  We present the topometric MST method to search for clusters of photons in the
LAT sky, which was used to obtain the seed list for the compilation of the
First LAT catalog. This method works well in non-dense field and can be
profitably used at energies higher than a few GeV. We describe the particular
techniques developed by us to improve the cluster selection criteria and the
estimate of the astronomical coordinates of the possibly associated gamma-ray
sources. A simulation technique to evaluate the confidence level of the source
detectionis presented.

High Energy Astrophysical Phenomena