DPOS question

[Tyler]

After comparing measured coordinates for multiple stations in the area to the NGS published coordinates of these stations, I am consistently getting 0.06 to .12 feet between the two values which seems great. However, I was wondering what is being held to establish these measured coordinates. Is it the base that is being held or some kind of satellite geometry? I am very new to using GPS and the equipment we are using is a Javad Triumph 1M base with a Javad Triumph LS rover. Hopefully someone is able to shed some light or perhaps help me rephrase my question if it seems confusing! If not I will be taking a GPS course this semester and maybe that can answer my question.

Thank you,
Tyler Brown

 

[Adam]

After comparing measured coordinates for multiple stations in the area to the NGS published coordinates of these stations, I am consistently getting 0.06 to .12 feet between the two values which seems great. However, I was wondering what is being held to establish these measured coordinates. Is it the base that is being held or some kind of satellite geometry? I am very new to using GPS and the equipment we are using is a Javad Triumph 1M base with a Javad Triumph LS rover. Hopefully someone is able to shed some light or perhaps help me rephrase my question if it seems confusing! If not I will be taking a GPS course this semester and maybe that can answer my question.

Thank you,
Tyler Brown

Tyler, it depends on how the NGS monument was established. I usually see .02 to .03 horizontally on monuments that were established by ncgs with Gnss. I see bigger variances on ones established by other means. It just depends.

It all eventually relates back to the CORS stations whether it's in real time with an rtn or after the fact with post processing. If you like you can use shift or mlocal to relate the survey to a specific monument.

 

[Alexey Razumovsky]

Give us an example (DPOS report) of the discrepancy

 

[Shawn Billings]

The base logs raw data which is processed with NGS stations known as the CORS (Continuously Operating Reference Station). These NGS stations have known coordinates published by NGS. When the data is processed between the base and the rover, vectors are formed from the CORS to the base. This produces redundant data as DPOS creates up to five vectors from the CORS, which means there are five potential positions of the base. The vectors are adjusted using least squares to determine the weighted average of the position of the base.

Observation duration is really your only trick (other than selecting a good location, free from overhead obstructions). The longer the duration, typically the better accuracy your vectors, and ultimately your final position, will have.

 

[Matthew D. Sibole]

It will also depend on how NGS derived the coordinates they are reporting. Are they first order, second order, GPS derived etc. It may also depend on what epoch they are reporting. They should be epoch 2010 but maybe something different.

 

[Nate The Surveyor]

Friday, I went out to work. I'd not charged my t2 in a while. It ran about 10 minutes, before batts died. I got one shot, about 110 seconds, before radio signal quit. (T2 had quit). Total t-2 time was about 10 minutes.
Well, I drove home, got the pigtail connector, and hooked the t2 to the 12 volt base battery, and continued work. This session was maybe 5 hrs long. I processed both the 10 min, and the 5 hr sessions. They were within 0.02' of each other.
I need to buy an extra pigtail connector, for this kind of event, to keep things running.
Also, I did not have a very good base site... It was in sort of a hole. 20 degrees up, and a tree. All my error eclipses were larger than normal, ( 0.09') as was shot spread. (Around a tenth, and one was 0.17')
My point is, short base times can be good, but the certainty factor goes down. And, base in a hole, does sort of the same... Larger errors per shot.

 

[David M. Simolo]

That's kind of funny Nate. I am in the process of ordering another pigtail for my T2 as well. Similar problem except I think the battery in my T2 is smoked (hopefully, the other options sound more painful). I was able to run it with a remote battery and my pigtail, even with an apparently very dead internal battery.

