I went to the?ÿHancock, Maryland, CBL today. It took me a hour to clear the line to the 1km mark. However, it??s clear now if anyone else wants to use it. Marks easy to find. It??ll grow over again soon.
Its always nice to see older EDMs still working better than the manual specs.
At 1 Km:
HP3805A: 5 ppm scale err.
Topcon GTS3B: less than 1 ppm scale err. (Same as in 2020 calibration check)
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Really nice. Here's how they look on NGS Data Explorer.
@mathteacher I felt lucky that a CBL is not far from me, and that it has published 2015 distances.?ÿ
@mathteacher?ÿ
Note the dates of measured/remeasured.
the Hancock CBL is fresh. And in 100 yr railroad bed, so I think pretty stable. But, gps has changed things.?ÿ
In my early days of messing around with map projections, I would build LDPs for CBLs. Then I could compare calculated distances using LDP coordinates to measured CBL distances. Hancock does not have lat/lon coordinates measured to 5 decimals in seconds, so I wouldn't have used it.
But distances, not coordinates, are the purpose of CBLs. We take what we can get when we have a different purpose.
Thanks for posting the pictures. That must be a beautiful spot.
@mathteacher yeah, it??s purely an instrument check.
And I thought I??d pass along: the line is clear. And the marks are easy. And parking available at both ends.?ÿ
Why did you setup that old EDM? For fun or an actual business need?
Checking my Topcon TS, and the 1973 HP came along. It??s gorgeous, and is still good. But only good if it has a current calibration sticker. And why not? It works great. Wouldn??t put it on a job these days, but it??s reliable and accurate.?ÿ
Then checking my own EDM check line, with both EDMs just seems like a good idea.
And I have those giant Wild reflectors, so they have a new cal sticker too.?ÿ
@robertusa It??s also a good exercise in precise distance measurement by non gps tools, and statistics. After the 1km CBL observation for scale with both EDMs I??ve compared both Topcon GTS3B and HP3805A on a 2km gps baseline. The GPS receivers are 20+ yrs old? The Topcon is at 25 yrs old and the HP3805 is 47 yrs old.
And it??s not a walk in the park. But at 2km the EDMs compared to ZSurveyor phase differential baseline is on the order of 2-3 ppm. It??s a good exercise at least. The HP came from a university sale and it??s like new. So it??s obsolete to be sure.?ÿ
