Johnson
Road Project
Silver Creek
Ranch
Goddard--p.73, September 30, 1854. "The road here winds up to the summit of the ridge near which is the small flat of Silver Creek Ranch; we had ascended 1,376 feet in the last two miles. The summit of the divide is about 100 feet higher [than] the ranch. Its level above the sea is 6,277 ft. being more than 300ft above Bigler Lake Valley." In his Meteorological Tables (p.19) the entry for Silver Creek Ranch he adds this further remark re its location.
In other places in the Report, Goddard uses the word "gulch" to describe what we would call a ravine. And in his Tables of Approximate Distances and Altitudes Johnson Cutoff Road (p.5) for the entry "Silver C. Ranch," Goddard adds the remark "Road descends to River." |
Here are some thoughts on two parts of the description; Goddard's determined elevation, and the "gulch."
Goddard's determinations of the altitudes are in error. However, his observation of 23.38"hg is apparently very good--it should mathematically reduce to 6,738', which is a near fit to the general area, as the elevation of where the bridge crosses Lyons Creek is 6,672' and Wilson Ranch is 6,610'. See below for details of these barometric determinations.
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From Google Earth, the gulch (ravine) leading down to
Fred's Place. |
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An overlay of topo and photo using TerraBrowser images sourced from Teraserver. Shown are Wilson Ranch, the meadow where the bridge crosses Lyons Cr., the wide turnout at Freds immediately below, and the slide that took out the Wrights Lake Road a few years ago. Note again the ravine heading on Lyons Creek and leading down to Freds. |
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An image rendered from USGS DEM (Digital Elevation
Model) files. |
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A topo image and a satellite image of the bridge and meadow. |
An examination of Goddard's reported elevations in the area of the present study of the Johnson Wagon Road.
Here are tables showing Goddard's barometric observations taken along the route of the Johnson Cut-off in 1855. This was early use of the aneroid barometer, and Goddard was uncertain of the appropriate mathematical reductions and temperature compensation to give the altitude. But, the aneroid of this form was only invented by Vidi [Vidie] in Paris in 1845, so this was still brand new technology in 1854. Very early use in a survey!
Goddard's instrument was marked "No. 264, J. W. Queen, Chesnut Street, Philidelphia." Queen. In the 1850s, Queen's anerouids were actually supplied by W. & L. E. Gurley in New York, who probably imported them for him from Vidi in Paris or Dent in London.
Aneroids,
a calibrated mechanical instrument, are useful because of their
portability, but are never intended to be used without reference to a
standard direct-reading mercury barometer. Goddard did make these
references with a mountain (ie, portable) iron cistern mercury
barometer by Henry & James Green to set his aneroid (Placerville)
at the beginning of his work, but constant problems with the fragile
mercury barometer eventually rendered it useless, so all observations
were made with the aneroid. He was, however, very careful of the
instrument--carrying it only on foot on a cushion in a basket in an
effort to have it maintain its zero.
In following Goddard's narrative, he gives his determined elevation for many places on the Johnson Road curriently under study. But these reported elevations at known locations are found to be in error, and are therefore not very useful in attempting to locate unknown locations.
But in Goddard's tables of Meteorological Observations are given the original observations of his instrument for each location in "hg (inches of mercury). This remains Good Data. It is only necessary to take the original data and reduce it mathematically by modern method to get a more accurate determination of the actual elevations.
Bear in mind, that not knowing what the sea level barometer was
reading on any particular day in question in 1855, it is necessary to
use an accepted seasonal daily mean pressure. So this does not take
into account weather variations which would have affected his
barometer. However, the registers over the corresponding days kept by
Dr. Logan in Sacramento and Mr. Lelner at Coloma slow very little
variation from a standard mean.
However, the temperature of Godard's instrument (which was not
temperature compensated) can have a larger effect than the daily
barometric variation.
