NO A A Technical M em orandum NO S NG S -58 G U ID ELIN ES FO R ES TA B LISH IN G G PS -D ER IVE D E LLIPS O ID H EIG H TS (STANDARDS:2 CM AND 5 CM ) VER SIO N 4.3 D avid B .Zilkoski Joseph D .D 'O nofrio Stephen J.Frakes Silver S pring,M D N ovem ber 1997 U.S.DEPARTMENT OF NationalOceanic and N ationalO cean N ationalG eodetic COMMERCE Atm ospheric A dm inistration S ervice Survey
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N O A A T e c h n ic a l M e m o r a n d u m N O S N G S - 5 8
G U ID E L IN E S F O R E S T A B L IS H IN G G P S - D E R IV E D E L L IP S O ID H E IG H T S( S T A N D A R D S : 2 C M A N D 5 C M )V E R S IO N 4 .3
D a v id B . Z i lk o s k iJ o s e p h D . D 'O n o f r ioS t e p h e n J . F r a k e s
S i lv e r S p r in g , M D
N o v e m b e r 1 9 9 7
U . S . D E P A R T M E N T O F N a t i o n a l O c e a n i c a n d N a t i o n a l O c e a n N a t i o n a l G e o d e t i cC O M M E R C E A t m o s p h e r i c A d m i n i s t r a t i o n S e r v ic e S u r v e y
BEST SCENARIO = 7 OR MORE GPSGLN “K” SATS WILL HAVE A CDMA (L3) FORMAT SIGNAL
SO – WHAT CAN I EXPECT FROM A RTN?
MOST RTN PRODUCE “GOOD” HORIZONTAL VALUES. OUR HORIZONTAL SYSTEM IS BASED ON ACTIVE REFERENCE STATIONS (NGS CORS), AS ARE THE RTN STATIONS.
BECAUSE ORTHOMETRIC HEIGHTS (‘ELEVATIONS’) ARE BASED ON PASSIVE MONUMENTS, THE RTN USER SHOULD, FOR THE MOST PART, CONSTRAIN THE PASSIVE MARK VALUES IN A LOCALIZATION.
CHOOSE THE RTN WITH A BUSINESS MODEL THAT BEST FITS YOUR NEEDS.
THE TECHNOLOGY SWEET SPOT• SBAS: 2 M H, 6 M V, 0.3 M SMOOTHED H, CHEAP• COMMERCIAL DGPS: FEW DM, $$• USCG BEACON: METER+, CHEAP• DIFFERENTIAL LEVELING: 1-3 CM, LABOR/TIME INTENSIVE, $$$• GEODETIC LEVELING: mm, LABOR/TIME INTENSIVE, $$$$$• USER BASE RTK: 1-3 CM H, 2-4 CM V, REQUIRES INITIAL INVESTMENT• RTN: 3-4 CM H, 5-7 CM V, REQUIRES INITIAL INVESTMENT(BUT ½ OF
RTK)• AERIAL MAPPING: .10 M H, .20 M V, $$$• LIDAR: 0.10 – 0.3 M V• SATELLITE IMAGERY: 0.5 METER H RESOLUTION, 3 M LOCATION, $$$• LOW ALTITUDE AERIAL IMAGERY: 2-4 CM H, 3-5 CM V, $$$• TERRESTRIAL LASER SCANNING/MMS: PROJECT SITES ONLY, 0.015 M
H, 0.02 M V, REQUIRES INITIAL INVESTMENT
PRECISION VS. ACCURACY
• “PRECISION” IS A COMPUTED STATISTICAL QUANTITY TO THE SOURCE OF THE MEASUREMENT - ALIGNMENT TO THE RTN OR PASSIVE MARK SHOWS PRECISION OF THE OBSERVATION (PER THE DATA COLLECTOR).
• “ACCURACY” IS A COMPUTED STATISTICAL QUANTITY TO THE REALIZATION OF THE DATUM - ALIGNMENT OF THE RTN OR PASSIVE MARK TO THE NSRS SHOWS ACCURACY (PER ESTABLISHED METHODOLGY)
•
Typical RTN precisions at the 95% confidence level are: horizontal 2-3 cm , vertical (ellipsoid) 3-5 cm, orthometric heights 5-7 cm (typical-using the NGS hybrid geoid model). Exceptional RTN derived precisions are at the current limit of the RT technology: horizontal : ≤ 1 cm, vertical (ellipsoid) ≤ 1 cm, possible orthometric height ≤ 2 cm.
Accuracy is a measure of how the positions are aligned to “truth”. NGS wishes to encourage all RTN to provide users with alignment to the NSRS as the representation of truth.
Since RTN positioning is a differential solution from a base station to a point of interest, the results are displayed in the data collector as measures of the precision, or repeatability, of the solution. On the other hand, the alignment of the base station to the user-selected datum (as part of the NSRS or otherwise) can be considered the level of accuracy.
PRECISION/ACCURACY
PASSIVE/ACTIVE• NAD 83 REALIZED THROUGH NATIONAL CORS
• NAVD 88 REALIZED FROM PASSIVE MARKS
• NGSIDB HAS 1,000,000 PASSIVE MARKS
• PASSIVE MARKS IN STATES HAVE MANY CAMPAIGNS OVER MANY YEARS WITH MANY ACCURACIES IN MANY SEPARATE ADJUSTMENTS. RTN MAY NOT AGREE WITH THESE REQUIRING CONSTRAINTS TO THE MONUMENTS FOR PROJECT WORK.
• PASSIVE MARKS COORDINATES ARE A SNAPSHOT IN TIME AND CAN BE RELIED ON TO BE ACCURATE ONLY AT THE RECORDED OBSERVATION TIME.
• 2020+ = NEW GEOMETRIC DATUM / 1 CM GRAVIMETRIC GEOID, VELOCITIES ON A GEOCENTRIC DATUM
• THE METHOD OF CHOICE TO ACCESS THE NSRS IS ACTIVE MONUMENTATION (CORS, OPUS, RTN)
THE PROBLEM WITH PASSIVE MARKS….
Normal to ellipsoid
THREE SURFACES
NEW NATIONAL VERTICAL DATUM
• A PURELY GRAVIMETRIC SURFACE• BASED ON A HIGH RESOLUTION, 1 CM GEOID FROM
GRAV-D PROGRAM
• ACCURATE TO 2 CM (ALLOWING FOR GNSS ERROR)
• ACCESSABILITY: BROUGHT TO A PROJECT SITE VIA ACTIVE REFERENCE STATIONS (NATIONAL CORS), DENSIFIED TO PROJECT ACCURACY NEEDS. (ALTERNATIVE: USE BMs PREVIOUSLY TIED TO THE DATUM-CAVEAT EMPTOR)
• ETD: 2022?
RTN GUIDELINES FOR GNSS POSITIONING–WILL NOT SPECIFY OR