 

[Tyler]

Give us an example (DPOS report) of the discrepancy

DPOS REPORT
JAVAD POST-PROCESSING SERVICE
=============================


USER: DATE: 25.04.2018
FILE: 5000 TIME: 15:40:24.268 GPS time


SOFTWARE: DPOS(2/5/2018 12:20:58 PM) START: 2018/04/19 04:03:51.000 PM
EPHEMERIS: brdc STOP: 2018/04/19 10:48:21.000 PM
OBS USED: 19994 / 19994 TIME SPAN: 404 min
ANT NAME: JAVTRIUMPH_1MR NONE Geometry factor: 0.8
ANT HEIGHT: Slant(SHMP) - 1.847 meters FIXED AMB: Yes
ANT DELTA N/E/H: 0.000/0.000/0.000 OVERALL RMS: 0.008 meters


REF FRAME: NAD83(2011)(EPOCH:2010.0000) IGS08 (EPOCH:2018.2973)

X: -2726602.060(m) 0.005(m) -2726603.036(m) 0.005(m)
Y: -4218088.753(m) 0.005(m) -4218087.241(m) 0.005(m)
Z: 3917929.098(m) 0.005(m) 3917929.226(m) 0.005(m)

LAT: N 38 08 35.12275 0.005(m) N 38 08 35.14080 0.005(m)
E LON:E 237 07 15.92454 0.005(m) E 237 07 15.85717 0.005(m)
W LON:W 122 52 44.07546 0.005(m) W 122 52 44.14283 0.005(m)
EL HGT: -29.692(m) 0.005(m) -30.195(m) 0.005(m)
ORTHO HGT: 2.570(m) 0.005(m) [NAVD88 (Computed using GEOID12B)]

UTM COORDINATES STATE PLANE COORDINATES
UTM (Zone 10) SPC (NAD83(2011) /0403 California zone 3)
Northing (Y) [meters] 4221698.233 685004.766
Easting (X) [meters] 10510610.703 1791493.492
Convergence [degrees] 0.07478871 -1.45644483
Point Scale 0.99960139 0.99995253
Combined Factor 0.99960604 0.99995719

US NATIONAL GRID DESIGNATOR: NONE


BASE STATIONS USED
PID DESIGNATION LATITUDE LONGITUDE DISTANCE(m)
NONE CASR(CORS) N 38 26 26.414 W 122 44 49.164 34990.0
NONE P183(CORS) N 38 18 49.174 W 123 04 07.940 25201.8
NONE P196(CORS) N 38 17 53.304 W 122 44 33.457 20943.9
NONE P198(CORS) N 38 15 35.534 W 122 36 26.767 27082.5
NONE TIBB(CORS) N 37 53 27.139 W 122 26 51.317 47098.3

RESIDUALS OF POSITION
VECTOR dX,m dY,m dZ,m dN,m dE,m dU,m
CASR - 5000 -0.006 0.001 -0.014 -0.012 -0.006 -0.007
P196 - 5000 -0.001 -0.004 0.016 0.010 0.001 0.014
P198 - 5000 0.007 0.004 -0.017 -0.009 0.004 -0.016
TIBB - 5000 -0.007 0.003 -0.002 -0.002 -0.007 -0.001

 

[Tyler]

I have posted the DPOS report above but I have a few questions to further my understanding:

1) This DPOS report shows different northing and easting values than what the NGS has published for these stations. Why is there a discrepancy in these values?

2) For a boundary survey, what should we be reporting to help someone else get on our basis of bearing? We are showing a control diagram and using two stations as our basis of bearing. Should these be from the published values for the stations?

3) I have attached an example of a GPS control diagram I am trying to put together for a survey but I do not know what a final product would look like to correctly represent our findings. Should we show the published values or our values from the DPOS?

4) Do we need to show a control diagram for a Record of Survey if we are on a grid coordinate system? Below is the PLS Act that states a control scheme must be used when coordinates are shown. We don't technically have coordinates shown but our mapping angle is based on the base point. Does this qualify for the need of a control diagram in your opinion?

"8771.5 When coordinates in the California Coordinate System are shown for points on a record of survey map the map may not be recorded unless it also shows, or is accompanied by a map showing, the control scheme through which the coordinates were determined from points of known coordinates."

Sorry for all the questions! Any advice would be helpful or if this control diagram does not make sense let me know what I can do to make it clear. Thanks again for the help!

Tyler Brown

 

[Tyler]

 

[Matthew D. Sibole]

1) This DPOS report shows different northing and easting values than what the NGS has published for these stations. Why is there a discrepancy in these values?

