Retracing the Thirty-Ninth Parallel to United States-Mexico Boundary Arc of Primary Triangulation
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LOS ANGELES SE BASE (NGS PID DX4752)
During the classical era of geodetic surveying, triangulation was the most efficient method to both extend and densify horizontal control networks over large areas. Positions can be determined mathematically through such a system if one knows the position at the point of origin and can reduce the heights of all the stations to a computational surface (an ellipsoid); triangulation networks also require two other key elements to determine the positions of the other stations in the network: scale and orientation.
In larger triangulation networks, scale and orientation is provided by periodically measuring base lines connected to the scheme of triangulation. Astronomic observations are also made at the respective ends of those same base lines to determine the orientation of that line, thereby strengthening the triangulation network regionally. Triangulation base line stations are somewhat analogous to initial points in the public lands survey system.
The role of the Los Angeles Base Line was to provide local scale and orientation to the national system through the `Thirty-Ninth Parallel to United States-Mexico Boundary Arc’ of primary triangulation in southern California.
During the 1870s, it became the USC&GS’s mission to establish a unified system of triangulation throughout the United States and the agency decided that two base lines would be required in California: one in the north and one in the south. Throughout the early era of coastal triangulation, similar base lines were measured but for various reasons, these older base lines were not considered suitable from the perspective of building a national triangulation network. Therefore, new base lines would need to be marked and measured in California.
These two new base lines in California would be located in Yolo County (west of Sacramento) and southeast of Los Angeles. The task of surveying these base lines and connecting them to the primary network of triangulation was assigned to George Davidson of the USC&GS. Sent to the Pacific frontier in 1850 along with a few other "energetic young men" with "reputation to make", Davidson would spend forty-five years along the Pacific Coast performing geodetic surveys and making nautical charts for the USCS and its successor agency, the USC&GS. During the `Thirty-Ninth Parallel Arc’ survey, he extended triangulation westward across the Sierra Nevada Range towards the Pacific Coast using sights that were in some cases over one-hundred miles long, earning these surveys the nickname `Davidson’s Quadrilaterals’.
Assistant Davidson was a renowned scientist but amongst his greatest technical achievements were the measurements of the Yolo Base Line and Los Angeles Base Line. During the 1880s, these were the longest measured base lines at the time in geodetic surveying with accuracies of better than one part-per-million. Both lines measured at just over 11 miles in length each.
Davidson measured the Yolo Base Line in 1881 and he decided that similar procedures would once again be used for the Los Angeles Base Line in 1889. Davidson describes for us the mark set at LOS ANGELES SE BASE:
"… in 1889 underground mark was small needle hole in place of silver wire, one-tenth inch in diameter, which was driven into head of 5/8 inch copper bolt which was fixed in melted sulphur in granite block. Block was 1 foot square on top, 3 feet long, and placed 5 feet below surface. Copper bolt was covered with glass evaporating dish, cemented onto block, and top of block was marked with letters `U.S.C. & G.S.’ Underground mark was surmounted by brick and cement pier, 72 inches square at base and 54 inches square at surface. Space around pier was filled in with coarse sand and charcoal. Surface mark was small needle hole in silver core of 5/8-inch copper bolt which was set in granite block, 26 inches square, flush with surface of ground and built into foundation pier … In 1890 brick pier, 35 feet high, was built over station. In 1896, this pier was removed and granite monument, 24 inches square at base and sloping to 12 inches at point 2 feet above base, with pyramidal top and small hole at apex of pyramid marking center, was cemented to surface block. Side of monument toward base line was lettered `U.S.C. & G.S. S.E. BASE 1889′."
As an appendix to the 1889 Annual Report of the Superintendent of the Coast and Geodetic Survey, Davidson left a detailed description not only of his survey of the Los Angeles Base Line, but sketches of the subsurface marks set at their ends.
After World War II, land development in Orange County boomed with many of the orange groves and farm fields being subdivided into residential housing tracts. A subdivision was built around LOS ANGELES SE BASE and almost miraculously, Davidson’s original monument remained intact until the late 1950s; the mark was located in a backyard in the City of Garden Grove.
During a recovery in 1957, the USC&GS replaced Davidson’s original surface monument constructed of granite with a less obtrusive bronze triangulation disk. As described in the 1957 recovery,
"… the station mark and one reference mark were recovered. All other reference marks were searched for but not found, and it is quite evident they have been destroyed as the area has been landscaped and houses built. The pyramid-shaped block of granite, the 26-in. block of granite and part of the brick foundation were removed at this time, and replaced with a standard disk in concrete flush with the ground. Two additional reference marks, 6 and 7, were established at this time … the station mark, a standard disk, stamped `LOS ANGELES SE BASE 1889 1957′, is set in the top of a 1-ft square block of concrete which is flush with the ground and protrudes down into the brick foundation 10 in."
According to the recovery note, when the USC&GS replaced the original surface mark in 1957 with a bronze triangulation disk, they apparently left in place Davidson’s original subsurface mark under the remains of the brick and concrete foundation.
