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.)
[USER=8882]@rfc[/USER] ... you are on the right track.
side note: I don't know if you will ever run into it, but it is very important to look at this line in the data sheets
AA8189* NAD 83(2011) EPOCH - 2010.00
I looked, both of your stations are adjusted on the same Epoch... if Not you would be mixing apples and oranges and it could drive you nuts. (been there, done that)
out in the West Coast with active fault lines a 10 year difference in Epoch can be significant.
Larry Scott, post: 366308, member: 8766 wrote: 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.)
The "Ah Ha!" moment has arrived (Thank you Larry).
So far, I've put aside getting too deep into GPS because I don't have the budget to explore it in real life, especially with so much else to learn regarding earlier surveying techniques. But I've always hoped that there would be a moment during the process of immersing myself in the sea of acronyms (PDOP, HDOP, GDOP, VDOP, CORS, OPUS, etc.), that how the whole system works as a whole would come to me.
I've always known that time is one of the key elements to making GPS work...extremely accurate time, and the differences in time it takes for signals to arrive at various places. But the other major piece (I've just discovered), is gravity. Upon reading about ECEF, it makes total sense that the earth must spin about a particular point, and every point on it's surface would never change relative to that point (tectonic plate shift notwithstanding). The center of mass is sometimes called the center of gravity, and all of the GPS satellites orbit about the earth, such that their orbits are highly predictable, and their positions are known RELATIVE TO THAT POINT at all times.
It makes total sense then that the entire reference frame for any GPS system then would originate at the center of mass of the earth. I know it's a lot more complicated than that, especially when it comes to putting it into practice and I can see where it would be easy to become a "button pusher", using it without knowing exactly how it works. But I would guess that a broader understanding would be useful when it comes to detecting when something goes awry. Certainly many stories here about those occurrences.
Anyway, back to astro. This GPS stuff is all for another day.
That's the beauty of it. GPS is geodetic and the difference between Astro geodetic is better known every few years.
For instance: the distance mark-to-mark from the data sheet is a very simple calc from ECEF coords. It's just a Pythagorean right triangle.
Då? = (xâ-xââ)å?+(yâ-yââ)å?+(zâ-zââ)å?
ECEF, for instance. XY is the equatorial plane. Therefore:
Longitude = ATAN (Y/X)
That's ECEF X and Y.
rfc, post: 366361, member: 8882 wrote: The "Ah Ha!" moment has arrived (Thank you Larry).
So far, I've put aside getting too deep into GPS because I don't have the budget to explore it in real life, especially with so much else to learn regarding earlier surveying techniques. ... .
You might find the following link to a basic text on geodesy to be helpful: http://www.xanedu.com/higher-education/educators/custom-books-catalog/geodesy-for-geomatics-and-gis-professionals/
The authors even include some discussion of NGS products like their datasheet.
While geodetic longitude is easy, the determination of the geodetic latitude as well as the ellipsoid height is much more interesting. The recommended book has a nice treatment of this issue as well as many other issues about which you have posted.
As for the statement about GNSS orbits being "highly predictable" it took a lot of work to get to the point we are today. If you review the elements in the navigation message you will see a large number of parameters are involved. See page 200 of this document: http://geodesyattamucc.pbworks.com/f/LN58.pdf (University of New Brunswich Lecture Notes No. 58). This was my text in a course on GPS. While dated, there is a lot of good information.
Even in the late 80s broadcast v. precise ephemeris for phase differential didn't make a lot of difference at the 5 PPM level over 10s of miles. GPS was great right out of the gate. Just not fast like today.
(I skipped geocentric to geodetic latitude cuz it's got more to it.)
Until April 1993, the US National Geodetic Survey in its most precise work used a technique known as "orbit relaxation" to account for the relative inaccuracy of then-existing GNSS orbits.
See: http://www.ngs.noaa.gov/CORS/Articles/SolerMexicoASCE.pdf specifically the last two paragraphs of page 86 in this Journal of Surveying Engineering article by Dr Tomas Soler. It was common to wait a week or more for precise ephemerides to become available.
The accuracy of modern IGS orbits is tabulated here: http://igs.org/products BTW, those who have not visited the site in a while should note that this is the current page.
While in geodetic work the most accurate orbits are always used, the following relationship can be used to determine whether an orbit product is good enough: BL error/BL length = SVerror/distance to SV (where BL is baseline and SV is satellite (actually space vehicle)). Given the phenomenal improvements in orbit accuracy even the broadcast, most users can use them for nearly all of their work.
I have always done it three ways, broadcast and post ephemirides, NASA's AUTOGIPSY. (Fixed integer, multiple receivers, dual freq.). Even at very long distances, kinematic and static. >100 mi.
It's a confidence builder to experimentally determine the effect. A few hundred times.
The big advance in GPS in past 8+ years has been in atmospheric modeling. The hazard in geomagnetic disturbances remains.
GeeOddMike, post: 365953, member: 677 wrote: FWIW,
Here is the INVERS3D output verifying Bill93's results:
Are the azimuths shown from this assuming the UTM grid?
That's the same az I calc for geodetic
I suggest you take a look at the NGS Geodetic Glossary here: http://www.ngs.noaa.gov/CORS-Proxy/Glossary/xml/NGS_Glossary.xml
UTM and State Plane are projected from geodetic coordinates: latitude and longitude. There is a two-dimensional version of the inverse program, INVERSE, that does not account for ellipsoid heights and INVERS3D which does. The output I provided was from INVERS3D. Neither UTM nor SPC were involved. While one can convert between geodetic and plane azimuths, the convergence angles are computed as part of the projection.
While the help provided by members of this board can address particular questions, reading a good textbook like the one linked above (better yet a class) are preferable.
There are many resources and opinions available on the Internet. Assessing the quality and correctness can be a problem.
HTH,
DMM
We have a problem, Houston.
I took 16 shots (8 each face), with standard deviations in the 4-6 second realm on each face. I'm confident in my work here.
That said, not only is the azimuth off, not by seconds, but by over 8 minutes, but the distance I measured to the mark was 698.27 meters.
The data sheet shows it should be 842.8 meters!
So, the possibilities are:
1. There's something seriously wrong with my EDM (This still wouldn't explain the Azimuth anomaly.)
2. I picked up a reflective sign on the highway, 150 meters closer than the mark.
3. One or the other of the marks have been moved, or is just not where the data sheet says it is.
I don't think it's 1. I've checked the thing recently via two peg.
2. Unlikely, as I was looking across the highway as it takes a gentle turn. Admittedly, this is the longest shot I've ever taken. There was significant heat shimmer even though it was 45 degrees F. But I took 8 shots (forward and reverse, just to be sure). The residuals were less than .003'
If this were some rogue reflective thing, I'd think some of the 8 shots would be wildly different.
As for 3., I don't know what to think. This Grasshopper's first instinct is certainly NOT to claim an NGS documented survey mark is in the wrong place!
The south one (AA8818) is in a bridge abutment, and while the markings don't match the data sheet precisely, it's close enough; I'll confirm with VT AOT this week. The bridge has been there as long as I can remember.
The north one could be suspect. It's labeled "Pomfret School" (which does happen to be another 150 meters further along the highway...that's not evidence, I know. Just pointing it out.) I'll look at the data sheets again, but I thought they were both checked by GPS in 2010.
So my next moves will be:
1. Take two separate consumer GPS units (my Garmin and my iPhone) and see if either mark appears to be off. At 150 meters thats more than a few seconds of latitude.
2. Measure the distance from AA8188 TO AA8189 and see if I get the same distance (and azimuth if the sun is out).
3. Check with the State of Vermont to see if they know if anything's up with these two marks.
If all that fails to turn up the reality of the situation, then....any ideas???
rfc, post: 366523, member: 8882 wrote: We have a problem, Houston.
I took 16 shots (8 each face), with standard deviations in the 4-6 second realm on each face. I'm confident in my work here.
That said, not only is the azimuth off, not by seconds, but by over 8 minutes, but the distance I measured to the mark was 698.27 meters.
The data sheet shows it should be 842.8 meters!
So, the possibilities are:
1. There's something seriously wrong with my EDM (This still wouldn't explain the Azimuth anomaly.)
2. I picked up a reflective sign on the highway, 150 meters closer than the mark.
3. One or the other of the marks have been moved, or is just not where the data sheet says it is.I don't think it's 1. I've checked the thing recently via two peg.
2. Unlikely, as I was looking across the highway as it takes a gentle turn. Admittedly, this is the longest shot I've ever taken. There was significant heat shimmer even though it was 45 degrees F. But I took 8 shots (forward and reverse, just to be sure). The residuals were less than .003'
If this were some rogue reflective thing, I'd think some of the 8 shots would be wildly different.As for 3., I don't know what to think. This Grasshopper's first instinct is certainly NOT to claim an NGS documented survey mark is in the wrong place!
The south one (AA8818) is in a bridge abutment, and while the markings don't match the data sheet precisely, it's close enough; I'll confirm with VT AOT this week. The bridge has been there as long as I can remember.The north one could be suspect. It's labeled "Pomfret School" (which does happen to be another 150 meters further along the highway...that's not evidence, I know. Just pointing it out.) I'll look at the data sheets again, but I thought they were both checked by GPS in 2010.
So my next moves will be:
1. Take two separate consumer GPS units (my Garmin and my iPhone) and see if either mark appears to be off. At 150 meters thats more than a few seconds of latitude.
2. Measure the distance from AA8188 TO AA8189 and see if I get the same distance (and azimuth if the sun is out).
3. Check with the State of Vermont to see if they know if anything's up with these two marks.If all that fails to turn up the reality of the situation, then....any ideas???
put them record in Google, see if you are anywhere near the correct location
MightyMoe, post: 366527, member: 700 wrote: put them record in Google, see if you are anywhere near the correct location
Bingo! There's another brook at the correct Lat/Long south of the bridge abutment I found. It matches the Latitude of the mark I thought I was on almost to the second. Tomorrow morning I'll investigate.
As for 3., I don't know what to think. This Grasshopper's first instinct is certainly NOT to claim an NGS documented survey mark is in the wrong place!
The south one (AA8818) is in a bridge abutment, and while the markings don't match the data sheet precisely, it's close enough;
In your message you state:
ÛÏThe south one (AA8818) is in a bridge abutment, and while the markings don't match the data sheet precisely, it's close enough; I'll confirm with VT AOT this week. The bridge has been there as long as I can remember.Û
NO! NO! NO! NO! NO!
If the markings donÛªt match the data sheet precisely it is most likely NOT the correct monument!
Both are indicated to be marked by a SURVEY DISK AA8188 is a VTAT disk; AA8189 is a VTSM disk. The stamping should be as shown in the data sheet.
I have also copied the section of the description that provides local measurements which are used to positively verify that the disk recovered is that in the data sheet.
Recovering an monument does not consist of merely going to the general vicinity and finding a likely disk. We use the local ties; we verify the stamping.
NGS requires the submission of a photo or rubbing of stations used in projects submitted to them to insure the correct disk was used. The gross distance difference makes me confident that you have NOT correctly identified one of the two (or even both) monuments. It is also the reason that mark destroyed notes are not treated as definitive when submitted to NGS.
BTW, I do not think any of the replies specifically indicated how to find the dsdata.txt file. Whenever you retrieve a data sheet from the NGS site a link to the file is at the top of the page. It is an invaluable guide to the contents of the NGS data sheet.
Using the mark-to-mark distance as the check over ellipsoid or plane distance requires you know why they differ. The ellipsoid distance between these two monuments differs from the mark-to-mark by 2.5 cm. Do you know why? Do you know what distance your EDM measurement should check?
