The galactic central black hole Sgr A* exhibits outbursts of radiation in the near infrared ( so-called IR flares ) . One model of these events consists in a hotspot orbiting on the innermost stable circular orbit ( ISCO ) of the hole . These outbursts can be used as a probe of the central gravitational potential . One main scientific goal of the second generation VLTI instrument GRAVITY is to observe these flares astrometrically . Here , the astrometric precision of GRAVITY is investigated in imaging mode , which consists in analysing the image computed from the interferometric data . The capability of the instrument to put in light the motion of a hotspot orbiting on the ISCO of our central black hole is then discussed . We find that GRAVITY ’ s astrometric precision for a single star in imaging mode is smaller than the Schwarzschild radius of Sgr A* . The instrument can also demonstrate that a body orbiting on the last stable orbit of the black hole is indeed moving . It yields a typical size of the orbit , if the source is as bright as m _ { \mathrm { K } } = 14 . These results show that GRAVITY allows one to study the close environment of Sgr A* . Having access to the ISCO of the central massive black hole probably allows constraining general relativity in its strong regime . Moreover , if the hotspot model is appropriate , the black hole spin can be constrained .