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Written by stephen
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Friday, 19 December 2008 21:26 |
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The new Idaho-Maryland roundabout destroyed the old U.S.G.S. Benchmark V213 dated 1934. The cap was well hidden under a pile of earth and rocks and had not been recovered either by N.G.S. in 1949 or by Caltrans in 2004. The City engineer was recently asking about remonumentation and the value of having the municipality maintain benchmark records. From what I've seen locally anyway, benchmark information varies widely between the agencies. Grass Valley conducted an aerial survey in 1980 and set some semi-permanent benchmarks at that time presumably with the idea of coordinating various public works projects onto a common vertical datum tied to U.S.G.S benchmarks. Most of the points set for control of this project were rebars, RR spikes or nails set in pavement and are getting hard to find and may have shifted. As far as I know, this is the only effort the City has made to establish a vertical control network. I have heard this referred to as the "Grass Valley Datum," but I don't know if it has been officially adopted as such. Benchmarks set and maintained by NGS are listed here: http://www.ngs.noaa.gov/cgi-bin/ds_radius.prl Data sheets at these points are useful and contain recovery information as well as Geodetic. There was also a FEMA study along Wolf Creek, establishing base flood information. Almost all of the benchmarks set during the course of this study have been destroyed, with the exception of the U.S.G.S.marker in the old Post Office building (now lawyer's offices). As far as the Nevada County and other public agencies that I am familiar with, including the City of Auburn, the City of Rocklin and the town of Loomis, are concerned, there are lists or binders in their offices of varying degrees of antiquity that show benchmark locations and elevations. As far as I know these were prepared quite some time ago and have not been maintained recently. There are no ordinances requiring the upkeep of these lists. I have discussed this with Tom Martin, the County Surveyor, and his feeling is that since the advent of GPS, absolute vertical elevations are much easier to obtain and they have gotten away from requiring ties to benchmarks on improvement plans. I believe that the benchmarks are still useful as checks and ties for the GPS observations, but maintaining a dense vertical control network may be more expensive than establishing tight GPS control on an as-needed basis.
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