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Do you ever do astronomic azimuths Poll is created on January 10, 2020 5:56 am

  
  
  
  
  

Astronomic Azimuths  

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Bill93
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This site may be of interest to those seeking ephemeris information.  I haven't tried it myself.

Ephemrides

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Retired PLS
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Surveying in the 1970's and 80's i would do a Sun shot on every survey. Sometimes 2 or 3 a day. Even had a Sunshot calc on my PLS test. I tried to teach others how to shoot the Sun and Polaris but they never got the hang of it. I still shoot Polaris  sometimes just for fun. 

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Jp7191
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Studied like hell back in early 90’s because there was a chance that it would be on the state (Ca) license test.  So I once could do it in theory but never did one for real.  Jp

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JaRo
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Starting about middle of the year 1990, every survey I did was oriented to grid north using a sun shot and calculating the theta angle. I used the program from the Elgin Knowles and Senne book keyed into my HP 41 and also used the 41 for time. I could set the time from a radio shack time radio and adjust for the positive or negative adjustment based on the number of ticks on the radio. I always did 8 sets, 2 direct leading edge, 2 direct trailing edge, and the same thing inverted. I had the 41 programmed to stop time with the push of a button so I could watch the scope with my finger on the button and mark time very accurately.

With the 41 app for smart phones, I'm guessing anyone with a smart phone could still do the same thing. It might be hard to get time.

James

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Mike Marks
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@jaro

My memory's fading but didn't the original HP41C factory Survey Module have a solar reduction routine based on hardwired Chebyshev polynomials for the sun instead of contemporary Ephemeris data which needs regular updating?  That meant the computation was accurate when the module was new (for a coupla years or more), but as the decades have passed the hardwired polynomials for the sun's position drifted to where the module is now useless. 😥  It could be i'm mis-remembering and I was using a keyed in (card reader) program from Joe Bell's excellent HP41 Journal.

Concerning accurate time an HP41 with a time module or HP41CX with integrated time module, after a few months of the correction routine for crystal  the 41C was the finest autonomous time standard available for civilian applications, like surveying; assuming you kept it reasonably temperature stable.

Observation of the Sun's limbs instead of its center by using a Roelofs prism introduces several errors, most notably pointing error and  to a lesser extent semidiameter error dependent on solar activity, and the extended duration of the observation as the sun moves involves atmospherics which cannot be corrected for.

That being said, even with meticulous technique I doubt astronomic north can be determined by solar observations using tripod mounted optics to within 10-15".  Yes, precision to within 5" has been anecdotal,  but really determining north by observing the Sun is essentially shooting a fast moving target as time passes. 

Much better is to observe Polaris.  The target is moving slowly, or briefly not at all for elongation observation.  Polaris is a pinpoint in the sky so pointing is not a problem.  It's night (although observation during the day is possible), so atmospherics are minimal.  Tripod, instrument thermal stability is maximized.  I'd guess accuracy can be 5" or less with careful procedures.  It's as if you have a target at near true north millions of miles away.  The stinker is you're observing a target at a high elevation, OK @ 20° latitude, useless @ 70° latitude because unavoidable instrument levelling errors degrade the observation.  But 99% of us are in CONUS so it's not a problem.

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Posted by: @mike-marks

@jaro

My memory's fading but didn't the original HP41C factory Survey Module have a solar reduction routine based on hardwired Chebyshev polynomials for the sun instead of contemporary Ephemeris data which needs regular updating?

I never used any of the survey modules. I wrote my own except for the sunshot program from Elgin Knowles and Senne.

I never used any of the survey modules. I wrote my own except for the sunshot program from Elgin Knowles and Senne and I modified it to give me North and then Grid North in my zone.

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Bill93
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Posted by: @mike-marks

 a solar reduction routine based on hardwired Chebyshev polynomials ...  That meant the computation was accurate when the module was new (for a coupla years or more), but as the decades have passed the hardwired polynomials for the sun's position drifted to where the module is now useless.

No idea what it used, but I have a picky point about Chebychev polynomials and the word drift.  Those polynomials are used to provide as good an approximation to a function as their degree allows over a selected period of time, wiggling between a small plus or minus bounded error.  Then outside that period of time the error doesn't drift, it shoot rapidly to huge values.

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@mike-marks

A few years ago, I observed a radio tower strobe light 5-12 mi distant, from 9 widely separated points. Using opus derived coordinates and dozens of solar observations, plenty of star observations, over many weeks. I have a coordinate on the strobe. Starnet returns residuals on the azimuths less than 8”, and only 8-10 rejections, leaving 40+ azimuths in the adjustment. 

