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Tie to NGS mon
Posted by jmh4825 on February 6, 2017 at 5:56 pmWhile doing boundary surveys, anytime I have a NGS mon. within 2,000′ of a job we are surveying I normally take a 3 min VRS shot on it to be able to show a tie on the plat. I also have pairs of control points set onsite within our closed traverse. Shot a mon last week to find the coordinates published differ from what I observed in the field by 0.18′. I return to the monument for another shot and get results 0.01′ different than my original shot. The data sheet says it was GPS and NAD83(2011). I am fairly new to GPS so any insight would help me understand why I’m “off” that much. What are your normal procedures for this just to show a tie? Pretty good sky visible and rods were just checked a week ago for bubble plumb.
john-hamilton replied 7 years, 8 months ago 6 Members · 7 Replies -
7 Replies
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Could simply be the difference between how your VRS coordinates were/are established and how the published NAD83(2011) 2010.00 position on the NGS datasheet was established (other than by GPS). I would suggest you contact your State Geodetic Coordinator, Gary Thompson, who likely could provide you answers given enough information.
Gary W. Thompson, NCDPS
Chief, NC Geodetic Survey
Claude T. Bowers Building
NC National Guard Complex
4105 Reedy Creek Road
Raleigh, NC 27607
Telephone: (919) 948-7844
Fax: (919) 733-4407
E-mail -
.18′ is really good if it’s not a GPS point, the datasheet will explain how the coordinates are generated. Most are only calculated from old NAD27 data, now if it’s a HARN point or it’s been located with GPS and updated, then the .18′ isn’t very good at all.
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Are you using a tripod setup or a pole and bipod? Are they in adjustment? I’ve seen poles with bad level bubbles that tilted as much as 0.3′ off plumb. Same with tribrachs with bad OP’s.
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Many possibilities. Here is another: you measured for 3 min with constellation configured as ‘x’, come back later and meas another 3 min with constellation configured differently, say ‘y’, and it gives you different results. a change in constellation could create a bias.
which measurement is ‘right’? that depends. measure some more times, grist for the mill, and you will have a more useful sampling
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Data sheet said it had been located with GPS. I’m using a pole and bipod that was just checked about a week ago. 2 different shots on 2 different days resulted in pretty much the same thing. Wouldn’t think I would ever need to do a rapid-static on a published mon. but have never seen this much difference before. Just wondering what else of mine could be off.
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Here is an NSRS monument that we occupied last week for OPUS-DB (project requirement). Note the network accuracies published. We hit the published values with VRS much closer than that (under a decimeter). This was GPS’d in 1984 (yes, 1984). I don’t believe there is any more recent GPS data on it in the NSRS.
KV5914 ***********************************************************************
KV5914 DESIGNATION – GULPH
KV5914 PID – KV5914
KV5914 STATE/COUNTY- PA/MONTGOMERY
KV5914 COUNTRY – US
KV5914 USGS QUAD – NORRISTOWN (1992)
KV5914
KV5914 *CURRENT SURVEY CONTROL
KV5914 ______________________________________________________________________
KV5914* NAD 83(2011) POSITION- 40 04 45.09545(N) 075 21 04.79918(W) NO CHECK
KV5914* NAD 83(2011) ELLIP HT- 39.586 (meters) (06/27/12) NO CHECK
KV5914* NAD 83(2011) EPOCH – 2010.00
KV5914* NAVD 88 ORTHO HEIGHT – 73.2 (meters) 240. (feet) VERTCON
KV5914 ______________________________________________________________________
KV5914 GEOID HEIGHT – -33.500 (meters) GEOID12B
KV5914 NAD 83(2011) X – 1,235,899.971 (meters) COMP
KV5914 NAD 83(2011) Y – -4,728,225.081 (meters) COMP
KV5914 NAD 83(2011) Z – 4,084,743.168 (meters) COMP
KV5914 LAPLACE CORR – 2.13 (seconds) DEFLEC12B
KV5914
KV5914 Network accuracy estimates per FGDC Geospatial Positioning Accuracy
KV5914 Standards:
KV5914 FGDC (95% conf, cm) Standard deviation (cm) CorrNE
KV5914 Horiz Ellip SD_N SD_E SD_h (unitless)
KV5914 ——————————————————————-
KV5914 NETWORK 111.95 24.03 45.20 35.83 12.26 0.94651141
KV5914 ——————————————————————-Another monument we hit very close horizontally (1.6 cm) but 60 cm off vertically:
AI8155 ***********************************************************************
AI8155 DESIGNATION – GPS 1
AI8155 PID – AI8155
AI8155 STATE/COUNTY- PA/MONTGOMERY
AI8155 COUNTRY – US
AI8155 USGS QUAD – VALLEY FORGE (1992)
AI8155
AI8155 *CURRENT SURVEY CONTROL
AI8155 ______________________________________________________________________
AI8155* NAD 83(2011) POSITION- 40 05 27.17691(N) 075 24 40.75949(W) ADJUSTED
AI8155* NAD 83(2011) ELLIP HT- 21.785 (meters) (06/27/12) ADJUSTED
AI8155* NAD 83(2011) EPOCH – 2010.00
AI8155* NAVD 88 ORTHO HEIGHT – 55.4 (meters) 182. (feet) GPS OBS
AI8155 ______________________________________________________________________
AI8155 NAVD 88 orthometric height was determined with an earlier geoid model
AI8155 GEOID HEIGHT – -33.632 (meters) GEOID12B
AI8155 NAD 83(2011) X – 1,230,734.875 (meters) COMP
AI8155 NAD 83(2011) Y – -4,728,694.473 (meters) COMP
AI8155 NAD 83(2011) Z – 4,085,724.748 (meters) COMP
AI8155 LAPLACE CORR – 2.46 (seconds) DEFLEC12B
AI8155
AI8155 Network accuracy estimates per FGDC Geospatial Positioning Accuracy
AI8155 Standards:
AI8155 FGDC (95% conf, cm) Standard deviation (cm) CorrNE
AI8155 Horiz Ellip SD_N SD_E SD_h (unitless)
AI8155 ——————————————————————-
AI8155 NETWORK 3.34 8.66 1.50 1.20 4.42 -0.06799731
AI8155 ——————————————————————-I am 99% positive this monument has not moved or been disturbed, just a blown HI when it was set in 1998.
Point is, you never know….But it is always a good idea to check the network accuracies, they can give a good idea if the point was tied tightly to NAD83 (2011) or rather loosely. Not all GPS stations in the NSRS are created equal.
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