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AZcachemeister

I have my theories...

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I recently visited SGC 17, or was it SGC 17?

I can't believe there are two marks in such close proximity with the same designation, but who knows. The terrain is flat, with sparse vegetation, so a wayward mark should be 'easily' findable. (I spent a half-hour looking.) This is a horizontal strain monitoring site, so there are many pits, wells, vertical pipes, and excavations in the area. But only one monumented disk.

My theory is that the original mark was monumented in 1970, but then replaced in 1978 (for reasons unknown). The NGS recovered the 1978 mark in 1981 (and again in 1984) and the 1970 mark in 1984.

I think the 1984 recovery on the 1970 monument is erroneous.

 

Opinions?

Edited by AZcachemeister

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It reads as though there are 2 different markers. Both are altitude adjusted and show different elevations. One is also location adjusted. One is south of the centerline of the highway & one is west of the centerline of the highway.

 

The one is about 1 1/2 feet from the witness post.

 

John

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I lean toward them being the same disk.

It's really difficult to tell though.

 

Besides the differences in the to-reach's, here's the specification differences:

 

DU1323* NAVD 88 - 478.138 (meters) 1568.69 (feet) ADJUSTED

DU1323 VERT ORDER - FIRST CLASS II

DU1323.The orthometric height was determined by differential leveling

DU1323.and adjusted by the National Geodetic Survey in June 1991..

 

DU1930* NAVD 88 - 477.921 (meters) 1567.98 (feet) ADJUSTED

DU1930 VERT ORDER - SECOND CLASS I

DU1930.The orthometric height was determined by differential leveling

DU1930.and adjusted by the National Geodetic Survey in November 1993..

 

The height difference of 1 foot and the later adjustment for the one with a lower class VERT order tend to imply that the marks are not the same ones, however. Interesting issue.

 

There doesn't seem to be any other marks in the SGC series in AZ.

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I re-visited the site yesterday, in hopes of noticing SOMETHING I missed before. I didn't.

I did take a couple more photos, at GZ for the older mark, to help show what the terrain and vegetation are like.

 

As mentioned in the NGS datasheet for the newer monument, this is an extensometer site (one of the two buildings has 'S.G. 16' spray-painted on the side), and this area is a known area for subsidence cracks. There is one such crack just a quarter-mile due south. The VERY expensive Central Arizona Project Canal is just about .15 to the northeast. Presumably the extensometer is there to detect ground movement that would threaten the integrity of the canal.

 

As for the possibility that the marks are one and the same, the stamping clearly is 1978, with no appearance of having been over-stamped. I do think there is a possibility that the same concrete post was re-used, since the rim of the disk is a few millimeters above the top of the concrete.

 

It now seems to me that the original monument did not have the precision needed for it's function to help locate the extensometer and/or it's assorted sensors, so a new mark was surveyed and created (possibly at the same location as the original mark), which would explain why they re-used the designation but of course it had to get a new PID.

 

Since the mark is a BOR disk, I have contacted the local office in hopes they will have some records of the marks.

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DU1323 reads "DESCRIBED BY NATIONAL GEODETIC SURVEY 1981 6.2 KM (3.9 MI) SW FROM APACHE JUNCTION."

 

DU1930 reads "DESCRIBED BY ARIZONA DEPARTMENT OF TRANSPORTATION 1984 2.99 KM (1.86 MI) EAST FROM APACHE JUNCTION."

 

Sounds like 2 different directions from Apache Junction?! Probably 2 different benchmarks.

 

John

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DU1323 reads "DESCRIBED BY NATIONAL GEODETIC SURVEY 1981 6.2 KM (3.9 MI) SW FROM APACHE JUNCTION."

 

DU1930 reads "DESCRIBED BY ARIZONA DEPARTMENT OF TRANSPORTATION 1984 2.99 KM (1.86 MI) EAST FROM APACHE JUNCTION."

 

Sounds like 2 different directions from Apache Junction?! Probably 2 different benchmarks.

 

John

 

Until you notice that CRISOMON (sic) road is two miles WEST of Meridian Road!

So you can either go south on Crismon, and then east along SR360 for 1.86 miles, OR you can go south along Meridian to the site, 321 feet WEST of the centerline.

Two different ways of getting to the same general area.

That being said, I now realize that .14 miles is MUCH more than 321 feet.

It appears my next search will concentrate considerably farther west than before. Let's hope their odometer wasn't too far off in 1970!

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Well, it seems this mystery is winding to a close. I had a chance to visit the site again yesterday, and did what I should have done on my first visit. I projected a waypoint 1.86 miles east from Crismon road and US60, and then went to the area about 160 ft south, which is right next to the right-of-way fence (just as with the recovered mark) where I found a big hole in the ground.

 

Many thanks to Black Dog Trackers and DaveD for following up on this!

 

1117ef7b-3307-4001-8b13-0d04b6e3c6aa.jpg

Edited by AZcachemeister

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I would say you answered the question as to how many marks there HAD been. You found the 1978 mark and the hole where the 1970 mark should have been located.

 

Excellent work!

 

Now the NGS can list DU1930 missing in action/probably destroyed.

 

Well done,

 

John

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Thanks, John!

 

What is bugging me now is the unanswered question of whether there were two marks here in 1984!

 

Ah, well...life goes on.

 

I'm sure the answer is locked away in some dusty file cabinet somewhere.

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