In this day and age of gps does anyone still do solar observations with a total station??ÿ
Not much (due to not having an objective filter, who wants to fry the EDM?), but I have observed Polaris for azimuth a couple times in the last 10 years or so, both on a contract basis for other firms, apparently a lost art...
I don't anymore.
I actually found that the one instrument capable of doing one seems to have no solar filter in the box anymore.
I noticed that a couple of years ago during the eclipse.?ÿ
I'd imagine that you can find the formulas and ephemeris online to do one, I used to pick up the Elgin and Knowles booklet and calculate them during lunch. I think they stopped putting them out about the time I did my last solar-2005.?ÿ
It has been at least 15 years since my last solar.?ÿ I do still have a nice Leica objective filter that I used during the eclipse.?ÿ I was using them along with a single GPS point to provide LiDAR verification under different canopies back in its infancy.
I came across this the other day.?ÿ From the subdivision regulations of a small town nearby.
If bearings are given on the final plat, they must be based on true meridian, either by solar or polaris observation. Bearings must be close to true meridian within plus or minus 0.01" method and date shall be listed on the final plot.
Interesting.?ÿ Whenever I used to do farm surveys I would establish bearing with a solar.?ÿ Stopped that with the advent of gps.?ÿ I was out in the field the other day and finished off staking a couple 1/16 corners with a total station and wondered if I did the whole section from the get go how would I establish the bearing.?ÿ No filter available.?ÿ
I used to buy my filters at Thousand Oakes Optical and of course the elgin knowles booklet.?ÿ How could we do it today??ÿ MICA from the naval observatory I guess.
Bearings must be close to true meridian within plus or minus 0.01"
I take it that " is a typo and ' was meant.?ÿ If not, the standard is too unrealistic to impose.
Under ideal conditions and skill sets, how close to true geodetic bearings can you get with a sun shot, using J-Mate, or whatever the best available method would be.?ÿ I've never done a sun shot, but I'm wondering what's the point unless you can get pretty tight with it.?ÿ If it's not that accurate, why not just guess with a compass to get it in the ballpark??ÿ Obviously I'm uninformed here so someone please set me straight.
There are two solar methods, hour angle and altitude. Each has it's own factors that limit accuracy. My impression is that the limiting accuracy is a few arc seconds but difficult to achieve that. Leveling errors are significant.
I haven't practiced solars (only a feeble attempt) and prefer Polaris, but it also has drawbacks. Above 40-some degrees latitude, depending on your instrument, you will need a right-angle eyepiece. It isn't convenient for work schedules. You can't use Polaris in the daytime unless you have top quality optics, very clear atmosphere, and an initial pointing guess within a degree or so. It's easy and not time-critical from dusk to dawn.
For solar, you need an instrument setup you can safely point at the sun. For a TS you need an objective filter out front. A Roelf's pri$m makes it easier to do the pointing. For a theodolite (no EDM to fry) you can use projection.
For solar hour angle, you need UT1 time to fractions of a second because the sun moves ~15 arc seconds per time second. For altitude, you may be limited by the ability to accurately predict refraction, which may exceed an arc minute, to a small part of that.
Here is an interesting paper defending the altitude method.
http://www.cadastral.com/papersl1.htm
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The sun traverses 360d in 24hrs. This means that every hour it is moving 15d more or less. Divide it all out and you have the sun moving 15" of arc for every second of time. Basically it's flying across the telescope as you are watching it. You observe the trailing and leading edges, you record the angles and time the recording as close as you can.
Then you have the location of the instrument. Back when I would do them that meant a plot on a quad sheet, an interpolation of the geographic location all in NAD27. The shift from 27 to 83 is around 150-200'. Round that to 2" of longitude.
I was always happy to be less than 30" with a solar bearing. Not that I knew for sure, it was later that I would tie into State Plane and have a check, of course State Plane had lots of error also.
Now there is GPS so time and geographic locations are more accurate. Ironically, because of GPS solar bearings can probably be more accurately determined, but I'm not going to find out, been there, done that, got the tee shirt.
However, to compare solar observations to compass bearings would be incorrect. I could do a solar very quickly, I even had for many years a program on my HP DC that would automatically do them as you turned the angles as long as the yearly data had been stored.
Half a minute of arc with a compass?
Compare the azimuth accuracy of GNSS.?ÿ If you measure two points a quarter mile apart with a sideways uncertainty of 0.05 ft that's 8 arc seconds. So that's why solars are becoming a lost art.
As has been said something in the 10" to 30" range with a sunshot was considered good work. That sun is a moving target and it's moving fast!
In practice it is a two person operation. One guy was the timer the other operated the instrument. The guy on the instrument would be tracking the sun with the screws and the moment he got it on the right spot he would simultaneously say "mark" and stop turning the screws. The timer notes the second he hears "mark". An error in timing of just one second is 15" on the measure. Plus, it took some special care to get your watch on the exact time in the first place. You could do solo, of course, but at the sacrifice of some precision in most cases.
Compasses with least counts in that range do exist but there is always local attraction of unknown magnitude and then there is the declination issue. So a sun shot is better than compass. But GPS is better than sunshot.
I practice solars a lot. Currently and for the past 10+ years. And its ?ÿgreat skill to keep alive.
I favor a radio tower 4-12 miles away and have many dozens of solars. Starnet has convinced me that +/-5? is true. And difference to gps is noise level. Must include: Deflection of the vertical, Laplace, proper curation if UT1, and practice practice practice.
and I??ve created a spreadsheet, developed my observation routines. It??s a great skill to keep alive.?ÿ
