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.
Here is a 3D image of the 1880 triangulation survey looking from the southeast to the northwest showing that the majority of the stations are on hilltops.?ÿ I'm checking the intervisible stations as shown on the 1880 map with a line-of-sight tool in my image processing software.
Photo of setup at Tri-Station 1 looking to the west where Tri-Station 2 is located.
Close-up of 1" drill hole for TS-1
Photo of setup at Tri-Station 2 looking to the southeast at the town of Central City.
Close-up of 1" drill hole for TS-2 with dimpled copper bolt!
Photo of Tri-Station 3 showing that it is located on a steep slope.
Photo showing the "3" chiseled on the side of the boulder.
Photo of 1" drill hole.
really cool stuff gene. i survey in colorado too, and its the best place for someone in our profession. that job sounds incredibly complicated to me, but also super cool. good luck!
Back to the question of why two 1" drill holes for Tri-Station 4.?ÿ I decided to scale the 1880 survey (scale factor of 1.000106) so that the base line matched my GPS measurements.?ÿ When I did so, I checked into another triangulation station (Tri-Station 58) which is approx. 8000 ft. to the south-southeast of Tri-Station 1.?ÿ I missed the scaled position of Tri-Station 58 from Tri-Station 1 by 3 arc-seconds and 0.81 ft.?ÿ Not too bad at all.?ÿ
Photo of Tri-Station 58 looking to the southwest.
Close-up of the 1" drill hole with dimpled copper bolt!
I also found that the westerly of the two drill holes for Tri-Station 4 fit the scaled 1880 map position within 0.02 ft.?ÿ That means that with the scaling I did, Tri-Stations 1, 2 and 4 are consistent with each other.?ÿ The map position was N 80?ø W, 0.21 ft. from the easterly drill hole.?ÿ As a retracement surveyor, I begin my work by assuming that the surveyor I'm retracing did everything correctly.?ÿ The burden of proof is on me to find more than a preponderance of evidence that he screwed up.?ÿ In this case, I can buy into a possibly far fetched story that the easterly drill hole was set first and sighted during the astronomical observations.?ÿ After everything was reduced and a true north line computed it is maybe, perhaps, sounds plausible that true north is 0.21 ft. to the west, therefore the reason for the second drill hole was to mark the true north line.
Reality is never neat and tidy though.?ÿ When I checked the found position of Tri-Station 3 on a steep hillside, I found that it was 1.59 ft. east-southeast of the 1880 map position (down the fall line).?ÿ So I guess I can pontificate as a Professional Geologist that the small boulder is being subjected to mass wasting processes and has moved by soil creep approx. 1.6 ft. since 1880.?ÿ As a PLS I don't have the luxury of standing on top of that boulder and waving my arms that something likely happened without proof; thereby embracing the pin cushion principle that monuments should be rejected whenever they are not neat and tidy.
A correction to my opening post.?ÿ I made an error in regard to which section of Davies talks about reduction to sea level.?ÿ "Reduction to centre" is not the proper section.?ÿ It deals with something entirely different; computing the position of a steeple or other object that cannot be occupied.?ÿ Sorry for the error.
The correct section is on pages 216 and 217.?ÿ The section (249) also includes the formula for the reduction.?ÿ I also just found this little tidbit about the use of "copper bolts" on pages 215 and 216 (first paragraph of Sec. 246).
246.?ÿ In measuring a base-line, every possible precaution should be taken to insure accuracy.?ÿ The line measured should be straight, to effect which it should be ranged out with the transit.?ÿ The ends of the base should be marked by a stone sunk in the ground, with a copper bolt let into it and the exact point of beginning and ending fixed by the intersection of two lines cut into the head of the bolt.
I've seen a pincushion in at least one set of private field notes from that decade (1880s).
Great post! My wife was looking over my shoulder at the pics.
In Section 284 of the 1883 text, the average diameter of the earth is given to be 7919 miles. Solving the formula for reduction to sea level for h, using 3959.5 miles as the radius of the earth and the ratio of the record distance to your ground distance gives an average height above sea level for the base line of 10,888 feet. This number won't be exactly right because of significant digit differences, but it should be close, within a couple of feet perhaps.
