Ivan's Place
In honor of the greatest moralist who never lived
Copyright © 2004 by Bill Becker

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1.        PBM Descriptions

2.        PBM Elevations       Source data for the    The Differences Study

3.        Level Net Map

4.        The Differences Study       Derived from    PBM Elevations

5.        Scanned original of Differences Study

6.        Scanned original of NAVD88 Study

7.        Geographic and Grid Coordinates

8.       Zone 5 coordinates, 1980 & 1994 elevations, and the differences between them.


First, all of the materials on the pages referenced above, and any additional such pages I add, are in the public domain.

Second, all elevations on these pages are in meters. Where they appear, California Zone 5 grid coordinates are in U.S. feet.

At the moment of the 1994 Northridge Earthquake, centered in the San Fernando Valley of the City of Los Angeles, I staffed the geodetic desk of the City's Survey Division. As I recall, I was then engaged in updating the basis of the City's entire Precise Benchmark Index from the National Geodetic Vertical Datum of 1929 (NGVD29) to the North American Vertical Datum of 1988 (NAVD88). Naturally, a large portion of my work went out the window.

The Survey Division immediately set out to re-level the San Fernando Valley. My job was to take the data recorded by the division's precise level crews, enter the numbers into the least-squares adjustment program LEVELOB on the City's mainframe computer, and create various reports on the vertical motion of the Valley as requested. I worked under the direction of the Engineer of Surveys at the time, Robert Packard.

At some point, the need for such analyses diminished, and I transitioned into a broader programming role for the Bureau of Engineering, as well as continuing to work closely with the Survey Division on other projects. Before I "shut down" the analysis program, however, I made copies of two separate analyses for my personal files. One of them I will call the "Differences Study;" the other I will call the NAVD88 Study. I have been carrying these files around with me since 1994. It is possible that copies of these reports, and perhaps others which I do not remember, are buried somewhere in the Survey Division's archives. But, as far as I know, they have never otherwise seen the light of day, except for a Ph.D. candidate or two whom I seem to remember sending the Differences Study to.

The files referenced above re-present the two studies. The organizing schema is based on the original Differences Study, which was designed according to the lines along which the City's Precise Bench Marks were observed by the precise level crews, and the sequences of the marks as the crews proceeded along the lines. These lines are shown on the map of the level network referenced above. According to this schema, two fields are the indexes for each record of the different datasets referenced above: the Line Label and Sequence Number. E.g.: AY-000 12-22751; AY-001 12-22810; AY-002 09-01500; ...

A note on the PDF originals: I was not able to scan the original study pages with the resolution or precision which would allow me to copy and then paste the text for further programmatic morphing into the files on these pages. For reasons I do not understand, the scanned values were garbage when copied and pasted. Thus I had to transcribe by hand virtually all of the elevations and other numeric values seen here. On the other hand, the PBM numbers and their descriptions were selected, copied, and pasted from the Survey Division's official Precise Benchmark Website.

For visitors with a lot of patience, my transcriptions of the printed pages can be checked against the PDF files themselves. I hope that I did not make any serious mistakes.

Finally, I hope that this material will be of use to those with a professional, academic, or simply curiosity-based interest in the earthquake. For more information, I can be reached at ivanklives @ earthlink.net.

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Page updated October 16, 2014