Conventional and Global Positioning System survey data from 1975-2022 at Bald Eagle ridge, Lake County, Colorado

As part of an effort by the U.S. Geological Survey (USGS) to better understand topographic and structural conditions that promote slow spreading and sagging of mountain ridges (Varnes and others, 1989), USGS researchers began surveying monitoring points at Bald Eagle ridge in the Sawatch Range of Colorado in 1975 (Radbruch-Hall and others, 1976; Varnes and others, 1990). Bald Eagle ridge forms the southeastern flank of the glaciated Busk Creek valley and is underlain by Mesoproterozoic St. Kevin Granite (Kellogg and others, 2017; Ruleman and others, 2020). Since 1975, surveys of points have continued at roughly decadal intervals, with the most recent survey occurring in 2022 (Table 1). Monitoring point locations on the ridge (Figure 1) were selected to determine if the long-term movement that created multiple uphill-facing scarps, trenches, and a graben on the ridge (features collectively known as sackungen, refer to Figure 1) was still ongoing. Points 1, 2, 3, 4, 6, and K (Figure 1) have been surveyed since 1975. Additional points along a profile line approximately perpendicular to scarps and trenches (points 431 through 438, Figure 1) were added in 1997 and were surveyed in 1997 and subsequent years (1999, 2012, 2013, 2022). 
 
Two generations of survey techniques have been used at Bald Eagle ridge, a conventional technique utilizing theodolites and electronic distance measuring devices from 1975 through 1989 (Varnes and others, 1990), and a Global Positioning System (GPS) technique from 1997 through 2022 (refer to Varnes and others, 2000, and Coe and others, 2013). Both the conventional and GPS surveying techniques used point 4 at the top of Bald Eagle ridge (Figure 1) as a fixed, stable point. The stability of point 4 was repeatedly demonstrated using surveys between point 4, and points 6 and K (Figure 1), which showed that the baselines and positions of points 4, 6, and K have remained stable during the monitoring period.
 
Conventional surveys from 1975-1989 required line-of-sight between points. That is, points on the northwest flank of the valley were needed to properly sight points on Bald Eagle ridge. The location of survey positions on the northwest flank of Busk Creek valley and additional details about the conventional methods used for these surveys can be found in Varnes and others (1990). Varnes and others (1990) estimated surveying errors associated with conventional methods to be in the range of 1 to 2 cm.
 
For the GPS surveys from 1997-2022 (refer to Varnes and others, 2000; Coe and others, 2013), measurements were made using static and rapid-static surveying techniques (Van Sickle, 1996). With GPS surveying, direct line of sight between points was not needed, so only points on Bald Eagle ridge (the points shown in Figure 1) were occupied with GPS receivers. The GPS receivers that we used varied over the length of the study. In 1997 and 1999, GPS surveys were done using Ashtech Model Z-12, dual frequency receivers. For surveys from 2012 through 2022, Magellan ProFlex 500 dual frequency receivers were used. The minimum occupation time at each point was 15 minutes, but some occupations were up to 2 hours long, depending on the number and positions of observable satellites, and the availability of people to move and manage the receivers. Our estimate of surveying errors associated with GPS surveying was +/-1.5 cm in horizontal and +/-3.0 cm in vertical.
 
GPS data were post-processed using Ashtech Precise Navigation (PNAV), v. 2.4.00 M software (PNAV, 1997) in 1997 and 1999, and Magellan Survey Solutions software (Survey Solutions, 2010) for 2012 through 2022. Standard errors of computed point positions derived from PNAV and Survey Solutions were less than 1.5 cm in horizontal and 3.0 cm in vertical. Additional details about the GPS methods used for these surveys can be found in Varnes and others (2000).
 
Surveyed point positions from conventional and GPS surveys from 1975-2022 are given in the SurveyData .csv and .xlsx files. Horizontal point positions from conventional surveys are given in North American Datum 1983 (NAD83) Colorado State Plane central zone coordinates. Horizontal positions from GPS surveys are given in the World Geodetic System 1984 (WGS84) and Colorado State Plane central zone coordinates. Vertical point positions for conventional and GPS surveys are given in the North American Vertical Datum 1988 (NAVD88) and as Ellipsoid Heights. Conversion of WGS84 coordinates to the Colorado State Plane coordinate system was done using Ashtech Prism software (Prism, 1997). Conversion of Ellipsoid Heights to NAVD88 elevations was done using the GEOID96 model (refer to https://geodesy.noaa.gov/GEOID/GEOID96/). The WGS84 position of point 4 was initially determined by a static GPS survey of point 4 and two benchmarks with known coordinates provided by the National Geodetic Survey—point "Vail Pass" at Vail Pass and point "V 4" near Leadville. The final WGS84 position of point 4 was determined by least squares adjustment of the triangle formed by the three points.
 
