  Performance of triple GEM prototypes has been evaluated by means of a muon
beam at the H4 line of the SPS test area at CERN. The data from two planar
prototypes have been reconstructed and analyzed offline with two clusterization
methods: the enter of gravity of the charge distribution and the micro Time
Projection Chamber (\muTPC). Concerning the spatial resolution, the charge
centroid cluster reconstruction performs extremely well with no magnetic field:
the resolution is well below 100 \mum . Increasing the magnetic field
intensity, the resolution degrades almost linearly as effect of the Lorentz
force that displaces, broadens and asymmetrizes the electron avalanche. Tuning
the electric fields of the GEM prototype we could achieve the unprecedented
spatial resolution of 190 \mum at 1 Tesla. In order to boost the spatial
resolution with strong magnetic field and inclined tracks a \muTPC cluster
reconstruction has been investigated. Such a readout mode exploits the good
time resolution of the GEM detector and electronics to reconstruct the
trajectory of the particle inside the conversion gap. Beside the improvement of
the spatial resolution, information on the track angle can be also extracted.
The new clustering algorithm has been tested with diagonal tracks with no
magnetic field showing a resolution between 100 um and 150 um for the incident
angle ranging from 10{\deg} to 45{\deg} . Studies show similar performance with
1 Tesla magnetic field. This is the first use of a \muTPC readout with a triple
GEM detector in magnetic field. This study has shown that a combined readout is
capable to guarantee stable performance over a broad spectrum of particle
momenta and incident angles, up to a 1 Tesla magnetic field.
