Adjustments to 1966-2001 Great Salt Lake water-surface
elevation records as a result of corrected benchmark elevations

Introduction

  Great Salt Lake is divided into a north and a south part by a rock-fill causeway. The U.S. Geological
Survey (USGS) operates gages that collect water-surface elevation data on the south part of the lake at
the Boat Harbor gage (USGS station 10010000), and on the north of the lake at the Saline gage
(10010100). It has been known since the mid-1980s that the difference in water-surface elevation between
the two parts of the lake as measured at the Boat Harbor and Saline gages was greater than the difference
measured directly at the causeway. Because the lake surface is considered to be relatively flat on calm
days and the gages were periodically checked against permanent benchmarks with surveying levels; the
difference was assumed to be an error in the given elevations of the benchmarks to which the gages are
referenced. During 1969-82 and 1997-99, a gage was operated on the south part of the lake at Promontory
Point (USGS station 10010050), referenced to the same line of benchmarks as the Saline gage. The difference
in water-surface elevation between the two parts of the lake as measured at the Promontory Point and
Saline gages generally agreed with the difference measured directly at the causeway.

Until this time (April 2001), there was no economically feasible way to verify the given elevations
of the reference benchmarks of the Great Salt Lake elevation gages. In 1999, a high-resolution Global
Positioning System (GPS) survey was conducted by the National Geodetic Survey (NGS) in Utah. The
U.S. Geological Survey and Utah Department of Natural Recourses, Water Resources Division, participated
in this survey to determine the elevation of five benchmarks around Great Salt Lake that are used for the
determination of water-surface elevations of the lake. The final calculations from this survey were provided
to the USGS by the NGS in March 2001. This survey provided the first direct check and comparison of
the elevations of all of these benchmarks.

When the Boat Harbor and Saline gages are adjusted to the new benchmark elevations, the difference
in water-surface elevation between the two parts of the lake measured at the gages generally agrees with
the difference measured directly at the causeway. The records of water-surface elevation will be adjusted
at the Boat Harbor, Saline, and Promontory Point gages according to the 1999 NGS GPS benchmark elevations.

Findings

  Water-surface elevations reported at the USGS Great Salt Lake gages are considered to be accurate to
within +/- 0.10 foot of the datum in use. Of the five benchmarks surveyed by the USGS as part of the larger
1999 NGS GPS survey, only three were considered by the NGS to be accurate to within 0.10 foot (FMK 77
1966, Saltair, and WES DES PUMPS).

The elevation of the FMK 77 1966 benchmark, located near the Saline gage, was found by the GPS
survey to be 4,231.155 feet National Geodetic Vertical Datum of 1929 (NGVD 29). Data from the establish-ment
of the Saline gage in 1966 to the present is currently adjusted to the FMK 77 1966 benchmark with
a given elevation of 4,230.888 feet. All Saline gage elevations (1966-2001) will need to be increased by
0.267 foot (0.27 foot, rounded) to account for the change in the given elevation of FMK 77 to 4,231.155
feet.

The Promontory Point gage was referenced to the FMK 73 1966 benchmark, which is on the same line
as the FMK 77 1966 benchmark. Because the GPS survey adjusted the FMK 77 benchmark 0.27 foot higher,
and the datums of FMK 73 and 77 have historically agreed, it is assumed that the given elevation of FMK
73 should also be raised 0.27 foot. This will be verified with surveying levels in the near future.

The Boat Harbor gage has been tied to two different permanent benchmarks since the 1960s. The first,
BM H-39 1922, was used from sometime before the 1960s until 1985. Sometime between 1985 and 1989
it was destroyed by the construction of Interstate Highway 80. After 1985, the primary reference benchmark
for the Boat Harbor gage was C-174 (1970). By using the new GPS survey elevation for the Saltair
benchmark (located at the Boat Harbor gage) and the surveyed height differences between Saltair, C-174,
and BM H-39 from previous levels, elevations for C-174 and BM H-39 corrected to the GPS survey
were computed. From this it was found that the previous given elevation for the BM H-39 was 0.14 foot
too high, and for the C-174 benchmark was 0.42 foot too high.

In addition to the changes in given elevations for the Boat Harbor gage reference benchmarks, all three
gages used during 1980-2001 settled. Here is a synopsis of the findings on the settling of Boat Harbor
gages from 1980 to 2001.

• During 1981-83, the gage settled 0.25 foot. This problem was discovered and corrected for
  in 1983.
• In 1985 the gage became inundated by the rising lake and had to be moved to a temporary location,
  attached to a large concrete sign nearby. This sign, and therefore the gage, settled about 0.44 foot during
  the period it was operated from 1985 to 1989. This problem was not discovered until 2001 because the gage
  was established with BM H-39 (1922), which had a given elevation 0.14 foot too high, and had its datum
  checked when it was discontinued against benchmark C-174 (1970), which had a given elevation that
  was 0.42 foot too high. Although the gage settled about 0.44 foot, it appeared to the surveyors
  at the time to be off by only about 0.12 foot, and no changes were made. A prorated correction for this
  settling will need to be applied to the Boat Harbor water-surface elevation data from July 1985 to
  August 1989. Because the current gage was established off of the settled temporary gage, a constant -0.44-foot
  correction will need to be applied to the data from August 1989 to September 1994, when the 0.44-foot error was removed.
• The (current) gage installed in August of 1989 also settled during the first 6 or so years it was used.
  Levels indicate that the gage settled about 0.55 foot from September 1989 to July 1993, and about 0.11
  foot from July 1993 to June 1995. The record was adjusted for part of this settling in 1995.
• In 1995, the 0.25-foot settling correction applied in 1983 was mistakenly applied to the 1984-1995
  records. No evidence could be found in 2001 that this 0.25-foot correction was needed during 1984-1995.
• There is no indication that the gage has moved since 1995.

Changes to the Great Salt Lake Elevation Records on May 1, 2001

Saline Gage:

Promontory Point gage:

Boat Harbor gage:

  It should be noted (as described above) that although GPS levels indicate that the benchmark
BM H-39 had a given elevation that was 0.14 foot too high, the record will not be adjusted prior to 1984
for this apparent error at this time. This 0.14-foot error likely entered the record in the 1950s (or earlier), and
not enough information is available to justify an adjustment back that far of such a small amount. Below
is a tabular summary of the corrections that will be applied to the Boat Harbor gage water-surface elevation
data on May 1, 2001. These corrections are actually the sum of a combination of corrections to the problems
described above in the "Findings" section. From May 1, 2001, forward, reported water-surface elevation
data will reflect the datum correction discovered by the 1999 GPS survey.

For information on these changes, please contact the U.S. Geological Survey at (801) 908-5000.

68K GIF	44K GIF
Location of water-surface elevation stations and reference benchmarks near Great Salt Lake, Utah.	Location of water-surface elevation stations and reference benchmarks near the causeway across Great Salt Lake, and near Saltair, Utah.

URL for this document is http://ut.water.usgs.gov/gsl%20corr/gslcorrection.htm