Are both the angle of elevation and the angle of depression measured from a horizontal line?

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Angles of elevation and depression are both measured from a horizontal reference line through the observer’s eye. The line of sight forms an angle with that horizontal baseline: the elevation angle is the angle up from the horizontal to the object, and the depression angle is the angle down from the horizontal to the object. Using the same horizontal reference keeps these two measures consistent and intuitive. If you were to measure from a vertical line instead, you’d be dealing with different angles that are effectively complements to the horizontal-based angles. For example, looking at a point above you with a certain vertical rise and horizontal distance gives the elevation by arctan(rise/distance); looking at a point below you gives the depression by arctan(hypothetical rise/distance), all relative to the horizontal.