Don D'Onofrio
Geodetic Consultant August 3, 1999

Ronald J. Neighbors
General Manager
Harris-Galveston Coastal Subsidence District
1660 West Bay Area Boulevard
Friendswood, TX 77546-2640

Dear Ron,

The County of Yolo has just completed the initial observation portion of a county-wide GPS survey to establish a subsidence monitoring network. The county and the other agency participants are asking for ideas about monitoring the network. I would appreciate your ideas about the need and frequency of repeat measurements to monitor subsidence. I am sending letter to others, like yourself, who are involved in subsidence issues. A listing is included in the attached.

There were 45 stations selected for the network inside the county and three stations outside the county in stable areas. The network also includes two CORS sites which have NAVD88 heights. Although one of the CORS sites is in the county, and subject to subsidence itself, it is continuously monitored with respect to other stable CORS sites. Yolo County is in the Sacramento Valley in northern California and is the county immediately west of Sacramento. It is primarily an agricultural county. Several feet of subsidence have been noted in areas of the county. The subsidence is primarily the result of ground water withdrawal.

I would be interested in your thoughts about: network (station) maintenance; frequency of re-observation; guidelines (e.g., can maintenance re-observation guidelines be less stringent than those used to establish the network?); factors affecting re-observations; and other issues you think germane to subsidence monitoring with the GPS technology. I would also be interested in your use and frequency of conventional leveling techniques for subsidence monitoring, and your estimates of relative cost differences between conventional leveling and GPS techniques.

I look forward to hearing from you. Thanks for your consideration.

Regards,
/s/
Don D'Onofrio
7228 Willowbank Way
Carmichael, CA 95608
(916) 944-7879
trishdon@pacbell.net

Identical letters were sent to:

Marti Ikehara
NGS State Geodetic Coordinator
C/o Caltrans Geometronics Branch, MS35
P.O. Box 942874
Sacramento, CA 94274-0001

Luis James
Water Supply Policy and Planning
Santa Clara Valley Water District
5750 Almaden Expressway
San Jose, CA 95118-3686

Edmund S. Miller, Jr.
Chief Surveyor
Port of Long Beach
P.O. Box 570
Long Beach, CA 90801

Ronald J. Neighbors
General Manager
Harris-Galveston Coastal Subsidence District
1660 West Bay Area Boulevard
Friendswood, TX 77546-2640

Garry Weldon
Chief, Geodetic Branch
Division of Land and Right of Way
California Department of Water Resources
P.O. Box 942836
Sacramento, CA 94236-0001