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Unmarked Point


California Bear

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I was looking at some of the stations near me and came across a station with the following in the description:

 

THE STATION IS AN UNMARKED POINT, DETERMINED BY BISECTING THE MEASURED DIAGONAL OF THE ROOF OF THE SQUARISH, SQUAT TOWER, THAT IS THE HIGHEST POINT OF THE HOAG MEMORIAL HOSPITAL. THE HOSPITAL IS A LARGE BUFF-COLORED BUILDING. THE STATION WAS LOCATED BY TRIANGULATION, RESECTION METHODS.

 

The PID for this station is: DX4566

 

The description says the station is an unmarked point. Does this mean there is no physical mark to be found? Was this a virtual benchmark that could only be found by reshooting all the reference points?

 

Thanks in advance.

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These are fun questions for Surveyors... Technical stuff…

 

An unmarked point has a couple different connotations, but here, I'll just dump a little of my brain out here and let you keep the parts you like.

 

This description is extreme engineerspeak for what is obviously a Landmark Intersection point, where several things can be done with it, and several things cannot be done with it. It is unmarked because there is no way to climb up there and cement a brass disc on it. I mean without special rigging you are not going to put a brass disc on a lot of things. This engineer just decided to call it as such. So you can intersect this station but you cannot set up on it, whereas with a brass disc you can do both. It can seem like a virtual benchmark as there is no disc, it is a landmark acting like a disc, and this was important as the Geodetic Surveying was done over long distances. These Land Mark Stations were used to help cross check other work. They were already elevated places and did not need to have tall towers erected over them to be part of an equation for a measurement.

 

Maybe I am stating the obvious, but for those who may not understand the process, Triangulation and Resection involve setting up a transit, theolidite or total station these days, with a tripod over the top of a Bench Mark station, with a Plumb bob dangled from under the instrument to center the instrument. Often we use the optical plummet, which is incorporated in modern instruments, and does the same thing. Either way we are centered over the mark. This is an instrument set up. Remember that. I will come back to that.

 

Now from the brass disc monumented in the ground to the center of the sight piece of the instrument is measured and called the height of instrument. (Height of the light) Then you add that measurement to the information you find on the Datasheet:

 

As an example:

 

*CURRENT SURVEY CONTROL

RD3152 ___________________________________________________________________

RD3152* NAD 83(1991)- 45 31 10.23551(N) 122 44 37.89866(W) ADJUSTED

RD3152* NAVD 88 - 290.77 (+/-2cm) 954.0 (feet) VERTCON

 

Ok, we are at 954.0 feet elevation to the monument plus the HI or Height of Instrument, which I will call 5.0 feet. So for the purpose of working all the Math equations, we are at this coordinate, and our measurements will be taken from an elevation of 959 feet. This is Station A

 

From the datasheet the instrument operator knows their Coordinate location, and swivels the instrument so it points at the unmarked point (Station C) as described. If we like, we can zero the instrument’s horizontal angle to this angle but we need not do it either. We do however need to take note of what measurement is indicated by the vernier. Then the measurement is what direction is the instrument pointing, Not necessarily a cardinal direction yet, just the number which is read off the horizontal vernier on the instrument, Zero since we zeroed here but will be the bearing or heading to the unmarked point once we determine it and what is the vertical angle when measured from that which is horizontal, above or below? The number which is read off the vertical vernier on the instrument.

 

Now as a frame of reference, we have determined our coordinate location, our elevation at the instrument, the vertical angle to the unmarked point, and an initial horizontal direction to the unmarked point.

 

Next, a second measurement will be made just like the one just made, of a different location. (Station B ) It will be the azimuth mark for this measurement. It could be a top point of Landmark that has been made a Station, which is also an unmarked point if we are trying to determine the point we are standing on. Or, since we are trying to determine the unmarked point, it can and should be another Bench Mark with a brass disc where a person holding a Signal, usually a prism (which acts as a reflector or target) mounted on a prism pole of known length which should be the same as the height of the measuring instrument, i.e., they should match each other (another height of the light) and the process is taken over again, the horizontal angle and Vertical angles are taken. As I said earlier, this station must be a known point, such as another Bench Mark Station. This Unmarked station has now almost been intersected.

 

Mathematically through Geometry and Trigonometry we can determine the bearing and distance to the other known station through comparison of the coordinates, though this is usually only done in geodetic work. Then, once this is known we can compare the second horizontal direction, which is now a known cardinal direction as well, to the first one we took to find the angle we measured. Once we add or subtract the angle to or from the bearing we know between the two Bench Marks, depending on if we are turning clockwise or counter clockwise to this bearing, we then determine the bearing to the unmarked point.