Now to check the coord on the tower I set a new opus point (not included in the LS adjustment), take a solar azimuth and compare that to inverse. On one such point, I have 5 solar azimuths observation. 3 return the same second and hit inverse to 1.5 seconds. And 2 obs are 4 and 5 secs off inverse. That’s not a one-off. However, there obs that look good but don’t fit for 10”. I repeat those, and find the first obs to be anomalous. 

Also deflection of the prime vertical was not easily had years back. Change in semi diameter due to Solar activity is well inside the noise  

I use a dedicated pps output from a GPS receiver for time. And Quartz stopwatch. I use the multi foresight method. I only obs the sun below 30 degrees vertical angle. 

I repeated the radio tower intersection, new points, solar shots, 6-12 mi distant. The coord from the first run is less than 30 cm from the second run. (The strobe lights appears to be at least a meter in size.) 

Sun shots are not that easy, or fast. But there’s no mathematical reason against it. True UT1 time is far more readily available as well. Time has to be curated carefully. 

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Mike Marks
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Posted by: @larry-scott

On one such point, I have 5 solar azimuths observation. 3 return the same second and hit inverse to 1.5 seconds. And 2 obs are 4 and 5 secs off inverse. That’s not a one-off.

Impressive.  I can't meet such repeatability (much less precision) using T2 type or better terrestrial theodolites on stable concrete fixed mounts with optimal targetry in the 2-4 mile range, even with favorable atmospherics (dam subsidence deflection surveys).  Be aware an arcsecond is approximately the angle subtended by a U.S. dime coin (18 mm) at a distance of  2.5 miles.  You state the strobe tower was @ 5-12 miles distant so you are claiming solar observation repeatability better than  field survey instrument capabilities by a wide margin. 

OTOH, maybe you're claiming after a least squares adjustment many  observables were not adjusted by more than 1.5",  and all were within 8" after adjustment  and you chucked the +10" observations (usually not statistically rejectable).  Well done precise survey but be a little humble and certify you located the strobe tower within a few feet if that was the contract.

My experience with solars is they're no better than 15" even with careful procdedueres & anybody who claims better is blowing it up their ass's clients.

 

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@mike-marks

To be sure repeatability 1-2 seconds cannot be counted on. I’ve worked hard on refinements of the observation procedure over the past couple of years. And a comprehensive reduction excel sheet.

Across OPUS derives points, at 11 mi, I do get repeatable at 1-5 seconds. And hit inverse to the same. Not just once  

So I realize that in a commercial project in today’s gps environment solar az is not financially viable or warranted, or even a good idea. 

It’s in the minutia. Eliminating micrometer scaling error, advanced leveling procedures, gps pps time source, multi foresight, practice practice practice, and multi day. USNO ephemeris. Im not pretending to get better than the instrument is capable in any one reading or set, or one-off lucky shots. 

I like the art of observation. 15” ‘best case’ is not my experience. 15” is a do-over.

 

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@mike-marks

Some Instrument operators are better than others, that's just the way it is. There is no doubt in my mind that Larry Scott and

A. Harris are 2 of many.

I would suggest you read " Astronomy Applied to Land Surveying by R. Roelofs" (1950). This will give you a very good

understanding of different types of errors in the Wild T2 and T3 theodolite or other instruments that are the same.

You should also read Prof. Art Peterson papers on astro. See ACSM-ASPRS annual convention 1986, Vol. 2, pp177-185.

See also "Accuracy of the Automatic Grid Azimuth Determination by Observing the Sun Using Kern E2 Theodolite, by

Nikola Solaric and Drago Spoljaric, in Surveying and Mapping 1988. The next paper you will want to read is Accuracy of Automatic

Grid Azimuth Determination by Astronomical Methods with the Leica-Kern E2 Theodolite by Nikola Solaric, Drago Spoljaric, and Zedenko Lukinac, Surveying and Land Information Systems, Vol. 54, No. 1, 1994, pp5-19

There are many other papers but you can find them. Then go out and practice taking at least 50  sun shots (different days)

using what you have learned from all the references above and more and let us know how you did.

 

JOHN NOLTON

 

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@mike-marks

Mike ...

An extraordinary claim needs evidence. I made a claim of higher accuracy solar azimuths than is expected. Attached is Starnet output listing file of my recent observations of a radio tower strobe light. Please give it a look, and note the global weight of azimuths, and the 95% confidence on the azimuths.   

