The Dakshinottara Bhitti Yantra measures the Meridian Altitude of a celestial object, once in a day, as the celestial object crosses the Meridian.
The Meridian or the arc defined by the North, South and overhead point, is in this plane. With the rotation of the Earth, when a celestial body crosses this arc, halfway between its rising and setting times, it is said to transit the Meridian.
The angular height of the object from the Horizon, as it crosses the Meridian, is its Meridian Altitude.
Dakshinottara Bhitti is an instrument built into a wall placed exactly in the North-South direction.
The instrument uses either the semi-circular arc built into the west facing wall, or the intersecting quadrants of the East facing wall, to measure the Meridian Altitude of a celestial object.
The instrument measures the Altitude of a celestial object as it crosses the Meridian. The circular arc of the west facing wall and the intersecting quadrants of the East facing wall have 90 degree Altitude marked at the bottom of the scale and 0 degree Altitude marked at the top ends of the scale.
The markings on the scales are in units of degrees that have been further subdivided into 10 main divisions and further subdivisions of 3 small units which yield a least count of 2/ of arc for the instrument.