New Ohio CBL-the way they should be!

There are pros & cons to pillars.

As you point out, with pillars there are no centering issues, etc. But the only thing you are checking is the gun along with your ppm & prism offset constants.

With ground stations you are checking out your total system the way it is normally used.

Probably the best thing is to have both, but then you're really asking for a lot.

> A lot of countries around the world build pillars for their EDM calibration lines. I never understood why we didn't do the same.

The article can be found here>>>>>

http://digital.bnpmedia.com/display_article.php?id=1292956

Many state societies don't have the money, the manpower or the will to create such a baseline. As the article states, it took "......one and a half decades and the involvement of multiple private and public agencies to be realized."

But, notice, the article states ".....surveying and mapping specifications adopted in 2011 required ODOT surveyors and consultants to check and certify that their total stations were working properly by measuring a NGS calibration baseline every six months. These checks had to be documented and were to be a part of the deliverables consultants provided with their plan submission." Ohio surveyors are lucky (if you look at it that way) to have such an incentive to verify the accuracy of their equipment. Large numbers of surveyors who work mainly for private clients have much less incentive to perform calibrations.

Generally, this is driven by requirements for work with public agencies, such as DOT's and Transit Authorities, etc.

Meanwhile, on a state statutory level, some states require certification once a year (as does NJ), and some states do not even require such certifications.

For example, New York has a regulation that states: "Equipment used by the surveyor should be maintained, checked, calibrated, and documented at reasonable intervals to achieve measurements and results compatible with the intended use, required technical criterion and industry standards."

Yes, there is an expense involved with that type of monument. The ones the ODOT constructed are quite massive.

I have always been fan of pillars (we call them peds, or pedestals). The first survey job I ever went on as a newly hired US Corps of Engineers survey tech in 1980 had 6 pedestals at the M J Kirwan Dam in Ohio. All sixteen of the Pittsburgh District reservoir projects have multiple pedestals (used for deformation surveys) and many of the 23 locks and dams have them as well. So I estimate there are 100+ pedestals that we use annually.

I built a 2 pillar baseline on my property. One, the more massive pillar, is used for GPS, astro obs, etc. A smaller one is used to check prisms, edm's etc.

I have made it a policy to check our equipment on an NGS EDM line at least once per year. Not a mandate, but since I pride myself on being able to do high accuracy surveys, it is important. The local pair of pillars we use to check our equipment more often. The nearest "complete" NGS line is 2 to 3 hours away (45 minutes by small plane, but then I have to walk the baseline).

And of course it is important to know HOW to use the line. I do a least squares adjustment to get the scale and offset for the gun. I also calculate the prism offsets for each prism. I have found that for the same style of prism (seco mini), the offset varies from -28 to -30 mm. And, I have a couple of larger prisms that are supposedly -34 that are actually -32. May not sound like much, but when the goal of the survey is ±3 mm for all stations, it matters.

CBL

John,

Do you have any idea what this cost the ODOT? I like the pillar idea. Very cool!