Are the published values from NGS on the actual CORS stations used to derive the DPOS position? I would like to see the NGS data sheet for the NGS control you are referring to.

2) For a boundary survey, what should we be reporting to help someone else get on our basis of bearing? We are showing a control diagram and using two stations as our basis of bearing. Should these be from the published values for the stations?

Basis of bearing is derived from the projection used. So around here this is how it is stated. “Bearings are based on Kentucky Single Zone State Plane Coordinate system NAD83 (2011) epoch 2010” regardless of where we are in relationship to the CORS stations. Being a tenth off or even two feet will not change your basis of bearing.

3) I have attached an example of a GPS control diagram I am trying to put together for a survey but I do not know what a final product would look like to correctly represent our findings. Should we show the published values or our values from the DPOS?

I don’t think it matters which one is used as long as you have documention for what you did and what you used. Remember this is just to leave footsteps for the next guy to follow. Can he find your points based on the information provided?

4) Do we need to show a control diagram for a Record of Survey if we are on a grid coordinate system? Below is the PLS Act that states a control scheme must be used when coordinates are shown. We don't technically have coordinates shown but our mapping angle is based on the base point. Does this qualify for the need of a control diagram in your opinion?

In my opinion a control diagram is not needed unless you specifically show coordinates. (No coordinates = No diagram)

"8771.5 When coordinates in the California Coordinate System are shown for points on a record of survey map the map may not be recorded unless it also shows, or is accompanied by a map showing, the control scheme through which the coordinates were determined from points of known coordinates."

View attachment 7936

Sorry for all the questions! Any advice would be helpful or if this control diagram does not make sense let me know what I can do to make it clear. Thanks again for the help!

Tyler Brown[/QUOTE]

 

[Tyler]

1) This DPOS report shows different northing and easting values than what the NGS has published for these stations. Why is there a discrepancy in these values?

Are the published values from NGS on the actual CORS stations used to derive the DPOS position? I would like to see the NGS data sheet for the NGS control you are referring to.

