An Attempt to Restore a Basis of Elevation using GNSS, Aerial Lidar, and Historical Survey Data
Brian Christensen, PLS
Introduction
The removal of four hydroelectric dams along the Klamath River between 2023 and 2024 marked one of the largest dam removal efforts in United States history. While the ecological and cultural impacts of this project have been widely discussed, far less attention has been paid to a quieter consequence: the loss of physical infrastructure that once anchored a century-old vertical datum used for engineering, mapping, and property boundaries in the Copco, California area.
This article documents the effort to recover and perpetuate that historical elevation datum using modern GNSS, unmanned aerial lidar, photogrammetry, bathymetry, and archival survey records.
Boundary Challenges After Reservoir Drawdown (2025)
In 2025, I was contracted by Resource Environmental Solutions (RES) to perform a boundary survey of two parcels within a 1966 subdivision along Copco Lake. With the reservoir drained in 2024, the absence of the former waterline, along with emerging vegetation and other natural surface changes, began to obscure features that had previously been clearly visible. This created an urgent need to re-establish the legal boundary between the parcels and the former reservoir.
The original property markers—iron pipes reportedly set at the shoreline corners—were diligently searched for but could not be recovered. Only the roadway-front parcel corners were found. The subdivision map provided no shoreline meander geometry, and the only way to mathematically extend from the front parcel corners to the shoreline was with a bearing and a “plus or minus” distance rounded to the nearest foot. Despite this critical boundary being defined by the 2607.5’ Sea Level Datum contour, the subdivision map never set benchmarks or referenced elevations on property corners.
Recovering the vertical datum used to define this contour became essential—not just to complete the survey, but to ensure accurate retracement in the future.
Historical Datum and Station CP-B1
Station CP-B1, a 2-inch brass disc set in a rock outcropping above the old Copco No. 1 dam site, became a critical anchor for re-establishing the datum. CP-B1 was part of a system of passive monuments used to monitor the dam while it was operational and tied to vertical control originally established by the US Bureau of Reclamation (1904) and the US Coast & Geodetic Survey (1921).
John C. Boyle used this datum in 1910 for the design surveys and construction of Copco No. 1 and No. 2 dams. The USBR and USC & GS leveling campaigns, however, did not meet third-order accuracies for inclusion in the USGS network. After 1921, these differential circuits and benchmarks were abandoned.
A 1959 USGS report from Siskiyou County records provided sparse details on the remaining benchmarks, often no more than a sentence describing distances along a river from historic landmarks or vague topo calls such as “…set in a rock, 15 feet above the road.” These references bore little resemblance to current conditions, making field recovery extremely challenging.
Going Back to 2018: Early Geospatial Surveys
Before the dam removal, GMA Hydrology, under contract to AECOM, had performed aerial lidar, close-range photogrammetry, and bathymetry surveys of the Copco No. 1 structure. During these surveys, they recovered and photographed two 1922 survey tablets atop the dam. I later obtained the positions and photos of these tablets from Cort Pryor, Branch Manager, Yurok Tribe, Fisheries Department, Design & Technical Services Program, Geomatics & Surveying Branch, formerly with GMA Hydrology, Inc.
Both tablets were stamped with engineer’s stationing, elevations, and the year “1922.” The 11+00 tablet, stamped 2612.98’, immediately caught my attention as a potential key to recovering Boyle’s original elevations.
Modern Geospatial Survey Work During Dam Removal (Late 2024)
After the dam removal, I was contracted by Kiewit Corporation to perform as-built surveys of all three dam removal sites. These surveys required heavy use of GNSS and unmanned aerial lidar. I constrained my ground surveys to existing project control provided by Cort Pryor. The control originated from GMA Hydrology’s survey work and was made available to me with permission from the project owner, Klamath River Renewal Corporation, and the data owner, AECOM. This network combined stations from Woolpert and GMA Hydrology surveyors.
I verified this network independently with a GNSS static survey, tying observations to the project’s contract datum and projection: NAD83 (2011), Epoch 2010.00, California State Plane, Zone 1, NAVD88 (Geoid 12b), US Survey Feet, via NGS CORS. This provided a reliable framework before beginning lidar acquisition.
Recovering the Datum (2024–2025)
Comparing the GMA survey to the 11+00 tablet, I calculated a 3.74-foot separation, which I applied to the 2607.5’ contour, converting it to 2611.24 NAVD88 (Geoid12B). Using post-drawdown aerial lidar provided by Yurok Tribe surveyors, I derived a surface and staked the adjusted contour along the former reservoir edge. The line hit the intended elevation and followed an escarpment along the terrain.
To further validate, I obtained a Structure from Motion (SfM) point cloud of Copco No. 1, coupled with multi-beam bathymetry of the dam face and lake bottom. Cross sections along the curved spillway surface confirmed the 19-foot grade separation from the 11+00 tablet, aligning with the 2,594’ spillway elevation shown on the 1954 USGS quadrangle.
While waiting for SfM and bathymetry permissions, I discovered a 2017 KPFF Consulting Engineers survey that had independently measured the 1922 tablets and CP-B1. KPFF’s measurements closely matched GMA’s survey, providing independent verification.
Conclusion
Without KPFF’s survey report, I would have never known the existence of Station CP-B1. This disk helped me conclusively recover the historic datum and perpetuate it by densifying benchmarks along the former lakefront, ensuring future successful retracements. A record of survey detailing the recovered evidence, survey methodologies, and narrative will be filed with the Siskiyou County Surveyor.
This process demonstrates how modern geospatial tools—GNSS, lidar, SfM, and bathymetry—combined with archival records—can solve complex historical surveying challenges, even after major infrastructure changes.
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