Additional information included in this data release includes the following. Field photos of survey points are included in the SurveyPointPhotos.zip file, organized by the point names and dates of photography. Photos were not available for survey points 435-438. With a few exceptions (as indicated in the photo file names), photos were taken by the first author of this data release. A Google Earth GPSSurveyPointLocations.kml file showing survey point locations, survey point names, and WGS84 coordinates is also included. Table1.jpg provides the survey technique used in each year that surveys were performed. 
 
The survey data included in this data release were used in the following interpretive paper:
 
Coe, J.A., Avdievitch, N.N., Allstadt, K.E., Collins, E.A., Jensen, E.K., Hoch, O.J., Schaefer, L.N., Ruleman, C.A., Godt, J.W., Matthews III, V., in review, Sackung at Bald Eagle ridge, central Colorado: an updated interpretation of ridge-spreading movement, structures, and mechanisms from 50 years (1975-2025) of U.S. Geological Survey research: Engineering Geology.
 
Disclaimer: Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

References
 
Coe, J.A., Ruleman, C.A., Godt, J.W., Matthews, V., Goehring, B.M., Lucha, P., Deuell, A.E., and Reeves, R.R., 2013, Long-term measurement of ridge-spreading movements (sackungen) at Bald Eagle Mountain, central Colorado [abs.]: Geological Society of America Abstracts with Programs, v. 45, p. 643. https://gsa.confex.com/gsa/2013AM/webprogram/Paper227500.html.
 
Kellogg, K.S., Shroba, R.R., Ruleman, C.A., Bohannon, R.G., McIntosh, W. C., Premo, W.R., Cosca, M.A., Moscati, R.J., and Brandt, T.R., 2017, Geologic map of the upper Arkansas River valley region, north-central Colorado: U.S. Geological Survey Scientific Investigations Map 3382, pamphlet 70 p., 2 sheets, scale 1:50,000, https://doi.org/10.3133/sim3382.
 
PNAV, 1997, Precise differential GPS navigation and surveying PNAV software user’s guide: Ashtech, Sunnyvale, California.
 
Prism, 1997, Ashtech Prism software, Ashtech, Sunnyvale, California.
 
Radbruch-Hall, D.H., Varnes, D.J., and Savage, W.Z., 1976, Gravitational spreading of steep-
sided ridges (“sackung”) in western United States: International Association for Engineering
Geology Bulletin, v. 14, p. 23–35, https://doi.org/10.1007/BF02634754.
 
Ruleman, C.A., Frothingham, M.G., Brandt, T.R., Shaw, C.A., Caffee, M.W., Brugger, K.A., and Goehring, B.M., 2020, Geologic map of the Homestake Reservoir 7.5’ Quadrangle, Lake, Pitkin, Eagle Counties, Colorado: U.S. Geological Survey Scientific Investigations Map 3451, scale 1:24,000, https://doi.org/10.3133/sim3451.
 
Van Sickle, J., 1996, GPS for Land Surveyors: Ann Arbor Press, Chelsea, Michigan, 209 p.
 
Varnes, D.J., Radbruch-Hall, D.H., and Savage, W.Z., 1989, Topographic and structural conditions in areas of gravitational spreading of ridges in the Western United States: U. S. Geological Survey Professional Paper 1496, 32 p. https://pubs.er.usgs.gov/publication/pp1496.
 
Varnes, D.J., Radbruch-Hall, D.H., Varnes, K.L., Smith, W.K., and Savage, W.Z., 1990, Measurement of ridge-spreading movements (sackungen) at Bald Eagle Mountain, Lake County, Colorado, 1975–1989: U. S. Geological Survey Open-File Report 90–543, 13 p.
https://pubs.er.usgs.gov/publication/ofr90543.
 
Varnes, D.J., Coe, J.A., Godt, J.W., Savage, W.Z., and Savage, J.E., 2000, Measurement of Colorado, II—Continuation of the 1975–1989 measurements using a Global Positioning System in 1997 and 1999: U.S. Geological Survey Open File Report 00–205, 23 p.
https://pubs.er.usgs.gov/publication/ofr00205.