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An 1880 Pin Cushion!
Well, I suppose it is. I do have an opinion on why there are two positions approx. 0.22 ft. east-west if each other.
First, I’ve been working on and off for the last few years researching, drafting, and retracting an 1880 control survey in the hills surrounding Central City and Black Hawk. Sometime in 1879 or 1880, it was decided that too many surveys had errors in their 1 to 2 mile connection to the east Range Line of T.3S., R.73W., 6th PM. From that acknowledgement, the Central City Triangulation Survey was completed in the fall of 1880. It consists of over 80 Tri-Stations mostly located on the ridges and hilltops around town. This allowed the mineral surveyors to tie their survey into a nearby Tri-Station and then compute the required tie to a rectangular PLSS corner.
Most of the Tri-Stations are 1″ drill holes in boulders (gneissic in composition) with some of the holes filled with lead plugs, copper bolts, and steel rods. For those interested, they do have DIMPLES! The remainder are corners of stone and brick buildings.
In order to get a handle on the location and accuracy of this old triangulation survey, I went out and found the corners of the base line (Tri-Stations 1 and 2) and the due North line (Tri-Stations 3. and 4). Static GPS sessions for the four stations varied from 2 ?« hours (TS-4) to 6 hours (TS-2). After post-processing the static sessions I found the horizontal ground distance between stations 1 and 2 to be 1613.67 ft. However, the 1880 survey showed a distance of only 1612.83 ft. (approx. 1:1915).
That didn??t make any sense because U.S. Deputy Mineral Surveyors were required to have a closure of ?« ft. in 1000 ft. and the base line missed that general closure error. After some head scratching I took a look through one of my old surveying text, ??Elements of Surveying and Leveling? by Charles Davies, revised by J. Howard Van Amringe, 1883. Bingo, BOOK VI, Trigonometrical Surveying gave the answer. Under Section 1, Making the Survey is a subsection entitled, Reduction to the Centre (page 226). The horizontal distances in the 1880 control survey had been reduced to sea level. After adjusting my ??ground? to an ellipsoid distance, I was within 0.17 ft. of the 1880 base line measurement!
Here is the snippet from the 1880 triangulation map showing the stations in the vicinity of the base line and north line.
And an 2014 aerial photo of the same area with the intervisible tri-stations shown by the light blue lines.
Back to the 1880 pin cushion I found, below are photos of Tri-Station 4, the northerly Tri-Station of the North Line. The first photo is looking to the southeast, the rest are taken from the south looking north.
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