  We improve the accuracy of photometric redshifts by including low-resolution
spectral data from the G102 grism on the Hubble Space Telescope, which assists
in redshift determination by further constraining the shape of the broadband
Spectral Energy Disribution (SED) and identifying spectral features. The
photometry used in the redshift fits includes near-IR photometry from
FIGS+CANDELS, as well as optical data from ground-based surveys and HST ACS,
and mid-IR data from Spitzer. We calculated the redshifts through the
comparison of measured photometry with template galaxy models, using the EAZY
photometric redshift code. For objects with F105W $< 26.5$ AB mag with a
redshift range of $0 < z < 6$, we find a typical error of $\Delta z = 0.03 *
(1+z)$ for the purely photometric redshifts; with the addition of FIGS spectra,
these become $\Delta z = 0.02 * (1+z)$, an improvement of 50\%. Addition of
grism data also reduces the outlier rate from 8\% to 7\% across all fields.
With the more-accurate spectrophotometric redshifts (SPZs), we searched the
FIGS fields for galaxy overdensities. We identified 24 overdensities across the
4 fields. The strongest overdensity, matching a spectroscopically identified
cluster at $z=0.85$, has 28 potential member galaxies, of which 8 have previous
spectroscopic confirmation, and features a corresponding X-ray signal. Another
corresponding to a cluster at $z=1.84$ has 22 members, 18 of which are
spectroscopically confirmed. Additionally, we find 4 overdensities that are
detected at an equal or higher significance in at least one metric to the two
confirmed clusters.