Relevant data from the data sheets for the two points you say you used were extracted below:
AA8188_MARKER: DD = SURVEY DISK
AA8188_SETTING: 38 = SET IN THE ABUTMENT OR PIER OF A LARGE BRIDGE
AA8188_SP_SET: ABUTMENT
AA8188_STAMPING: GULF STREAM 1993
AA8188_MARK LOGO: VTAT
AA8188_MAGNETIC: N = NO MAGNETIC MATERIAL
AA8188_STABILITY: B = PROBABLY HOLD POSITION/ELEVATION WELL
AA8188
AA8188 HISTORY - Date Condition Report By
AA8188 HISTORY - 1993 MONUMENTED VTAT
AA8188 HISTORY - 19970626 GOOD VTGS
AA8188'THE MARK IS SET IN THE SOUTHERN BRIDGE ABUTMENT. IT IS LOCATED 113.2
AA8188'FT (34.5 M) S10E FROM POLE 8-1/8-1, 16.4 FT (5.0 M) S70E FROM THE
AA8188'CENTERLINE OF TH1, 1.6 FT (0.5 M) FROM THE EAST END OF THE SOUTHERN
AA8188'ABUTMENT, 1.3 FT (0.4 M) S70E FROM THE STEEL GUARDRAIL, AND 1.0 FT
AA8188'(0.3 M) OUTE 4, AND 1.6 FT (0.5 M) NORTHEAST OF FIBERGLASS WITNESS
AA8188'POST.
ÛÓÛÓÛÓÛÓÛÓÛÓÛÓÛÓ
AA8189_MARKER: DD = SURVEY DISK
AA8189_SETTING: 7 = SET IN TOP OF CONCRETE MONUMENT
AA8189_STAMPING: POMFRET SCHOOL 1993
AA8189_MARK LOGO: VTSM
AA8189_MAGNETIC: R = STEEL ROD IMBEDDED IN MONUMENT
AA8189_STABILITY: C = MAY HOLD, BUT OF TYPE COMMONLY SUBJECT TO
AA8189+STABILITY: SURFACE MOTION
AA8189'THE MARK IS SET IN THE TOP OF A 30 CM DIAMETER CONCRETE MONUMENT. IT
AA8189'IS LOCATED 77.1 FT (23.5 M) S 60 E FROM THE SE CORNER OF A 1 1/2 STORY
AA8189'WOOD HOUSE, 66.3 FT (20.2 M) N 40 E FROM POLE 3/30/29, 54.5 FT (16.6
AA8189'M) S 75 E FROM AN 18 INCH HEMLOCK, AND 13.8 FT (4.2 M) N 90 E FROM THE
AA8189'CENTERLINE OF TH1.
One other comment. You have occupied a point you say is an NGS-published monument with a good position. From this point you have a geodetic azimuth (derived from your astronomical azimuth) and a distance. There is a tool in the NGS that allows you to use the information to determine a position. It is named FORWARD (and, of course, a three-D version INVERS3D).
Many years ago when I was at school the geodetic inverse and forward problems were a big deal. Of course you could go the easy way and use state plane coordinates. Convert your azimuth to plane and your distance to grid.
GeeOddMike, post: 366537, member: 677 wrote:
As for 3., I don't know what to think. This Grasshopper's first instinct is certainly NOT to claim an NGS documented survey mark is in the wrong place!
The south one (AA8818) is in a bridge abutment, and while the markings don't match the data sheet precisely, it's close enough;In your message you state:
ÛÏThe south one (AA8818) is in a bridge abutment, and while the markings don't match the data sheet precisely, it's close enough;"NO! NO! NO! NO! NO!
If the markings donÛªt match the data sheet precisely it is most likely NOT the correct monument!
Both are indicated to be marked by a SURVEY DISK AA8188 is a VTAT disk; AA8189 is a VTSM disk. The stamping should be as shown in the data sheet.
Relevant data from the data sheets for the two points you say you used were extracted:
AA8188_STAMPING: GULF STREAM 1993
AA8188_MARK LOGO: VTAT
OK. Rookie error #749.:-$
Went south about 150m to investigate the other bridge abutment, and found:
This one actually says what the data sheet says it should say.
Oh, and get this:
The bridge it's on actually crosses "Gulf Stream", not the Barnard Brook the other one is on. Argh!
Lesson learned. Unless I can find a record of the location of the monument I used at the State, it looks like we're doing the solar over again.
If my post seemed harsh it was because your situation brought back bad memories of observers occupying wrong disks and adding work days to projects. Occupying reference marks instead of the primary station is always a concern. Rubbings or photos are invaluable. While one can compute the reference mark's position from the BoxScore, unless the primary station is unusable, observe the primary station.
As for the need to re-observe the solar, you have determined an azimuth for the wrong disk. If you observe the angle between the wrong and correct disks from the point you observed the Sun you can add the angle to determine the azimuth for comparison with published values.
HTH,
DMM
