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."

Meteorological Tables (p.19) of Goddard's Report

Place
Date
Hour
Aneroid
Ther
Remarks

"Foot of hill"

Sept 30

7.30

24.50

56

"Silver Creek Ranch"

Sept 30

9.30

23.38

76

"On head gulch falling into S[outh]. fk [American R.]"

This is the follow-up to the original post on Goddard's barometrical observations and determinations of elevations.

On the September 5 outing of the Johnson's Road Group, this simple experiment was conducted. It does not require the recalculation of Goddard's observations, but is rather a reenactment of the observations Goddard made on September 30, 1854 in order to find if his data can be reproduced.

The point of beginning, or lower station, at the foot of the hill was at position A--a position we estimated to be at the elevation of the original road, approximately 80' above today's highway at the turn off to Wrights Lake.

On this day, the atmospheric pressure indicated on the aneroid barometer was 24.42"Hg, which was .08" less, because of daily fluctuations, than what was indicated on Goddard's barometer (24.50"). That .08" was taken as index error and subtracted from Goddard's indicated 23.38" at his upper station, giving a target pressure of 23.30"Hg.

We drove up the Wrights Lake Road until the 23.30" target was reached, which was at position B, which was right at the head of the gulch falling into S[outh]. fk [American R.]--the summit before entering the Silver Creek drainage.

This just repeats Goddard's observation of a reduction of 1.12"Hg between the point where he started to ascend and the upper point of his arrival. So it doesn't matter how he, or I, reduce that observation to an elevation above sea level.

Goddard's determiinations of the elevation of position B was 6,177'.

My previous recalculation (above sea level) of Goddard's original barometrical observation was 6,738'.

Calculating the elevaion change between point A and point B on Sept. 5, 2007 (difference = 62900 log10 pointB/pointA) gives an elevation change of 1,283'. This added to the elevation of A gives for point B 6,770'.
This is how it should be done.

The actual elevation by survey Bench Mark is 6,767'.
Goddard's barometric observations--as opposed to his elevation determinations--are good data.


Email received Sept 8, 2007

Interesting -- Point B has a timber landing on it (and has, since I began working here in 1989), but would have been immediately above Johnson's Cut-off, where it ascends the canyon and takes a left-hand turn to the west, atop the ridgeline. It's a very short hop down from there to the grasses / meadows along Lyon's Creek (much of which is now overgrown with conifers). I think we found a piece of historic ceramic in that landing, about 12+ years ago. I'll see if I can find any reference to that in any of our field notes from way-back when. And down the ridge facing Lyon's (a few hundred meters from Point B), we found a hole-in-top tin can. Might be fun to take a closer look at that area, although there is a lot of deadfall in there now.

Krista Deal
Pacific District Archaeologist
Eldorado National Forest


NOTE: Returning that same day to the Sacramento Valley on the evening of the outing yielded an interesting technical point related to errors in aneroid barometers. In the morning before driving up the mountain I had referenced my portable barometer to my Standard barometer at home. After returning, having completed a closed cycle of decompression and recompression, the portable barometer did not return to the same indication of the Sacramento based barometer--it indicated .05" less.

This is is caused by an elastic property of the of the metal of the barometer bellows called hysteresis--when deformed, the metal does not immediately return to its original state. This is temporary--overnight it did gradually return to comformity with the reading of my standard barometer. The actual technical term for this is creep.

This was not a problem for Goddard, as his ascents and descents were made over more time--not at modern freeway speeds. But this is a problem for altimeters in aircraft, as one can fly the plane into the ground on landing after a descent. It is why pilots contact airports for the current state of their barometer and reset their altimeters.