A Humbling Lesson

[+m&h]

In a recent thread we characterized ourselves as fairly well informed and careful, among other things. We didn’t mean to sound boastful or smug, but the marker gods may have thought we did; they have brought us a lesson that seems worth sharing in some detail. If this gets too long-winded, we hope you’ll move on and forgive us.

 

For several weeks in August and October we were in the Coburg/Eugene area of Oregon, and found data sheets for survey markers in the curbing and road surfaces of several overpasses that cross Interstate 5. We got to three of them, found them, photographed them, and reported to NGS and to this site that we had found them in good condition. The fourth was not so straightforward.

 

The marks in question are PTS I5 15, (PID QE2191), in the Sprague Road overpass; PTS I5 17 (PID QE2154), in the Van Duyn Road overpass; PTS I5 18 (QE2155), in the Wilkins Road overpass; and, finally, PTS I5 19 (QE2156), in the Coleman Road overpass, which we reported not found as described, because the stamping on the disk we found was 1992, and the datasheet told us to look for a disk stamped 1969. Furthermore, while we found reference marks for the other disks, those for this last one appeared to have been covered by the installation of a metal guardrail and its anchor posts.

 

Lane County, Oregon has an unusually well-staffed and responsive surveyor’s office. We heard from Bob Thurman there that in 1992 this mark was included in a GPS Control project and assigned a NAD83/91 coordinate value, and restamped at that time. It was brought into the High Accuracy Reference Network in 1995. However, and it’s a big however, he adds:

 

"Sometime after that the Oregon Department of Transportation needed more clearance on their bridges and jacked up all the bridges along the I-5 corridor.

 

"This brings us up to today and the position on that monument is probably better than the original NAD27 position that was done in 1969 but is off from the last County adjustment in 1995."

 

Jacked up the bridges? Goodness gracious. So we asked the ODOT about all four of the overpasses mentioned above, and received this:

 

"The history of the overpasses over Interstate 5 is as follows:

Coleman Bridge # 081687 was raised 1' 8" in 1996

Wilkins Bridge # 08170 was raised 1' 7" in 1996

Van Duyn Bridge # 08172 was raised 1' 5" in 1996

Sprague Bridge # 08177 was raised 1' 7" in 1988 or 1989.

 

If we can be of further service please don't hesitate to call.

Jack Lee

District Manager

odot.state.or.us"

 

All four of these marks are survey disks of second-order horizontal accuracy; their vertical accuracy is not specified in the data sheets.

 

Although we reported the first three of these as good in August, we will be in touch with Burt Smith, who is taking Deb Brown’s mail while she’s away, to see what the NGS decides to do about these. It’s an interesting call, but elevating a disk a half-meter, more or less, has a somewhat destructive sound to us, no matter how easy it is to walk onto the overpasses, find the disks, and take photos of them. A steep spot on the learning curve.

[+PFF]

Wow! Just when we thought we'd had the last benchmarking surprise....!!!

 

It will be interesting to see how this situation is resolved!

 

Keep us informed.

 

-Paul-

[+seventhings]

WOW!!!!

Long-winded? No - superb story!

Them "marker gods" are a mischievous lot - they play jacks with the universe.

Wow!

 

Will

[+m&h]

Well, on December 23, we heard from Burt Smith at NGS that he will be adding a note to these marks saying that their elevations have changed and that they should be used with caution. We guess that's reasonable enough, since the marks are not of a high order of accuracy to start with. It seems reasonable also to place such a cautionary statement in any recovery note where such elevation can be documented.

 

Happy new year to all.