displaystyle OP}
   be a horizontal line such that 
  
    
      
        O
        ,
        P
        ,
        Q
      
    
    {\displaystyle O,P,Q}
   are in a vertical plane. Then if 
  
    
      
        Q
      
    
    {\displaystyle Q}
   is above 
  
    
      
        0
      
    
    {\displaystyle 0}
   , the angle 
  
    
      
        ∠
        Q
        O
        P
      
    
    {\displaystyle \angle QOP}
   is the angle of elevation of 
  
    
      
        Q
      
    
    {\displaystyle Q}
   observed from 
  
    
      
        P
      
    
    {\displaystyle P}
   , and if 
  
    
      
        Q
      
    
    {\displaystyle Q}
   is below 
  
    
      
        O
      
    
    {\displaystyle O}
  , the angle 
  
    
      
        ∠
        Q
        O
        P
      
    
    {\displaystyle \angle QOP}
   is the angle of depression.
Often when using angle of elevation and depression we ignore the height of the person, and measure the angle from some convenient 'ground level'.