We use a lot of pedestals for deformation surveys. A pedestal (I learned the term back in the early 80's when I started surveying in the Corps of Engineers) is a concrete filled cylinder that sticks out of the ground 3 to 5 feet and has a stainless steel metal plate on the top with a 5/8" #11 threaded rod sticking up about 1/2". Others might call them pillars or piers. there are over 100 on the projects we work on in the Pittsburgh district. We bluebooked a bunch of these over the years. I set a pair at my house/office to use in calibrations and testing.?ÿ
I set one at the office I used to work at, and bluebooked it (made it a HARN) so that everyone could use it. One day I look out the window and I see a surveyor from the utility company trying to setup a tripod over it. I went out and told him all you have to do is screw the tribrach on. He looked at me and then realized how to do it. Duh.?ÿ
So I though that was just one person.?ÿ
Last week I went to one of the projects and used the ped. I downloaded the data sheet, and saw this recovery notice...
DL1906 STATION RECOVERY (2016)
DL1906
DL1906'RECOVERY NOTE BY PA DEPT OF TRANSP 2016 (ECH)
DL1906'RECOVERED AS DESCRIBED. MARK PROJECTS SIGNIFICANTLY FROM THE GROUND
DL1906'MAKING USE OF BIPODS DIFFICULT.
Maybe he didn't have a tribrach...
?ÿ
There is a relatively new calibration baseline in Ohio that has four pillars...Great to use because you eliminate centering errors...
Wow, very impressive.
Hard to imagine someone not recognizing the use and purpose of the threaded rod sticking out of the middle.
What's the rock-like object near the foreground pedestal in the last photo?
It's a trap for people who ask questions.......they get to carry it to the next pedestal. ??ÿ
Either that or somebody's grave stone.
The official title of the baseline is Culver-Davies. Usually they are named for the nearest town (New London).?ÿ
I had a feeling that the stone bore some sort of identification for the baseline, but that's quite impressive.
?ÿ
The first calibration baseline I ever used, in Surrey, B.C., Canada, had the marks on pedestals like that. In fact the pedestal tops were all at the same elevation. Which meant that some of them were?ÿ more than a little above ground level. A couple at the low end had platforms with steps for access.?ÿ ?ÿ?ÿ
The only thing "wrong" with that baseline is that the 0 and 120 m peds are about the same elevation, so when sighting from the other end you cannot have prisms on both peds at the same time. the other two marks (900 m and 1320 m) are at different elevations so that is not a problem.?ÿ
We go to this CBL once a year, even though it is a 2 1/2 hour drive. I wonder how many companies feel this is important enough to do regularly? I believe NJ has a regulation saying it MUST be done once a year...
Here are the results I got using a Seco mini prism (nominally -30 mm, I use a value of -29 mm), using my 2006 high accuracy S6. I also shot my newer SX10 on the longest line (1320 m) and it differed from the S6 distance by 1 mm.
I believe the scale error being over 1 ppm is due to my inability to accurately measure the temperature. I had a Brunton ADC pro with me, but it was WAY off (hadn't been used for about a year), so I used temperature from the nearest weather station. Pressure is measured by the S6. I have a weather station at my baseline at the office, but I didn't think to check the ADC pro before heading out.?ÿ
Once I go to the baseline each year, I do the same at the 145 meter baseline between peds at my office to use for checking prisms, etc. We use almost all of the same prisms (seco minis), but we do have a few larger oddball prisms. One that we use occasionally for longer shots supposedly has an offset of -34 mm, but I found it to be -31 mm. Millimeters matter in deformation surveys.?ÿ?ÿ
<pre>
SCALE ERROR = -1.6459 PPM
STANDARD ERROR OF SCALE = +0.7233 PPM
CONSTANT ERROR = +0.0002 METERS
STANDARD ERROR OF CONSTANT = +0.0006 METERS
VARIANCE OF UNIT WEIGHT = +0.6847
COVARIANCE = +0.3360
CORRELATION = +0.8394
PUBLISHED OBSERVED DIFFERENCE RESIDUAL
(METERS) (METERS) (METERS) (METERS)
119.9800 119.9800 +0.0000 -0.0000
899.9878 899.9898 -0.0020 -0.0007
1319.9739 1319.9738 +0.0001 +0.0021
119.9800 119.9803 -0.0002 -0.0003
780.0078 780.0100 -0.0022 -0.0011
1199.9941 1199.9958 -0.0017 +0.0001
899.9878 899.9879 -0.0001 +0.0012
780.0078 780.0105 -0.0027 -0.0016
419.9871 419.9873 -0.0002 +0.0003
1319.9739 1319.9758 -0.0019 +0.0001
1199.9941 1199.9968 -0.0027 -0.0009
419.9871 419.9864 +0.0007 +0.0012
<pre/>
