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Determination of Altitudes
and Notes on the Barometrical Observations
taken on the Carson and Johnson Immigrant Roads
over the Sierra Nevada in 1855

by George H. Goddard, Civil Engineer

The origin of the survey
(Note: The instruments shown are only similar to the unstruments used by Goddard and Day.)

The altitudes given in the following tables have been calculated from a series of observations made with the aneroid barometer during the journey. These observations are given in full. The aneroid barometer used was No. 264 J. W. Queen, Chestnut Street, Philadelphia.

Note: In the 1850s, Queen's aneroids were actually supplied by W. & L. E. Gurley in New York, who probably imported them from Vidie in Paris or Dent in London. Te view here also shows the mechanism of an example marked Queen and dating to the period.

It will be seen by a comparison with the Register kept by Dr. Logan of Sacramento which follows that the aneroid was two tenths of an inch lower than the Doctor's barometer on the 1st August. There is reason to believe that the latter even was too low, but in the absence of a standard barometer, it is impossible to tell what the true height of the mercury should be in this country at the sea level. Certain it is that the barometer in general use here, mostly of English construction [Ship Barometers] stand very low, while those which have been prepared with care, and set up in San Francisco, on the contrary, appear to stand too high. It is possible that the quicksilver may be impure in the latter and its specific gravity in consequence too little. We await with impatience, the arrival of a Standard Barometer which is now on its way to the country sent by the Smithsonian Institution to the San Francisco Academy of Natural Science.

In reducing the observations, into altitude, I have considered it more advisable to depend rather upon the results obtained by successive difference from station to station, taken within a few hours of each other under nearly similar atmospheric circumstances, than upon those obtained by any other mode. Thus, instead of taking the mean of all the observations at any camp as the reading of the upper station and the present uncertain base of our sea level as the lower reading. I have in all cases preferred taking the differences between the last observation on leaving Camp and the first observation made on the road and so on, the difference between each observation and its last preceding one, until our arrival at the evening Camp when the first observation is the one used. The successive differences thus obtained being put together give the heights furnished in the table. Although this method carries an error once committed throughout the entire series, still I conceive it gives a nearer approximation to the truth than a mean derived from the [word?] observation made at any camp and compared with what I consider as an altogether imaginary base, could possib[ly] give.

In addition to the aneroid, I was furnished with one of Green's Iron Cistern Mountain Barometers*, but from the difficulties attendant upon setting up this instrument. it could only be used at the principle Camps by way of a test on the aneroid. Unfortunately, I did not receive it in time to set it up in Sacramento, so as to compare it with the aneroid before leaving on out journey. This comparison was, however, made at Placerville and at the Astronomical Station in Clear Lake Valley [now Caples Lake Reservoir], the highest permanent camp on the journey, and in both instances the two instruments coincided, as will be seen by the Register. At this Camp, the Mountain Barometer was unfortunately broken, and so no further comparison could be instituted. The fluctuations of the Aneroid are more considerable than those of the Mercurial Barometer--particularly those caused by the non-periodic variation of atmospheric pressure.

*James Green of Baltimore, and later New York, was one of the foremost makers of barometers in America. "Green's iron cistern mountain barometer" was of a type which he had made for J. H. Alexander. It was an improvement on one designed by Ferdinand Rudolf Hassler of the U. S. Coast and Geodetic Survey--one of Frémont's mentors. Ten years earlier, Frémont was much better equipped.

In 1856 Green designed an improved portable barometer based on Fortin's design which was used in surveys and by stations of the US Weather Bureau for many years. It was a Green's mountain barometer that was used by Robert Stockton Williamson on his survey published as On the Use of the Barometer on Surveys and Reconnaissances , D. Van Nostrand, New York, 1868.

Williamson recommended against the use of the aneroid:

"The aneroid is a very convenient little instrument. But for nice work, as hypsometrical work, as at present constructed, an inferior instrument."

On page 131 of the above work he published a table of comparisons of an un-named aneroid with Greens Cistern barometer No. 1343.

The correction of the Horary Variation and extreme air temperature given in the very valuable Barometrical reports of L. Blodget of the Smithsonian Institution and Dr. George Engleman of St. Louis, and used in the reduction of the observation made on the exploration of Lt. Whipple and Beckwith, and which appear to be required for the California Climate, however applicable they may be to the mercurial Barometer do not appear to answer for the Aneroid. There is indeed an uncertainty as to the amount of correction the aneroid requires for difference of temperature, and indeed as to what formula is most applicable to the reduction of [the] observation 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.

There is another source of uncertainty with the aneroid--it appears to be highly sensitive to wind, and the amount of correction required can only be considered as approximately known. Hourly observations should be taken with these instruments at the principle points to get data for future use: my time, however too much occupied with the other duties of the Survey to allow me to give that attention I would have wished to give to this subject.

Yet notwithstanding all these uncertainties, and sources of error, I have had several most surprising instances of the correct working of the Aneroid used. It is with considerable satisfaction and pleasure that I give the following comparisons with actual measurements since made by the Hon. Sherman Day over the same ground in the Survey of the Immigrant Wagon Road.