I have also included 2 examples of observation data reduction.

I did this survey specifically to see how well solar obs work when all of the details are addressed, and to learn what observing techniques are required. Please look at the Mils angle input in the excel sheet and that the micrometer is left at a single value, advancing the instrument by integer mils only. This is to addresses micrometer scale error. (My T2 has about 0.4"/minute scale error.)

It's purely an academic endeavor. I repeated the survey 3x. The first running was not this good. After I wrote the excel sheet, and revised my procedure, the 2nd and 3rd return nearly identical coords of the strobe light (20-30 cm). 

The real reason behind the survey was to see how consistent and accurate solar azimuths could be. So, at 8-12 miles, and a large number of azimuths (all 3 runnings have on the order of 100 sets), I figured the residuals would demonstrate statistical significance, not flukes.

(Stars, Vega, Fomalhaut, Antares, Procyon, Capella, Polaris, were observed in the 1st and 2nd running, to further check the strobe light position.).

Please give them a look. It's just an appreciation for detail, and the azimuths and zenith angles tell the story. I'd be interested in comments.

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Mike Marks
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@larry-scott

I stand corrected.  I reviewed Nolton's list of papers (abstracts) and it's clear with attention to detail azimuth accuracy on the order of a few seconds is achievable using field survey theodolites.

I erred because I based my opinion on my experiences with T-2 solars in the field (hour angle method), where no extraordinary procedures were implemented; just one set of D&R observations against targets often less than 1,000' distant, tenth of second time accuracy using the hp41c time module that has undergone a month of the "Correct" process against WWV, lat-lon scaled from a quad sheet, and no observations within an hour of local solar noon.  Typical results when compared to reality (2 NGS stations or equivalent tests) was 5-10" with the occasional 15" outlier especially near local apparent Noon or bad atmospherics. 

I'm curious how you guys are achieving 5-10 times better accuracy and I suspect it's procedural. Obviously multiple sets (6?) distributed around the plate, no observations above 30° altitude, and observer skill concerning pointing, accurate  time coincidence ticks and low micrometer reading personal bias is important but can be ameliorated with practice.  I doubt a magical observer can tighten things up much compared to a seasoned surveyor familiar with the process.

I'd like to pick your brains concerning procedures/equipment:

  • Do you use geodetic grade tripods?  They're heavy.  If not, do you avoid unstable setup situations?
  • Do you shade the instrument and wait 20 minutes for it to acclimate, more if it just came out of the truck?  
  • Do you use a striding level to tighten the leveling of the horizontal axis?  Or do modern Total Stations compensate for levelling error so a striding level is old school.
  • Do you think a Roelofs prism introduces excessive horizontal pointing error and is not needed for the hour angle procedure where simultaneous observation of the vertical angle is not salient?
  • Do you base your lat-lon on quad sheets?  Does that introduce multi second errors?  Using a handheld GPS to get within 30 feet +- sort of negates the concept of autonomy; might as well whip out your GNSS receiver.
  • Concerning foresight targetry, are they free of phase error and stable?  If a sight is not evenly lighted on both sides, the instrument man will tend to point toward one side. This phenomenon, called "phase", can be reduced by using a target with a flat surface pointed directly toward the instrument.  Concerning your experiment a tower strobe at 9 miles is certainly subsecond resolvable but with differential heating and wind loads may dance around several feet.  A range pole @ <1,000' distant beware, phase error can be considerable.

Your and the literature quoted above involve multiple redundant observations from many stations over a long time span which can be least squares resolved to 1-3" accuracy.  I believe it, but question whether a single solar observation (6DR&RV) to a moderately distant target using solar to determine azimuth is better than about five seconds @95% confidence level.

But that's just me, I'm old school.  Could be wrong.

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@mike-marks

I would like to add something for you and others about Realistic azimuth results. Lets take 1st order work;

we go to Classification, Standards of Accuracy and General Specifications of Geodetic Control Surveys"

and see on page 4 that 1st order work requires 2 nights with 16D&R each night with a Standard error of 0.45

seconds. The real accuracy of 1st order azimuth is around 1.5 to 1.7 seconds of arc.

One of the papers I referenced does say a SOLAR obs. (that means SUN) will be 1.9 seconds "Standard Error"

or Standard deviation if you prefer.

Where did you come up with "5 seconds at 95% confidence level" ?

Question for all surveyors reading this:  Just how accurate of an azimuth do you require? Give it in seconds of arc

and with a 1, 2 or 3 sigma error.