Is it fairly close to elevation elevation in 1880?
The average of the ellipsoid heights of Tri-Stations 1 and 2 is 8732 ft.?ÿ The difference between 8732 and 10,888 is approx. the 106 ppm scale factor that I ended up with. The earliest topographic map of the are that I was able to find on the USGS historical topographic map site is a 1:62500 scale map from 1910.?ÿ It has 100 foot contours that match fairly well with my ellipsoid heights.
Sadly, the only remnants of the 1880 trigonometrical survey (Davies term) are two aperture cards in the BLM Public Room.?ÿ One is the 1880 map that has a date in the lower right of "Nov 1880" and the other is a tabular sheet showing the bearings and distances to all 80 numbered triangulation stations from corners on the east Range line of the township.?ÿ Oddly, the date of the tabular sheet is July 16, 1902.?ÿ It was prepared by F. Edgar Frantz, a daughtsman in the Colorado Surveyor's General office.?ÿ I asked an older friend about any history of the survey, and he mentioned that the work was likely done by a New York surveyor who did a similar survey for the Idaho Springs area, which is approx. 3 miles to the south.
Unless there is a magic box filled with the survey data and original maps buried somewhere in the Denver Regional Archives, that is all of the available information.?ÿ The National Archives is not keen on researchers being in the actual stacks, so it is doubtful that I'll ever uncover the original map and station listing.?ÿ The only other information regarding the individual Tri-Stations is in mineral survey field notes dated after August 1904.?ÿ At that time the field notes had to include a Report section that described the Tri-Station.?ÿ The descriptions are nominal, and not much help when looking for a 1" drill hole in a "rock" somewhere on top of a knob.?ÿ Transforming the 1880 survey into Colorado SPCs has been a help.?ÿ It should come as no surprise however, that the error propagation is not uniform throughout the triangulation network.?ÿ I mentioned earlier that I missed the position of a tri-station 8000 feet to the south-southeast by only 0.8 ft.?ÿ Two stations in the southwest portion of the network miss by over 5 ft.?ÿ That precludes using a 2D transform to accurately position the tri-stations that have been destroyed by past mining activity, new roads, etc.
I'll post some sketches of the entire network and the Range Line ties in a bit.
?ÿ
Neat stuff Gene!
Over the years I have seen/recovered/used quite a few Triangulation Networks ranging from the 1880s up into the early 1900s (all mining district related).
Some were reduced/expressed at "sea level."
Some were reduced/expressed at the "elevation" of the Base Line (along the face of the stamp mill).
One was reduced/expressed at the "1200 foot level" of a particular shaft.?ÿ
ALL of them were very well done (and monumented), and of course very valuable in retracing the various Mineral Surveys in the area.
Loyal
I'll post three CAD sketches to show the system and a photo from The Mining Reporter, a weekly mining trade journal that ran a series of articles on the mineral resources of Gilpin County, Colorado in 1899.?ÿ Sorry, I got the trade journal from Google Books so the photo is a tad on the poor side to say the least.?ÿ It does show the extensive mining activity in the heart of the triangulation network.
This is a sketch showing the "map projection" of the network.?ÿ The "coordinates" are bearing and distance from 4 section and quarter-section corners on the Range Line to each triangulation station.?ÿ The network occupies Secs. 11, 12, 13 and 14 and the northern parts of Secs. 23 and 24 of T. 3 S., R. 73 W., 6th P.M.?ÿ The dashed red lines are quarter section lines.?ÿ The lines in green were obtained from the tabular sheet recreated in 1902 by Frantz.?ÿ The blue text are the Tri-Station numbers and names.?ÿ There is one oddity in the southwest corner; that being the Virginia Canon USLT.?ÿ Some here know of U.S. Mineral Monuments (USMM), and U.S. Locating/Location Monuments (USLM), but what is a USLT.?ÿ This is the only example I have come across of a U.S. Locating Tree!?ÿ In March of 1880 it was described as a lone pine, 30" in diam.?ÿ Unless that tree lived long enough to have the blaze heal over with an ample supply of resin, it is likely that the only remnants of that USLT is a 2 foot diam. bare spot amongst the second/third growth pines that are there now.