In my preliminary report dated Oct 5 and published on the following day, I give the approximate height of Cary's Mill in Carson Valley [Woodfords] and the head of the Carson Canyon in Hope Valley. The difference of elevation between these two points was 1456.2 ft. Mr. Day on his return in the last week of December gave me his measurement with theodolite and chain up the Carson Canyon between the same points, which amount to 1455.4 ft, thus proving the wonderful and absolute accuracy of the instrument in the observation.

In the height of Luther's Pass above the same point in Hope Valley, the difference will be seen to be 695.5 ft by my observation. Mr. Day's measurement makes it 715.6 ft. There is an apparent error here of 20 ft, but the height given by me was at the lowest point of the Divide where the road crosses a small ridge slightly elevated above the former which makes the observation almost if not quite identical. In the height of the Slippery Ford Hill, we correspond again exactly. In some points, however, there are discrepancies. Mr. Day makes the descent from Marlett's Flat [Grass Lake] to Biggler Lake [now Lake Tahoe-Goddard's drawing at right] Valley 40 feet greater than I do. In the height also of the Johnson's Pass Mr. Day's height will I expect also exceed mine by about 100 feet. There is, however, a little uncertainty as our points of observation were not the same. I took the lowest point on the road of the summit of the divide, as the top of the pass while the old road [Johnson's] winds along on the divide for a quarter of a mile at an increased elevation.

I may also mention that in the same preliminary report the difference between the elevation of the Mormon Station [Genoa] and of Cary's Mill [Woodfords] is within five feet of that made by reckoning it from the successive differences along the road.

So many instances of accuracy absolute and approximate gives me a great reliance in the Aneroid Barometer when used with care, but at the same time it is an instrument so liable to change its zero from a jar or a blow, that it ought be carried in a basket by hand, by a person on foot, and not strapped across the shoulder or carried by a horseman. There being no means of detecting the amount of error caused by a change o zero, it is impossible to be too careful in its use. It is an instrument so adapted to an exploration of this nature, from the ease and rapidity with which an observation can be made, that it is certainly deserving of that care and attention that shall insure accuracy in its results.

The means of testing it at all the principle Camps should likewise be provided, so as to obtain data for its temperature and horary corrections as well as to be able to reset it., should its zero have been altered. The mercurial barometer should be used for this purpose, as the errors to which the measurement of height by the boiling point [see hypsometer at right] is subject are so very serious, that little dependence can be placed on the results so obtained.

Note: This was the method resorted to of necessity by Frémont in this region in 1844

On my return to Placerville I handed over the Aneroid to Mr. Day, who has had it with him ever since testing it on several occasions. It has, however had its zero altered by rough usage several times. Still Mr. Day states that the difference between certain points on the road are very similar to those given by me.

For more readily formed conception of the relative level of the country arranger at table of the altitudes of all the principle points on the old Carson and the Johnson Roads in the order of elevation.

I have also inserted amongst these tables one of the heights along the Sonora and Walker River Immigrant Road made from a series of observations taken with the Aneroid barometer by me when acting as Civil Engineer to the Railroad expedition made under Lieut. Moore in Oct 1853.

 The observations were tested by the boiling point at all the Camps, which invariably gave a yet greater elevation; I am under the impression however that the heights given in the table are too great for all the points on the eastern slope of the mountains, high winds having lowered the instrument on several occasions. The mode of reduction I have employed was that of the means of the observations against the monthly mean of Dr. Gibbons' Barometer at San Francisco as a base. As previously stated, this method of reduction I have since abandoned, for that of successive differences, which if applied to heights on the road in question would lower them from 100 up perhaps to 400 or 500 ft in the more eastern portions of the journey.

George H. Goddard

An examination of the some of Altitudes Determined



The determined elevation 1855

Actual elevation

Reduced by modern formula*

Bartlett's Bridge
S. Fk. American River below Pacific House





Top of Slippery Ford Grade
Camp Sacramento





Luther Pass





Carson Pass





Clear Lake
Now Caples Lake Reservoir





Hope Valley
Top of Carson Canyon


7070'was the evation determined by R. L. Williamson about 1860



Johnson Pass










Wrights Lake Road at Highway 50





* Z= 62900 log10 Po/P using sea level mean for month.

This was real pioneering use of the aneroid in survey work. Although the elevations determined by Goddard and Day are in all cases much to low, their confidence in the accuracy and practicality of their aneroid barometer appears to have been entirely warranted. It was the formulas used for reducing their observations that were not up to the task. Using a modern formula their original observations from their instrument yields excellent results.

Further examination of Goddard's data in the Eldorado National Forest Johnson's Road Project.

But six years later, in September, 1861, William H. Brewer, Chief Assistant of the Whitney survey reported the following on an ascent of Mt. Diablo.

"We carried to the top a new barometer, one made for the government topographical engineers, and an aneroid barometer of the finest construction, to test the accuracy of the other instrument. We found it far less accurate than the mercurial barometer, and it required as much care to carry it."

Do it yourself! Formulas and spreadsheet download
The Mountain Barometer. And a remarkable field repair by Frémont in 1842
Australian Exploration and the Introduction of the Aneroid Barometer by Julian Holland
Frémont's determinations of elevations
go Some of the historic roads through the canyon of the South Fork of the American River along the route that the Frémont Expedition traveled between February 23rd and 26th in 1844. From 1852 until present, this has been an area of intense road building.

©1999, 2007
Bob Graham