 

JOHN NOLTON

 

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@mike-marks

  • Do you use geodetic grade tripods?  They're heavy.  If not, do you avoid unstable setup situations?
Good wood. Regular tripod. 
 
  • Do you shade the instrument and wait 20 minutes for it to acclimate, more if it just came out of the truck?  
Well solar azimuths the instrument has to be in the sun, so no shade. But, letting the setup sit for a while is important. Mostly because I think the thermal stability of the tripod is key. 
 
  • Do you use a striding level to tighten the leveling of the horizontal axis?  Or do modern Total Stations compensate for levelling error so a striding level is old school.
I don’t have a striding level. I level using the vertical bubble. The split image of that vial is far more sensitive and can be observed far more critically than the center vial. The split image can be used to level the alidade far better. 
 
  • Do you think a Roelofs prism introduces excessive horizontal pointing error and is not needed for the hour angle procedure where simultaneous observation of the vertical angle is not salient?
I would like to use a Roelof. I don’t think for azimuth it’d be any better. I only observe the trailing edge. Reading the leading edge is far too noisy. The semi diameter is as good as center. I haven’t never obs the leading edge. 
 
  • Do you base your lat-lon on quad sheets?  Does that introduce multi second errors?  Using a handheld GPS to get within 30 feet +- sort of negates the concept of autonomy; might as well whip out your GNSS receiver.
The point of this survey is to determine the coord of the strobe light, and the accuracy of azimuths. So, only OPUS or published points are occupied - in this example. I’m sure a CA code position from a hand held will be fine (3-5 m). And a true coord would be available, if not same day. 30 ft in latitude longitude pretty small impact. Xi Eta is much greater: can be more than 10 seconds! Astronomic latitude longitude is a big deal! Ignoring Xi Eta can be 20 seconds error, by my obs from when I didn’t include Xi Eta. In this survey Xi Eta Laplace goes from (7.3, -10,7, 8.9) to (1.8, -3.2, 2.7) That's many seconds of latitude longitude! 
 
  • Concerning foresight targetry, are they free of phase error and stable?  If a sight is not evenly lighted on both sides, the instrument man will tend to point toward one side. This phenomenon, called "phase", can be reduced by using a target with a flat surface pointed directly toward the instrument.  Concerning your experiment a tower strobe at 9 miles is certainly subsecond resolvable but with differential heating and wind loads may dance around several feet.  A range pole @ <1,000' distant beware, phase error can be considerable.
Again, this example I’m observing a strobe light against blue sky to investigate azimuths. Job site conditions have their issues that can’t be avoided. 
 
If you looked at the spreadsheet, what is not apparent is target obs procedure. I obs the target D at least 4x, and R 4x at the start. Then at the end I re-observe D 4x and R 4x. So I point and read the target 16x. This has proven to be a big deal. At 10+ mi every time I come back to target I get 1-3 seconds different, and it’ll drift (+) and (-). So it’s atmospheric. I average the 4x D from start and end, 8 readings. And all 8 R readings of the target. 
 
I generally only take one set. With multi foresight, every foresight reading is to a different plate mark. I have taken 4-6 sets, in rapid succession and the azimuths are nearly the same. So to take 4 true sets, each should be separated in time: AM PM, or next day. 
 
As in all things, when it’s critical there has to be redundancy. So repeat as necessary. 
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@mike-marks

The survey that I presented is intended to: a) determine the coord of the strobe light, b) evaluate the azimuths. 

Re: your question about individual azimuths, not adjusted azimuths. 

so today I did a quickie. Just setup read one set, per the procedures I described. This obs is not in the adjustment. It’s unadjusted azimuth, right out of the spreadsheet: T4-strobe 

95-48-08.2

the azimuth from the adjusted net is:

95-48-09.9

I’ve been doing that for quite a while. And that’s why I contend these days. UTC time, Xi Eta Laplace, solar azimuths are very good. Not 10-20” as has been chatted up for a long time. 

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Mike Marks
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@larry-scott

I'm not doubting your results; you've provided indisputable data that shows an accuracy of 1-2" or so.  My worst case of 10-20"with 5" or so typical is merely an observation of the accuracy I obtain when doing solars (1 D&R obs) in the course of a mundane field traverse, with shorter legs, some shots near solar noon (>+- 1 hour) with no waiting for the instrument setup  to thermally stabilize, targetry a mix of plumb bob strings, range poles & prisms, etc.

Thanks for responding to my questions, may I followup?

