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Q. Who do I notify if I find that a horizontal or vertical control
monument has been destroyed?
A. You can call the Geodetic Unit @ (714) 834-3804
or send an email to: Art.Andrew@rdmd.ocgov.com. It is very important
that you notify us. This information is used to update the database,
which we would like to keep as current and accurate as we can.
Q. Where can I find the local ties, if any, to the horizontal control monuments?
A. When the original 2050 GPS horizontal control network was done in 1990, Monument Records were prepared on all of the stations. These records can be found at the Survey front counter located at 300 North Flower Street, Santa Ana. These records contain a simple sketch with the local curb ties and their measured distances. Monuments can be reset from these ties as long as the tie distances agree within an acceptable tolerance. Additionally, Record of Surveys were recorded for all GPS points. UPDATED: Local ties are now filed as Corner Records and may also be found at: http://www.ocgeomatics.com/landrecords/ .
Q. Can you give me the Horizontal and Vertical control in my area?
A. A. All control point information can be accessed on the Geomatics website via the “Orange County Public Records Retrieval System” located @ http://www.ocgeomatics.com/landrecords/. Select “Geodetic Control” on the pull down menu located on the left side.
Q. What is the elevation of my property?
A. We do not have the elevation to your property but we can give you a benchmark in your area that can be used by a surveyor to establish an elevation on your property.
Q.
Why are there two different elevations on a benchmark data sheet? Which one is related to Mean Sea Level?
A. Very simply stated, NGVD29 is very close and can be considered Mean Sea Level. NAVD88 has no relationship to Mean Sea Level in this area. NAVD88 differs from NGVD29 by about 2.34’ in Orange County.
Q. What is the 1991.35 epoch on the Horizontal Data Sheet all about?
A. 1991.35 is the year decimal equivalent of May 8th, 1991. This date represents when the horizontal stations were surveyed and the published coordinates represent where the stations were relative to each other on that date.
Q. How do I establish the Mean High Water line?
A. By following the proper procedures which are outlined in the document “Procedures for Establishing the Mean High Water Line Boundaries” prepared by the Geodetic Unit. This document can be used as a guideline and is in no way the only correct way to establish the Mean High Water line.
Q. NAVD 88 or NGVD 29, what
is the difference between them?
A. "The National Geodetic Vertical Datum of 1929” (NGVD29) which is the name, after May 10, 1973, of (the) Sea Level Datum of 1929."
"Sea Level Datum of 1929: A vertical control datum established for vertical control in the United States by the general adjustment of 1929."
"Mean sea level was held fixed at the sites of 26 tide gauges, 21 in the U.S.A. and 5 in Canada. The datum is defined by the observed heights of mean sea level at the 26 tide gauges and by the set of elevations of all bench marks resulting from the adjustment. A total of 106,724 km of leveling was involved, constituting 246 closed circuits and 25 circuits at sea level."
"The datum (was) not mean sea level, the geoid, or any other equipotential surface. Therefore it was renamed, in 1973, the National Geodetic Vertical Datum on 1929."
The North American Vertical Datum of 1988 (NAVD 88) is the vertical control datum established in 1991 by the minimum-constraint adjustment of the Canadian-Mexican-U.S. leveling observations. It held fixed the height of the primary tidal bench mark, referenced to the new International Great Lakes Datum of 1985 local mean sea level height value, at Father Point/Rimouski, Quebec, Canada. Additional tidal bench mark elevations were not used due to the demonstrated variations in sea surface topography, i.e., the fact that mean sea level is not the same equipotential surface at all tidal bench marks.
For further information, please click here to view the NOAA website.
Q. . Which Datum do I use?
A. . The NGVD 29 and NAVD 88, OCS 1995 adjustments are both referenced to the geoid (mean sea level). Because of the errors in the leveling and distortion from the sea surface topography, NGVD 29 is not an accurate sea level datum. Since the adjustment was warped to fit the local tidal stations, NGVD 29 does most closely fit "local mean sea level". NAVD 88 is a more accurate orthometric height because the errors that NGVD 29 contain are not present. The two datum’s relative differences between each benchmark should be equal. Certain areas will differ due to the constrained benchmarks in each adjustment. Benchmarks held fixed in the NAVD 88 adjustment are not the same as the benchmarks held fixed in the NGVD 29 adjustment. Of course, the absolute difference between each datum will differ greatly, average 71 cm (2.3 ft.). County of Orange, Caltrans, and Federal Emergency Management Agency (FEMA) are using NAVD 88. The American Congress on Surveying and Mapping and Federal Geodetic Control Subcommittee recommends using NAVD 88. Switching from NGVD 29 to NAVD 88 vertical datum is similar to switching from NAD27 to NAD83 horizontal datum. As to what vertical datum should be used, it is up to the Surveyor and the requirements of each project. NGS will only be maintaining the NAVD 88 network in the future. The OCS 1995 adjustment will be the last NGVD 29 adjustment that the County of Orange will maintain.
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Scripps Orbit and Permanent Array Center (CORS data and information)
National Geodetic Survey
Recent earthquakes in California and Nevada
California Spatial Reference Center
Orange County's Web Based Records Retrieval And Viewing System
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