Recovering LOS ANGELES SE BASE
The previous NGS recovery note dated from 1990 and it was uncertain whether this station had ever been occupied using GNSS. According to NGS records, another station, LA SE BASE 2 (NGS PID DX4774), was established in 1974 approximately 1500 feet away as a replacement for LOS ANGELES SE BASE. We felt there was a good chance that LOS ANGELES SE BASE might still exist, so an attempt was made to determine its status.
Not knowing exactly what to expect, I drove to the residence to meet the landowner in person. I knocked on the front door and an older woman answered. After introducing myself, I asked if she was the original owner of the home and she explained that she was. I then asked if the survey marker was still located in the backyard and if it was, could I please have her permission to look at it. Her next statement was almost surreal and caught me off-guard; she told me that she knew that a surveyor would visit someday asking to see the monument and that she was expecting someone like me to show up at her doorstep. She then gave me permission to enter the backyard and visit the station.
She led me into the backyard and showed me a USC&GS triangulation disk in pristine condition located in the middle of a nicely manicured lawn. If someone were glancing quickly, they might mis
take the monument for a sprinkler head and the yard looked more fitting for a garden party than a geodetic station. The station had not only been forgotten but had also become a gentrified part of suburban America; apparently, LOS ANGELES SE BASE had been hiding in plain sight all these years.
Next, I recovered reference mark 7 and then asked the whereabouts of reference mark 6. She reluctantly told me that her husband removed the original concrete stairs (the setting for reference mark 6) some years ago and replaced them with masonry steps leading to the rear of the home. She described her apprehension for having disturbed the reference mark when the family built the brick stairs and reminded me of the punishment, "$250 fine or imprisonment for disturbing this mark", embossed into the disk.
I explained that as a licensed California land surveyor, I would be more than happy to take care of this situation by filing the appropriate paperwork on her behalf; I assured her that no one was in any trouble. I asked if she still had the monument that had been removed from the steps "yes" and if she would allow me to keep it "of course". She seemed relieved.
I then asked permission to survey the station mark, but she was somewhat reluctant. She was concerned that we might damage her lawn and proceeded to describe what occurred the last time the station was surveyed.
During the early 1970s, she told me that surveyors "lived" in her backyard for a week, pitching tents, and causing all kinds of commotion. She described a scene where the surveyors stayed up all night while working out of a trailer located in the vacant lot next door to her home. According to the recovery note, NGS last surveyed this station in 1974. In my mind, I could imagine a 1970s-vintage NGS field party working to breakout of the relatively small, confined yard (while at the same time, dealing with less than happy landowners) using an eccentric setup in the vacant lot to connect LOS ANGELES SE BASE with other stations in the control network. The 1974 NGS recovery of LOS ANGELES SE BASE tells the entire story: "LA SE BASE 2 was established because a tower could not be erected over the station mark in the rear lawn of the property owner." In what might be the ultimate irony of this story, LA SE BASE 2 is now listed as not found in the NGS recovery records while the mark it was intended to replace, LOS ANGELES SE BASE, still exists.
I explained to the landowner that this type of surveying is very different today because we now use satellites. Telling her that we would only need to visit the yard for a couple days, I gave my assurance that we would work during the day and that we would be gentle with her well-kept lawn. Her only remaining issue was that she intended to turn on the backyard sprinklers during the early morning and she expressed concerns that the surveyor might get wet as result. I responded that getting a little wet from the sprinklers would probably feel nice on a hot summer day and it would not be an issue because field surveyors worked outside. The landowner, finally convinced that we would be respectful visitors, granted my request and Caltrans was ready to survey LOS ANGELES SE BASE one week later.
The last question that I asked before leaving was if she had any photographs of in her words, "the ugly old thing" that was once located in the middle of her backyard. She told me that her children would often play atop the monument, but "no", she did not have any old photographs of the original granite pyramid set by George Davidson. I left some historic documents telling the story about "her" monument and said farewell. I next saw the landowner a week later when I introduced the field surveyor who would soon go into the backyard with a GNSS receiver to occupy LOS ANGELES SE BASE.
Final Thoughts
Some might question why in the twenty first century anyone would survey old monuments that are no longer the basis for geodetic control. The simple answer is: this is what surveyors do. While it is very true that the four triangulation stations highlighted in this article are of little practical value as high-production geodetic control, they are of considerable value from the historic perspective because they are a vital and perhaps only viable links that remain to past surveys.
Some may contend that these stations have either little or no value from the perspective of moving coordinates from one place to another; one can readily concede the point based solely upon that very narrow interpretation, but one can also remark perhaps that such a statement misses the point.
Moving coordinates from one point to another might be one role of the land surveyor, but maybe there are other equally if not more important roles that the profession fulfills. While it might be true that these older triangulation stations no longer have the same roles once held in past geodetic surveys, it cannot be denied that they were once the basis of such surveys over many, many years. Because they are historic monuments of such pedigree, these particular triangulation stations are irreplaceable from the perspective of relating things to one another, which is maybe the true the role of the land surveyor both now and in the future.
Jay Satalich, PS, is supervisor of the Geospatial Branch at Caltrans District 7 located in Los Angeles, California. He is a licensed land surveyor in six western states.
A 3.194Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE