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Can I use two NGS Marks for Azimuth "control"?
Posted by rfc on April 6, 2016 at 3:51 pmI’ve been “dialing in” my astro abilities, now with solar, but will include Polaris and other stars soon. I’m currently using bearings shown on nearby surveys to compare my results with, but would like to go one step further, to prove both my abilities, and the excel (lent) spreadsheet (by Larry Scott), I’m using.
I’ve identified two NGS Horizontal control marks nearby: AA8188 and AA8189. They’re about .47 miles apart, along a long, flat wide open stretch of road, and easily intervisible (although I’ve never shot anything 2500′ away). I’d like to do a grid inverse (which I’d convert to geodetic), to derive an azimuth I can use as my back sight, for comparison to my observations.
I’m not really familiar with how to read these sheets, and uncertain as to how to go about it in Starnet, so I’ll just ask: What is the potential azimuthal error between the two (at the 95% confidence level), based on the “FGDC Geospatial Positioning Accuracy Standards” of each (observing one from the other). Or, how do I go about calculating it?
There is another mark not too far away, with it’s own paired azimuth mark, but I’d have to dig it up as it’s in someone’s lawn. I’d prefer not to go that route.:excruciating:
rfc replied 8 years, 3 months ago 10 Members · 44 Replies 
44 Replies

read the ngs datasheet and the ngs ‘decoder ring’. some of the datasheets publish network accuracy, others will be specified by order and class, those would be FGDC standards, i believe. either way, the quality indicators are there

Moe Shetty, post: 365886, member: 138 wrote: read the ngs datasheet and the ngs ‘decoder ring’. some of the datasheets publish network accuracy, others will be specified by order and class, those would be FGDC standards, i believe. either way, the quality indicators are there
Yes. They’re there. I just don’t know how to interpret them. Does the “horizontal accuracy” represent the whole ellipse or half of it (+/)? And what’s the difference between “horiz” and “Ellip”? I remember seeing a “Cliff Notes” (or decoder ring, as you call it), on how to read these things, but have looked all over the NGS site for it without success.
If this drawing is correct, I can do the math: 
http://www.ngs.noaa.gov/cgibin/showdoc.prl?Doc=dsdata.txt
rfc, post: 365889, member: 8882 wrote: Yes. They’re there. I just don’t know how to interpret them. Does the “horizontal accuracy” represent the whole ellipse or half of it (+/)? And what’s the difference between “horiz” and “Ellip”? I remember seeing a “Cliff Notes” (or decoder ring, as you call it), on how to read these things, but have looked all over the NGS site for it without success.
If this drawing is correct, I can do the math: 
.47 miles is close for hor. control, are you sure they arent benchs? If so, they arent useful, if they are 2nd order or better then they will work well, superior to solars

Moe Shetty, post: 365899, member: 138 wrote: http://www.ngs.noaa.gov/cgibin/showdoc.prl?Doc=dsdata.txt
Thank you! What page on the ngs site had that link on it?
I think this is exactly what I’m looking for:“The Local Accuracy of a control point is a value that represents the uncertainly of its
coordinates relative to other directly connected, adjacent
control points at the 95percent confidence level. This value
represents the relative positional error which surveyors can
expect between survey marks in a locality. It also represents
an approximate average of the individual local accuracy values
between this control point and other observed control points
used to establish its coordinates although, in general, all
of the immediately surrounding stations will not necessarily
have been used in the survey which established the original
coordinates.”Using the sketch I made, I’m coming up with a little less than 9″ of uncertainty, just due to the relative location of the points.

MightyMoe, post: 365904, member: 700 wrote: .47 miles is close for hor. control, are you sure they arent benchs? If so, they arent useful, if they are 2nd order or better then they will work well, superior to solars
They’re not called Benchmarks. The coordinates were established by GPS observations and adjusted in 2012. Here’s the first page of one of them:

You are good to go with those.
The 4 second Laplace correction is very small.
Those are your best AZ control 
You are quite lucky to have two good points close to you. Most people don’t.
I think the values given are +/ and not the arcs shown in your figure. Semimajor axis of the error ellipses. But unless there is reason to believe the errors of the two stations are anticorrelated, it isn’t so bad. I’d calculate sqrt(sum of squares) for the values and get +/ 0.0232 meters at 95% confidence, which for a distance of 750 meters gives +/ 6.3 seconds at 95%.
But I could be wrong.
. 
rfc thanks for posting the datasheet. I tried earlier but could not get it let alone print it.
Look at the Laplace correction 3.80 seconds. Do you know what this is and how to apply it? If not I would 1st read a good book on
astro. One to get you started is “A manual on Astronomic and Grid Azimuth by R.B. Buckner”, Chapter 7 for sure, but the whole book is best.
What type of instrument are you going to use?JOHN NOLTON

JOHN NOLTON, post: 365926, member: 225 wrote: rfc thanks for posting the datasheet. I tried earlier but could not get it let alone print it.
Look at the Laplace correction 3.80 seconds. Do you know what this is and how to apply it? If not I would 1st read a good book on
astro. One to get you started is “A manual on Astronomic and Grid Azimuth by R.B. Buckner”, Chapter 7 for sure, but the whole book is best.
What type of instrument are you going to use?JOHN NOLTON
Yes; absolutely. I can (finally) say…”this ain’t my first rodeo”:D At least when it comes to astro. Of course I’m not doing the math longhand this time, as I did about a year ago. Don’t want to do that more than once or twice. Then went to Jerry Wahl’s spreadsheet but then the Ephemeris therein ran out. Now Larry Scott has developed a brilliant piece of work that applies La Place, and Convergence to SPC. Also corrects for DUT. It means multiple observations, direct and reverse; reveals the deviations for each observation (and provides a switch to toggle off outliers). With his permission, once I get this done a few times, I might be able to post my results (but only if they’re good enough).:stakeout:

http://www.ngs.noaa.gov/cgibin/ds_mark.prl?PidBox=aa8188
http://www.ngs.noaa.gov/cgibin/ds_mark.prl?PidBox=aa8189Did I type something wrong into INVERSE or is the distance really 842.7767 meters (on ellipsoid) or 0.52 mile instead of 0.47 mile? About 2 or 3 cm longer at ground elevation.
If I typed it right, the azimuths between them are 355 25 58.7423 and 175 25 56.6755 where the 2.07 second difference from 180 degrees between them is due to convergence. Not much since they are close to northsouth.
. 
Bill93, post: 365938, member: 87 wrote: http://www.ngs.noaa.gov/cgibin/ds_mark.prl?PidBox=aa8188
http://www.ngs.noaa.gov/cgibin/ds_mark.prl?PidBox=aa8189Did I type something wrong into INVERSE or is the distance really 842.7767 meters (on ellipsoid) or 0.52 mile instead of 0.47 mile? About 2 or 3 cm longer at ground elevation.
If I typed it right, the azimuths between them are 355 25 58.7423 and 175 25 56.6755 where the 2.07 second difference from 180 degrees between them is due to convergence. Not much since they are close to northsouth.
Couldn’t say.
Here’s the other sheet data:
*CURRENT SURVEY CONTROL
AA8189 ______________________________________________________________________
AA8189* NAD 83(2011) POSITION 43 38 58.73830(N) 072 31 34.49405(W) ADJUSTED
AA8189* NAD 83(2011) ELLIP HT 185.405 (meters) (06/27/12) ADJUSTED
AA8189* NAD 83(2011) EPOCH – 2010.00
AA8189* NAVD 88 ORTHO HEIGHT – 212.9 (meters) 698. (feet) GPS OBS
AA8189 ______________________________________________________________________
AA8189 NAVD 88 orthometric height was determined with geoid model GEOID93
AA8189 GEOID HEIGHT – 27.902 (meters) GEOID93
AA8189 GEOID HEIGHT – 27.456 (meters) GEOID12B
AA8189 NAD 83(2011) X – 1,388,012.497 (meters) COMP
AA8189 NAD 83(2011) Y – 4,409,255.619 (meters) COMP
AA8189 NAD 83(2011) Z – 4,380,135.328 (meters) COMP
AA8189 LAPLACE CORR – 2.83 (seconds) DEFLEC12B
AA8189
AA8189 Network accuracy estimates per FGDC Geospatial Positioning Accuracy
AA8189 Standards:
AA8189 FGDC (95% conf, cm) Standard deviation (cm) CorrNE
AA8189 Horiz Ellip SD_N SD_E SD_h (unitless)
AA8189 ——————————————————————
AA8189 NETWORK 1.91 2.65 0.85 0.70 1.35 0.01506052
AA8189 ——————————————————————But I can definitely feel the quicksand moving…I’m getting over my head here. Here’s what I got for the inverse:
I put the NAD 83 X,Y,Z in and just did an inverse. The vertical distance is certainly NOT 608 meters; It’s more like a meter. The azimuth is whacko too. From Google earth, it’s about .45 miles at 355 “true”.
I obviously don’t have a clue how to use these numbers. 
On the matter of guidance for surveyors interested in astronomy, there are a number of good books on the subject. Many are hard to find or outofprint. A quick scan of pdf versions available free and online, yields this: http://gbennett.customer.netspace.net.au/FieldAstronomy2/Field%20AstronomyPDF.pdf
Looking through it, I found a lot of good information and find it clearly written. It was written in the late 1970s.The UNB text is also good/free/available as a PDF.
Hope this helps,
DMM

FWIW,
Here is the INVERS3D output verifying Bill93’s results:

Thank you (and thanks Bill93). I really didn’t want to get side tracked trying to figure that all out. I should have known there’s an NGS tool to do that.o.O

rfc, post: 365948, member: 8882 wrote: Couldn’t say.
Here’s the other sheet data:
*CURRENT SURVEY CONTROL
AA8189 ______________________________________________________________________
AA8189* NAD 83(2011) POSITION 43 38 58.73830(N) 072 31 34.49405(W) ADJUSTED
AA8189* NAD 83(2011) ELLIP HT 185.405 (meters) (06/27/12) ADJUSTED
AA8189* NAD 83(2011) EPOCH – 2010.00
AA8189* NAVD 88 ORTHO HEIGHT – 212.9 (meters) 698. (feet) GPS OBS
AA8189 ______________________________________________________________________
AA8189 NAVD 88 orthometric height was determined with geoid model GEOID93
AA8189 GEOID HEIGHT – 27.902 (meters) GEOID93
AA8189 GEOID HEIGHT – 27.456 (meters) GEOID12B
AA8189 NAD 83(2011) X – 1,388,012.497 (meters) COMP
AA8189 NAD 83(2011) Y – 4,409,255.619 (meters) COMP
AA8189 NAD 83(2011) Z – 4,380,135.328 (meters) COMP
AA8189 LAPLACE CORR – 2.83 (seconds) DEFLEC12B
AA8189
AA8189 Network accuracy estimates per FGDC Geospatial Positioning Accuracy
AA8189 Standards:
AA8189 FGDC (95% conf, cm) Standard deviation (cm) CorrNE
AA8189 Horiz Ellip SD_N SD_E SD_h (unitless)
AA8189 ——————————————————————
AA8189 NETWORK 1.91 2.65 0.85 0.70 1.35 0.01506052
AA8189 ——————————————————————But I can definitely feel the quicksand moving…I’m getting over my head here. Here’s what I got for the inverse:
I put the NAD 83 X,Y,Z in and just did an inverse. The vertical distance is certainly NOT 608 meters; It’s more like a meter. The azimuth is whacko too. From Google earth, it’s about .45 miles at 355 “true”.
I obviously don’t have a clue how to use these numbers.you don’t want to try to inverse those numbers, convert the lat, long to state plane and inverse them for the grid AZ

MightyMoe, post: 365962, member: 700 wrote: you don’t want to try to inverse those numbers, convert the lat, long to state plane and inverse them for the grid AZ
Got it. I just realized they do half the work for you…on the second page of the data sheet, they give the SPC VT Coordinates. I inversed them and I’m very close (but I don’t have my scale factors right yet). Does Unvers3D output UTM grid azimuth?

I figured if you are doing astro measurements you wanted geodetic azimuths.
Using the SPC coordinates from the data sheets, I get SPC grid azimuths 175 27 02.0 or 355 27 02.0 The arctochord correction will be negligible for your purposes at this distance.
By combining geodetic azimuth with convergence angle from the data sheets I check the azimuths I posted above within a tenth or two of a second.
The UTM azimuth would be different. Which are you using?
. 
Bill93, post: 365993, member: 87 wrote: I figured if you are doing astro measurements you wanted geodetic azimuths.
Using the SPC coordinates from the data sheets, I get SPC grid azimuths 175 27 02.0 or 355 27 02.0 The arctochord correction will be negligible for your purposes at this distance.
By combining geodetic azimuth with convergence angle from the data sheets I check the azimuths I posted above within a tenth or two of a second.
The UTM azimuth would be different. Which are you using?
SPC and Geodetic. I know how to get from one to another. I didn’t realize (and haven’t done a fraction of the reading I need to yet) that Inverse3D outputs in Geodetic.
Thanks very much for all the guidance. The hand writing is on the wall though. Sooner or later I’m going to have to grab the ellipsoid by the horns and figure that all out.

rfc, post: 365948, member: 8882 wrote: Couldn’t say.
Here’s the other sheet data:
*CURRENT SURVEY CONTROL
AA8189 ______________________________________________________________________
AA8189* NAD 83(2011) POSITION 43 38 58.73830(N) 072 31 34.49405(W) ADJUSTED
AA8189* NAD 83(2011) ELLIP HT 185.405 (meters) (06/27/12) ADJUSTED
AA8189* NAD 83(2011) EPOCH – 2010.00
AA8189* NAVD 88 ORTHO HEIGHT – 212.9 (meters) 698. (feet) GPS OBS
AA8189 ______________________________________________________________________
AA8189 NAVD 88 orthometric height was determined with geoid model GEOID93
AA8189 GEOID HEIGHT – 27.902 (meters) GEOID93
AA8189 GEOID HEIGHT – 27.456 (meters) GEOID12B
AA8189 NAD 83(2011) X – 1,388,012.497 (meters) COMP
AA8189 NAD 83(2011) Y – 4,409,255.619 (meters) COMP
AA8189 NAD 83(2011) Z – 4,380,135.328 (meters) COMP
AA8189 LAPLACE CORR – 2.83 (seconds) DEFLEC12B
AA8189
AA8189 Network accuracy estimates per FGDC Geospatial Positioning Accuracy
AA8189 Standards:
AA8189 FGDC (95% conf, cm) Standard deviation (cm) CorrNE
AA8189 Horiz Ellip SD_N SD_E SD_h (unitless)
AA8189 ——————————————————————
AA8189 NETWORK 1.91 2.65 0.85 0.70 1.35 0.01506052
AA8189 ——————————————————————But I can definitely feel the quicksand moving…I’m getting over my head here. Here’s what I got for the inverse:
I put the NAD 83 X,Y,Z in and just did an inverse. The vertical distance is certainly NOT 608 meters; It’s more like a meter. The azimuth is whacko too. From Google earth, it’s about .45 miles at 355 “true”.
I obviously don’t have a clue how to use these numbers.FYI:
The XYZ coordinates in the data sheet are called ECEF. Earth Centered, Earth Fixed. The XY plane is the equator, and Z is the spin axis of the globe. And 0,0,0 is the center of mass of the earth.(Differential GPS measurements returns delta XYZ mark to mark.)
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