  • Yep, a good heavy wooden tripod outperforms metal ones.  It's a good thing you can setup in stable terrain for your observations; I must accept whatever the terrain allows and observe quickly before settlement and torsion degrades the observation.  
  • Concerning instrument shading I was commenting on my field operations, where it may be several observations in the traverse are in tree cover or shaded by nearby terrain, then entering a meadow with clear sky suitable for a solar, which may be tens of degrees warmer and suffer from direct sun on one side of the setup.  Obviously a solar shot must be done with the instrument in the sun, and allowing time for the instrument setup to stabilize is important.  Not enough time to do so in my commercial traverse situation.
  • Striding levels are utilized to reduce errors in trunnion axis perpendicularity compared to the the horizontal plate axis based on plate bubble orientation.  They have nothing to do with vertical angle errors except secondarily as you posit, a coincidence split bubble on the altitude axis D&R is effective.  Personally I've never used one and suspect it's not a factor in modern instrument observations if the instrument is well calibrated.
  • Obviously a Roelofs prism performs no better than a simple sun filter if only observing the sun's leading/following limbs, and is only useful for a simultaneous check for altitude to look for timing blunders.
  • Agreed that the lat-lon of the observing station introduces minor errors unless it's hundreds of feet.

So I'm puzzled why my solars are so sh*tty compared to your super good strobe location after hundreds of obs from NGS stations surrounding the target.  I deny I'm a lousy observer through the 'scope and strongly suspect  the problem is I neglected Laplace corrections for 5-20 mile traverses, not available back in the seventies.  As you opined, that introduces  10" or more into the results.

One final question, why do you consider leading limb observations inferior to trailing edge?

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@mike-marks

not transforming geodetic to astronomic (Xi Eta) can be a big source of grief when the values are large. I learned the hard way. In my area Eta varies by over 7” in less than 10 miles.

And it takes time. On small project you can’t easily spend the time on one setup. Or have the weather when you need it. Or the T2, because TS is how we work now.

The leading edge shows up like a surprise. So I’m always late on the stop watch. And it’s just not worth it. The trailing edge is in view in the sun as it crosses the cross hairs like a sweep second hand. Just better hand eye coordination. And semi diameter is not in question, so there’s no uncertainty in the diameter. SD is as good the plate marks in the gun. If you looked at the residuals in the spreadsheet you can see that the trailing edge was captured. The leading edge has no information.

any class/workshop on solar is heavy with calculation, not nearly as much on observation. I haven’t seen any real software since Elgin & Knowles, 1985.

Part of the problem is before 1980, we were told to expect 10+ seconds best case. In college level textbooks there’s no reason that sun obs have any mathematical reason to suffer, except that Xi Eta Laplace for everywhere wasn’t available, unlike now with NGS DEFLEC.

Given that accurate time is easier to curate, Xi Eta Lap. is modeled for everywhere, we’re all getting better with tools like Excel, we have to revise what is possible and practicable. 10+ sec accuracy is obsolete. 

I have a tiny GPS receiver with an LED pps output. Absolute time. And it took a long time to create my spreadsheet, over a year of various versions and improvements. And I set out to see what is practicable.

The strobe light is visible across the whole Shenandoah valley, so there’s no skimming terrain. I can choose the setup location. I can get 1cm opus solution, collecting gps data while making the observation, so time on station is efficient. The strobe is far enough away that +/- a couple feet is still only 1-2 seconds. So I became obsessed. I’ve used Starnet since v.1 came out in the 80s. I was curious to see if I could establish an intersection station as well as an agency. And I have the gear: Ashtech receivers, and T2 (T3, T1, T16...).

as I said before, first go around I was happy with a bunch of seconds residual. But noticed that I had good one and bad ones, per Starnet. (Gotta love Starnet) little things like using the vertical level vial to level the gun. Which at least gets the standing axis true. The trunnion axis can’t be bad, it’s a T2! Observing the target D and R, multiple times at the start, observing the target multiple times D and R at the end, averaging D start and end, and R start and end, that is a significant improvement on 10 mi sightings. 

”You’ve got to accentuate the positive, eliminate the negative.”

53CE5155 8AC7 4905 AF34 65FA2B42AB34

 

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John Hamilton
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@larry-scott

A few comments about possible error sources. Nowadays time should be a very small error source. 