Goddard: "There is indeed much uncertainty as to the amount of correction the aneroid requires for differences in temperature, and indeed as to what formula is the most applicable to the reductions of observations made with this instrument. After trying several, I found the well known formula of Baily For. 38 Astro Table to give the best result, and therefore have used it in all cases. "
Note: I have located the formula and tables by Baily that Goddard used in Smithsonian Meteorological Tables [Based on Guyot's Meteorological and Physical Tables], Smithsonian Miscellaneous Collections No.1032. I should have a copy of the 1893 second edition in hand in the near future.
The formula I have used is a simple isothermal calibration
standard from the Bureau of Standards.
Z= 62900 log10
Po/P
where Z=altitude in feet, P=Pressure at the upper limit in any
units, and Po=pressure in same
units as above corresponding to zero altitude.
It makes assumptions for the temperature of the column of air then
that we cannot know.
|
observations 1855 |
|
|
|
Johnson Pass |
22.84"hg |
6,752' |
7,376' |
7,377' |
Strawberry |
24.12"hg |
5,136' |
5,886' |
5,900' |
Wrights Lake Rd |
24.50"hg |
4,901' |
5,459' |
5,427' |
Silver Cr. Ranch |
23.38"hg |
6,177' |
6,738' |
|
Divide bet. S. Fk and Silver Cr. |
23.27"hg |
6,277' |
6,866' |
This ridge follows along at approx. 6,800' |
Peavine Hill |
23.39"hg |
6,410' |
6,726 |
6,645' |
Brockless Trading Post |
24.34"hg |
5,638' |
||
Bartlett's Br. |
26.75"hg |
2,532' |
3,059' |
3,000' |
Aneroid instruments are calibrated in "hg, but often have a scale
of elevation in feet/meters. The elevation scale is only approximate,
and is useful between stations. Some modern units designed for hikers
are calibrated only in feet/meters. One can set the indicator to
correspond with a place of known elevation, such as a contour on a
topo map, and navigate to a place on another contour. Accurate
estimates actually require that observations of pressure and
temperature are made at both upper and lower stations and are then
reduced using log tables. But
the moveable scale (stem setting) on mine--an early 20th C instrument
by Short & Mason, was designed by mid 19C Astronomer Royal,
Professor G. B. Airy and is much more useful.
In using Airy's scale, 0' is always set to 31"
The formula used is then D = (H-h) (1 + [T+t-100] / 1000)
D being the distance between upper (H) and lower (t) stations in inches, and T/t is the temperature at upper/lower stations in Fahrenheit.
This is how Goddard's elevations were done--the elevation of each station was based on the previous station and all stations eventually added together. The elevation of Placerville, from multiple observations of the mercury barometer, was the referenced base.
But calculating elevations from Goddard's
observations isn't even necessary to follow him.
The next time I am in the area of these readings, I intend to
bring my own pocket aneroid. By setting it to correspond to Goddard's
stations, it should give the corresponding reading at the
others--this would be purely comparative--no necessity to
reduce to altitude.
Before GPS, I used to carry a modern Swiss made altimeter by
Thommen. It's scale has 20' increments to 15,000'--this resolution
requires a gear train so that the indicator revolves five times
covering the range. But I soon stopped using it when I found that
that the action of snapping/unsnapping the deluxe leather carrying
case could cause an indicated elevation change of 100 feet or
more.
Too much mechanism and false precision. My weighty old Short
and Mason scale that has scale divisions in 50' divisions is much
more robust and, with reasonable care, holds its zero!
Some notes.
The history of Day and Goddard's surveys excerpted from From Trails to Freeways.
How modern mathematical reduction of historical barometric observations can identify disputed sites.
An overview of 19th Century barometric determinations of altitude.
Lt. R. S. Williamson's essay on The Use of the BArometer on Surveys and Reconnaissances--ground breaking work conducted in the Sierra Nevada in the 1860s. This is online, and difficult to access, but contains much historical data of great importance to climatologists.