USGS
Triangulation Station
"EAST BASE"

1891

Jerry Penry
May 30, 2010

As with any stand-alone triangulation network, there needs to be a measured base line. The Rapid City Base Line was located in the valley of Box Elder Creek which was the most suitable location offering a prolonged area of flat terrain. The base line was oriented in a mostly east-west direction. The west end being just northeast of Rapid City and the east end being just south of the present-day location of Ellsworth Air Force Base. The base line was 25,796.711 feet long which is 4.886 miles. It was measured three different times with a 300-foot steel tape which was one of the latest measuring devices at that time.

The location for the East Base monument now resides within the right-of-way of Interstate I-90. A calculated location which adjusts the 1893 latitude/longitude position into the modern NAD83 position places the location just inside the south right-of-way fence. An examination on the ground at this location revealed no traces of the base line stone nor any traces of the three reference points that were set. The reference points were each set 25' from the end of the base line. One was on the baseline west of the end point, and the other two were north and south at 90° angles to the base line. It appears that grading was done in that area during the construction of I-90.

Looking east along the south right-of-way fence of I-90 where the east end of the base line would have been located.
The terrain at this location begins to slope downward, so it was the appropriate location for the east end of the base line.

Looking west along the south right-of-way fence of I-90. The East Base stone and three reference stones would have been located in this area, but were most likely graded out during the construction of I-90.

Looking southeast across I-90 toward the location where the East Base location was once located as shown by the red arrow.

Aerial view of the Rapid City Base Line that was established in 1890. The East Base location is now near the southerly right-of-way fence of Interstate I-90 south of Ellsworth Air Force Base.

The layout for the Rapid City East Base location of the base line consisted of a marked stone for the end of the base line and three marked stones for reference points. Each reference stone was located 25' from the end point on the base line. One reference point was on the base line toward the West Base and the other two were located 90° from the base line.

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© Jerry Penry 2010