  Gas detectors are one of the pillars of the research in fundamental physics.
Since many years, a new concept of detectors, the Micro Pattern Gas Detectors,
allows to overcome many of the problems of other types of commonly used
detectors, as drift chambers and microstrips, reducing the discharge rate and
increasing the radiation tolerance. Among these, one of the most commonly used
is the Gas Electron Multiplier. Commonly deployed as fast timing detectors and
triggers, due to their fast response, high rate capability and high radiation
hardness, they can also be used as trackers. The center of gravity readout
technique allows to overcome the limit of the digital pads, whose spatial
resolution is constrained by the pitch size. The presence of a high external
magnetic field can distort the electronic cloud and affect the spatial
resolution. The micro-TPC reconstruction method allows to reconstruct the three
dimensional particle position as in a traditional Time Projection Chamber, but
within a drift gap of a few millimeters. This method brings these detectors
into a new perspective for what concerns the spatial resolution in strong
magnetic field. In this report, the basis of this new technique will be shown
and it will be compared to the traditional center of gravity. The results of a
series of test beam performed with 10 x 10 cm2 planar prototypes in magnetic
field will also be presented. This is one of the first implementations of this
technique for GEM detectors in magnetic field and allows to reach unprecedented
performance for gas detectors, up to a limit of 120 micron at 1T, one of the
world's best results for MPGDs in strong magnetic field. The micro-TPC
reconstruction has been recently tested at very high rates in a test beam at
the MAMI facility; preliminary results of the test will be presented.