1) note that what is needed in the reduction is astronomic latitude and longitude. So the xi and eta are not just for computing the laplace correction, they can be used to convert a geodetic position (i.e. from GPS) to an astronomic position. The effect of an error in latitude is minimal when the azimuth to the star or sun is 0° or 180° (i.e. on the meridian). An error in longitude (and time) is minimized when the object is at elongation, which is the opposite of being on the meridian. So both can't be met at the same time, but polaris makes a very good target when at elongation, which is why it is best to observe polaris at elongation and time can be off by a few seconds without causing any significant error, ditto with latitude and longitude.  

2) inclination of the standing axis: this is an error that is NOT corrected by taking D+R shots. The magnitude is m*Tan(V), where V is the vertical angle (not zenith distance) and m is the mislevelment in seconds. My (newer) T2 has a pendulum compensator (similar to the compensator in an automatic level), this can be used to either get the instrument close to being level, or, more practical, to measure the inclination of the vertical axis. Procedure with a compensator is to set the scope at any vertical angle, turn 90° left from the object (sun, polaris), read the vertical circle. Then turn 90° right, read the vertical circle. Take the difference, divide by 2, that gives m. This is done after each set. A T2 has a bubble with a sensitivity of 20"/2 mm division on the bubble. If the altitude of the sun (or polaris at 40° latitude) is 40°, and the bubble is off by 1.5 divisions, then the azimuth will be in error by 25", and the only way to correct that is by the above formula. If using an older T2, without a more sensitive striding level, it can be difficult to get it level enough to reduce this error to a negligible amount. I have a T3, which has a bubble sensitivity of 6". I have in the past used a different T3 that had a calibration of the spirit level bubble and I would carefully record the bubble position after every position when doing polaris. I also have a Trimble S6 and a Trimble SX10, both of which have dual axis compensators supposedly accurate to 0.5", but I have not yet done any astros with these instruments, although I plan to when the weather is warmer. What I really want to try is to use the SX10 for a polaris observation, it does not have a telescope to look through, it is all done by video. Not sure how well it will work, but I think it will at least work with the moon for lunar azimuths. 

#2 can be a very significant error source unless the instrument has dual axis compensation. And as I said , IT IS NOT ELIMINATED by taking D+R. 

I very much believe like Larry that a solar can be observed to an accuracy better than 5" without too much effort. Time is critical (easy to get), accurate ephemerides are available (I use USNO MICA or the astronomical almanac), and modern instruments can greatly reduce the error from mislevelment. 

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I level using the vertical circle vial.

I bring the halves into coincidence, rotate 180. Then the vertical vial will no longer be in coincidence. I cut that in half with 1 foot screw. Realign the vial. Repeat that 2-3 time until the vial remains in coincidence in F1 F2. Rotate 90 and repeat the process using 2 foot screws.

when it’s ‘level’ the split image remains in coincidence in all directions. And I keep an eye on it during the obs. A very small misalignment in the split image image of the vertical vial is addressable, while imperceptible in the long horizontal vial. I do the exact same Procedure with T3. With TS, I lock the scope and level so that the vertical angle is unchanging when rotated  

When the vertical vial is stationary when rotated, the hor vial is stationary too. That’s standing axis, not trunnion axis. 

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@mike-marks

And these days, solar obs may not have a place in the work flow.

I refer to my survey of the strobe light as a “civil reenactment”. More nostalgia than productive.

And single handed, I can’t leave gear a 1/2 hr drive away. So the strobe light was sort of plan B. I’d rather have 10 mi OPUS or published point pairs. But I’ve only managed that once.

and winter is good solar az season. The sun is never too high.  

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John Hamilton
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@larry-scott

Well, if the GNSS systems ever go down (it could happen, either by hostile attack in space, jamming, or by a massive solar flare) then astros will be king again. Not likely to happen , but the possibility exists. 

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@mike-marks

You know, your solar az might have been observed better than you think. It sounds more like the data reduction is at fault, than the field obs.

 

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A Harris
Posts: 8399
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(@a-harris)
5,000+ posts
Joined: 10 years ago

I mostly used SMI's local hour angle method with the HP41 and HP48.

With the canopy density of some of the locations around here it was nice to obtain a static position on as many points around a forest and make a sun shot from them and either traverse thru the static point and the azimuth point we had set as we measured around the project tract.

My best results for sun shots were when I would turn to a point a few seconds in time past the sun and mark the time when the sun's leading edge touched the vertical hair of the instrument.

Mostly after 6 repetitions, the computed azimuth was ± a few seconds at most from any of the single shots.

I believe the ephemeris ended a few years after SMI ended making programmed cards. Suppose I should give it another try between two points in a rather NS direction from one another that I have observed static data on and see how it compares.

Correct time and a well leveled instrument are key to that method.

0.02

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