THE ASTRONOMICAL JOURNAL VOLUME 114, NUMBER 6, PAGE 2822 DECEMBER 1997 SEEING TESTS AT FOUR SITES IN SUPPORT OF THE NPOI PROJECT D. J. HUTTER AND N. M. ELIAS II U.S. Naval Observatory, 3450 Massachusetts Ave., NW, Washington DC 20392-5420 Electronic mail: djh@fornax.usno.navy.mil, nme@sextans.lowell.edu E. R. PETERSON, W. B. WEAVER, AND G. WEAVER Monterey Institute for Research in Astronomy, 200 Eighth St., Marina, CA 93933 Electronic mail: peterson@relgyro.stanford.edu, bw@mira.org, glen@sonic.net D. MOZURKEWICH Naval Research Laboratory, Code 7215, Washington, DC 20375 Electronic mail: mozurk@rsd.nrl.navy.mil F. J. VRBA U.S. Naval Observatory, Flagstaff Station, P.O. Box 1149, Flagstaff, AZ 86002-1149 Electronic mail: fjv@nofs.navy.mil R. S. SIMON National Radio Astronomy Observatory, 520 Edgemont Rd., Charlottesville, VA 22903-2475 Electronic mail: rsimon@nrao.edu D.F. BUSCHER AND C.A. HUMMEL Universities Space Research Association, 300 D St. SW, Suite 801, Washington, DC 20024 Electronic mail: david.buscher@durham.ac.uk, cah@fornax.usno.navy.mil ABSTRACT This paper reports astronomical seeing measurements made at four sites in the western United States during 1991-92. This study was conducted as part of the site selection process for the Navy Prototype Optical Interferometer. An extensive series of observations were made at Chews Ridge and Anderson Ridge, CA (~36,000 equivalent FWHM seeing estimates). Observations made from Chews Ridge over 49 nights yield an average FWHM seeing value of ~ 1.1", while those made from Anderson Ridge over 34 nights yield an average FWHM seeing of ~ 1.4". However, observations made from Anderson Ridge at a location away from any nearby trees indicate that more open sites in that area may enjoy seeing comparable to that measured at Chews Ridge. Simultaneous free-air, ground-level observations at the U.S. Naval Observatory's Flagstaff Station and at Anderson Mesa, both near Flagstaff, AZ, yield similar seeing values to each other (<= 0.1" difference).