Here is one of the sheets for the TIBB station as shown on my control diagram above. Thank you for the quick reply!
The NGS Data Sheet
See file dsdata.pdf for more information about the datasheet.
PROGRAM = datasheet95, VERSION = 8.12.5
1 National Geodetic Survey, Retrieval Date = SEPTEMBER 12, 2018
DO2389 ***********************************************************************
DO2389 CORS - This is a GPS Continuously Operating Reference Station.
DO2389 DESIGNATION - TIBURON PENINSUL CORS ARP
DO2389 CORS_ID - TIBB
DO2389 PID - DO2389
DO2389 STATE/COUNTY- CA/MARIN
DO2389 COUNTRY - US
DO2389 USGS QUAD - SAN QUENTIN (1995)
DO2389
DO2389 *CURRENT SURVEY CONTROL
DO2389 ______________________________________________________________________
DO2389* NAD 83(2011) POSITION- 37 53 27.13938(N) 122 26 51.31741(W) ADJUSTED
DO2389* NAD 83(2011) ELLIP HT- -20.557 (meters) (12/??/12) ADJUSTED
DO2389* NAD 83(2011) EPOCH - 2010.00
DO2389 ______________________________________________________________________
DO2389 GEOID HEIGHT - -32.375 (meters) GEOID12B
DO2389 NAD 83(2011) X - -2,704,026.134 (meters) COMP
DO2389 NAD 83(2011) Y - -4,253,051.245 (meters) COMP
DO2389 NAD 83(2011) Z - 3,895,879.305 (meters) COMP
DO2389
DO2389.Formal positional accuracy estimates are not available for this CORS
DO2389.because its coordinates were determined in part using modeled
DO2389.velocities. Approximate one-sigma accuracies for latitude, longitude,
DO2389.and ellipsoid height can be obtained from the short-term time series.
DO2389.Additional information regarding modeled velocities is available on
DO2389.the CORS Coordinates and Multi-Year CORS Solution FAQ web pages.
DO2389
DO2389.The coordinates were established by GPS observations
DO2389.and adjusted by the National Geodetic Survey in December 2012.
DO2389
DO2389.NAD 83(2011) refers to NAD 83 coordinates where the reference frame has
DO2389.been affixed to the stable North American Tectonic Plate.
DO2389
DO2389.The coordinates are valid at the epoch date displayed above
DO2389.which is a decimal equivalence of Year/Month/Day.
DO2389
DO2389.Significant digits in the geoid height do not necessarily reflect accuracy.
DO2389.GEOID12B height accuracy estimate available here.
DO2389
DO2389.The PID for the CORS L1 Phase Center is DO2390.
DO2389
DO2389.The XYZ, and position/ellipsoidal ht. are equivalent.
DO2389
DO2389.The ellipsoidal height was determined by GPS observations
DO2389.and is referenced to NAD 83.
DO2389
DO2389. The following values were computed from the NAD 83(2011) position.
DO2389
DO2389; North East Units Scale Factor Converg.
DO2389;SPC CA 3 - 656,143.264 1,828,709.348 MT 0.99993214 -1 11 32.6
DO2389;SPC CA 3 - 2,152,696.69 5,999,690.59 sFT 0.99993214 -1 11 32.6
DO2389;UTM 10 - 4,193,850.969 548,572.252 MT 0.99962906 +0 20 21.4
DO2389
DO2389! - Elev Factor x Scale Factor = Combined Factor
DO2389!SPC CA 3 - 1.00000323 x 0.99993214 = 0.99993537
DO2389!UTM 10 - 1.00000323 x 0.99962906 = 0.99963228
DO2389
DO2389_U.S. NATIONAL GRID SPATIAL ADDRESS: 10SEG4857293850(NAD 83)
DO2389
DO2389 SUPERSEDED SURVEY CONTROL
DO2389
DO2389.No superseded survey control is available for this station.
DO2389
DO2389_MARKER: STATION IS THE ANTENNA REFERENCE POINT OF THE GPS ANTENNA
DO2389
DO2389 STATION DESCRIPTION
DO2389
DO2389'DESCRIBED BY NATIONAL GEODETIC SURVEY 2012
DO2389'STATION IS A GPS CORS. LATEST INFORMATION INCLUDING POSITIONS AND
DO2389'VELOCITIES ARE AVAILABLE IN THE COORDINATE AND LOG FILES ACCESSIBLE
DO2389'BY ANONYMOUS FTP OR THE WORLDWIDE WEB.
DO2389' ftp://cors.ngs.noaa.gov/cors/README.txt
DO2389' ftp://cors.ngs.noaa.gov/cors/coord/coord_08
DO2389' ftp://cors.ngs.noaa.gov/cors/station_log
DO2389' http://geodesy.noaa.gov/CORS

*** retrieval complete.
Elapsed Time = 00:00:04

 

[Shawn Billings]

Looks like the same coordinates. The DPOS report stops at 3 decimal places while the NGS report carries the value out to five.

 

[Matthew D. Sibole]

CORS

37 53 27.13938(N) 122 26 51.31741(W) ADJUSTED
DO2389* NAD 83(2011) ELLIP HT- -20.557 (meters) (12/??/12) ADJUSTED

DPOS reported position of CORS station
N 37 53 27.139 W 122 26 51.317 47098.3

As Shawn stated the values are the same out to 5 decimal places.

 

[Tyler]

CORS

37 53 27.13938(N) 122 26 51.31741(W) ADJUSTED
DO2389* NAD 83(2011) ELLIP HT- -20.557 (meters) (12/??/12) ADJUSTED

DPOS reported position of CORS station
N 37 53 27.139 W 122 26 51.317 47098.3

As Shawn stated the values are the same out to 5 decimal places.

Okay so this leads me to my next question... Does the DPOS report use the published values from the NGS website and just truncate the lat and long to 3 decimal places? Or is the DPOS report showing measured base station locations from our GPS session which could theoretically be different from the NGS values? Simply stated, is the DPOS report showing record or measured values?

 

[Alexey Razumovsky]

A1. Yes.
A2. No.
A3. Record.