 

The Tally determined so far: From the datasheets Our Coordinate location and elevation, as well as the Coordinates of the other Bench Mark Station. A known, calculated bearing between them. A stated elevation for both stations and a vertical angle with distance we can use to double-check both elevations for accuracy or changes. An angle which gives us a Bearing to the unmarked Location and a vertical angle between Station a and c with no distance.

 

The coordinance of the unmarked station, Station C, a roof peak, are found by closing the horizontal triangle. There will be 180 degrees in a closed triangle and starting off we measured the angle between the 2 stations, B and C, taken at station A so we move the instrument setup from Station A to Station B and measured for the angle between A and C at station B. We can also measure the vertical angle from B to C for elevation comparisons.

 

Now that we know two of the angles we can find the third angle which is the intersected point at station C the unmarked location. Using the Trigonometric formulas alone with the Sines, Cosines and Tangents, we can then determine the location and elevation of Station C

 

Now that is old school and a very simplified explanation but it is still being done for us in the computers on our instruments. We just basically need only tell a total station where it is and carry the reflective prism around and it will tell us where the prism is on the earth. A GPS has several satellites helping triangulate it’s location all the time, in real time and the more Satellites, the better the accuracy due to multiple triangulation comparisons. The GPS’s internal computer just uses those signal’s origin locations in relation to a Known Datum of the Geoid to mathematically determine it where it is. Many hand held GPS units can triangulate your location at 500 MPH… Whereas It just took 10 minutes to read about just one triangulation.

 

I hope this answered some of the questions. And truly yes Bear, in order to truly find it Mathematically is to re-shoot it. But since we have Reference data containing the physical description of what the location looks like, we only need that to recover the station.

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Bear,

 

That is the case. Physical verification is the preferred method. As a couple examples, I have a church near me which has a tall pointy steeple and is on a ridgeline. In early 2003 the steeple was located mid way on the southerly side of a roof with a peak that runs east to west. I have a picture of that as I was going to recover the landmark. At the time there was scaffolding erected in the front yard of the church. By late 2003 the steeple on the church had be moved from its former location to the North side of the roof peak and in the front of the building on top of a new addition. I waited to report it. I went back took another Pic and a new GPS fix for it. I will report it as POOR DISTURBED with an explanation similar to this. It is important as it can serve as a good azimuth mark for many Surveys. Is it Destroyed? Yes really in terms of an exact location it is, but since there is a steeple in place on the same building, a choice may be made to adjust this position so it can still be of future service. That is above my pay scale.

 

In 3 similar cases I have in my pending file, there are or were actual discs cemented to the roofs, some with RM discs and some without. Until I obtain permission to access the roof to physically see them and check their measurements, find out if roofing tar has buried them since last recovery etc, then all I know for sure is that the building is still there. In one of these cases the Building had physical discs cemented in the roof but has been torn down. So I need to put the case together to accurately as possible explain what happened to them. I have to contact the municipality who owns the place and find out what I can of the history.

 

In one other case, I have an old School that had a 30 foot stack which was a Landmark station which was unmarked. I checked it and the chimney now only protrudes 10 feet. This is a common thing that happens when a buildings boilers are converted from Oil which needs a chimney that will draught properly, to Natural Gas where the Chimney is relegated to being a vent pipe. Again, the station is now POOR Disturbed and I need to explain why.

 

When you know a building is there, but you don't go compare the description to the current reality, it may seem trivial but there is a lot we can miss... Is the Station GPS usable? Can it be occupied? Has foliage or trees rendered the station a difficult place to see anything from? Have other obstructing Buildings been erected near it? Has the station become obscured by something else? Has a recent remodel changed it somehow? I suppose one question we could think about when we recover a station of any kind is; What kind of work could this station be used to do right now? If there is no longer a view to the north due to recent construction in the area, then that is a good information to make note of...

 

Sometimes when you are on location, things are still hard to say!

 

Rob

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My 2cents

 

These intersection stations are sometimes not easy to identify. It make take observations from known points to confirm if the building is the same today and the location monumented is unchanged. With a handheld GPS you cannot get accurate enough to know if the position is the same. While the bldg is a landmark, the point is the actual reference. I skip over these marks if I cannot with 100& certainty confirm it.

 

IMO this would be one of those stations to skip over. You could log it if the building is still there and there has not been an renovations to it.

 

fwiw

The is a new water tank not far from me. The original was almost 100 yrs old and in a Nat't Historic Park. It was the fire protection for the city but was inadequate and because of this, residents and the city were paying high insurance rates. So after fighting with the NPS for years, they finally agreed they will build a new larger water tank next to the original and to be a copy of the original down to the color, ie. the tank look 100 yrs old. That took place about 5 yrs ago but the USPSQD guys have reported the old tank as being recovered 3 times since, they have not idea is not the same apparently. Only someone local who knows the old tank was razed when the new tank went into service would know.

Edited by elcamino
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