This is a sketch of the triangulation network showing the intervisible Tri-Stations as magenta lines.?ÿ Again the blue text are the Tri-Station numbers and the red lines are the protracted section lines (this township was not subdivided until the 1980s).?ÿ You will notice that the northwest area is not well connected to the rest of the network.?ÿ Several Tri-Stations only have two ties to them.?ÿ Often they represent brick or stone building corners and therefore are secondary stations in the network.?ÿ This is very obvious in the southern part of the network.?ÿ For example, Tri-Station 74 is described as, "N.E. Cor. of H.F. Sawyer house in N. branch of Willis Gulch." and Tri-Station 8, "S.W. Cor. East Brick Powder House on Bates Hill."
I mentioned in the OP that many mineral surveys done in the 1870s had erroneous ties of 1 to 2 miles to the Range line.?ÿ After the triangulation network was completed I found approx. 120 of these old mineral surveys that had amended connections made circa 1881-1882.?ÿ The sketch below shows some of those at the northern end of the network.?ÿ The solid red lines are ground distances from a corner of the mineral survey to a shiny new 1" drill hole (i.e. Tri-Station).?ÿ The intervisible lines of the triangulation survey are are magenta dotted lines.?ÿ The dash-dot-dot green lines are the 1880 bearing and distance (reduced to sea level) from the range line to the Tri-Stations.?ÿ The dashed blue lines are the computed ties from the mineral surveys to the range line.?ÿ These are a mix of the ground distance from the mineral survey to the Tri-Station and the reduced to sea level distance from the Range line to the Tri-Station.?ÿ I ended plotting these to verify the mix of ground and grid distances and found that the great majority fit within 0.02 to 0.10 ft.?ÿ That means the ties between the mineral survey to the Range line are off by 2 to 4 feet for mineral surveys that are 1 to 2 miles from the Range line.?ÿ No big deal in 1880 I imagine.?ÿ Oh, and the black lines and text are mineral survey outline, cor. no. and sur. no.
1899 photo of Quartz Hill, which is to the southwest of Central City. The high point on Quartz Hill is the location of Tri-Station 33 the other high spot to the east is the location of Tri-Station 32.?ÿ The photo is likely taken somewhere near Tri-Station 60 looking to the northwest.?ÿ The old saying goes that monuments control over course and distance.?ÿ Of course, that is with the caveat that the monument is in its originally established position.?ÿ I would have a hard time justifying that a stone I found amongst all the rocks was never disturbed.?ÿ The tri-stations may be the best available evidence today for the locations of the lode mining claims on that hillside!
Okay, now for why I think it might be best to rely on those Tri-Stations and why in heavily mined areas that at one time might have been completely free of vegetation (that photo of Quartz Hill looks like mining on the Moon).?ÿ The magenta lines are the intervisible Tri-Stations.?ÿ The black lines are the current county parcel map for the area.?ÿ The parcels are a good indicator of the overlapping nature of lode mining claims.?ÿ It is thoughty for the modern surveyor to have minimal expertise in junior/senior rights when working in certain areas.
I had a nice chat on Friday with the county regarding this. I'm hoping they will support the effort although that will likely only be words of encouragement. Support for good ideas usually ends at the point of financial support! ????
If things work out I might get some compensation should the county decide to use my work to upgrade their GIS. I won't hold my breath in the meantime.
I agree that these old control surveys established in mining districts can be invaluable (and are often very accurate). Even landowners tend to appreciate that my survey uses a control monument their land was originally tied to in 1880.
One final piece of eye candy of the area around Quartz Hill.?ÿ This snippet is from the "Map of Gilpin Co. Colo." compiled from official records by U.S. Deputy Mineral Surveyors E.E. Chase and S.A. Rank (last updated Sept. 15, 1891).?ÿ This shows the heavy development on the north side of Quartz Hill in the area called "The Glory Hole" or "The Patch."?ÿ Tri-Stations 32,?ÿ 33 (just south of the text for the lode claim, Mitchell 682), 60, and 61 are marked by triangles and the station number.?ÿ The 1899 photo above shows that the mining activity greatly increased in 8 years.