Got a call about an old survey; the guy couldn't understand why he wasn't matching my bearings very well. The internal angles were all good, but I had referenced the survey as state plane and he wasn't matching.
I went to look up the survey it was from 87. I asked how far "off" his bearings were, seems about 30-40 seconds.
Soooo, NAD27 bearings from?ÿold DOT control which I know tied into some third order control (since deceased)?ÿalong the highway. Needless to say I didn't dig into it to find out what was "wrong".
I guess they were usually matching 10 seconds or less when they retrace one of our old SPC surveys, RTK to RTK.
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I suppose there could be a variety of things here:?ÿ NAD27 to NAD 83 ( NADCON), and Then the whole NAD83 2011, IGS reference frame, etc etc etc.?ÿ I gave up trying to retrace old surveys exactly once I really started to understand to do so, I'd have to use the same methods and materials to recreate exact circumstances and measurements, and just relied on the adjustments provided by math from people smarter and even harder working than I am. It is fun though to chase out the math when you have time to?ÿ see what exactly they are missing or conversely, what I'm missing to find out where something is supposed to actually be geo-spatially versus where my eyes can plainly see it.?ÿ?ÿ
The DOT had an old extensive network. I would think much of the control in the area at the time was based from two local 3rd order NAD27 monuments that were installed along that highway.
No doubt I set on some of it to do the survey and accepted the control, imagine everything was traversed then compass rule adjusted, then I set on something in the middle of it all and further adjust my traverse.?ÿRunning between the two local base monuments with GPS I saw about 4 seconds of rotation between 27 and 83, pretty sweet if you think about it.?ÿ
But further away and lessor level of control and adjustments mixed in, things can slide. I remember traversing to one of those 3rd order?ÿpoints during a 70's township survey and missing it by a few feet.?ÿI knew then it was problematic, but the process back then was to accept and adjust to official government SPC?ÿmonuments.
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One thing that put a damper into some of the observed sun shots from 25 to 30 years ago was from using the difference between actual true time and GPS time. GPS time was so easily available with use of the handy handheld Garmins and other brands.
Locally that was enough to represent 43?ñ seconds of difference between computed azimuth and true azimuth.
With the improved use of GPS and all of my work being referenced to WGS84 aziumths and comparing that to work from other surveyors in the area, I have not found an example that has exceeded 8 seconds of difference.
Personally, I am very happy to fit any information within 1 minute as opposed to the past when it was usually as much as 4 degrees of difference between bearings and when it comes to DOT info, some of that stuff is 15 degrees off.
I don't recall seeing any GPS receivers, survey grade or handheld, that displayed GPS time rather than UTC (i.e. with the leap seconds since 1980 offset not applied). The leap second info is broadcast by the satellites, so any receiver has access to it to display UTC rather than GPS.?ÿ
Of course I could be wrong...
I don't recall seeing any GPS receivers, survey grade or handheld, that displayed GPS time rather than UTC (i.e. with the leap seconds since 1980 offset not applied). The leap second info is broadcast by the satellites, so any receiver has access to it to display UTC rather than GPS.?ÿ
Actually, using the hour angle method where time is critical the use of GPS time would be quite erroneous as the time difference got larger, especially anywhere close to local noon.?ÿ
Of course I could be wrong about the display....
Time was one issue doing solar shots, also location was an issue. Normally you scaled your location on a quad and plotted the lat, long. They were in NAD27 so you have the error inherent with the 27 system. I liked sitting down and calculating through them at lunch, and we had a programmer that set-up?ÿHP data collector so you just had to point and push a button. That was cool. He posts on this board every so often.
Radio Shack TimeCube and Knowles/Elgin programmed into my HP-41cx...current Leitz ephemerical publication at hand...better than a stiff compass needle in the eye ??
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http://www.rollanet.org/~eksi/Handbook.htm
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As for a current ephemeris, my old source ( I would edit the year in the URL):
http://www.cadastral.com/2010jan.htm
does not include a 2018 version.
As for time, remember the DUT/UT1 corrections see:?ÿ https://www.nist.gov/pml/time-and-frequency-division/atomic-standards/leap-second-and-ut1-utc-information. Many years ago I did a lot of altitude solar observations using a Roelof??s prism.?ÿ
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