Understanding North American Geocentric Reference Frames in Geographic Calculator

Dr. Qassim Abdullah, Woolpert, Inc.

The 2015 Blue Marble User Conference October 6th, 2015 Arlington, VA

Are You Confused? We Too

The American Reference Systems

•North American Datum of 1927 – ugly and Obsolete – Do not bother with it!!•North American Datum of 1983 – Newer more accurate

….. But Which NAD83?NAD83 (86)NAD83 (HARN)NAD83 (CORS93)

NAD83 (CORS96)

NAD83 (CORS94)

NAD83 (NSRS2007) NAD83 (NSRS2011)

The Story of NAD83• There have been seven realizations (versions) of NAD 83 in the United

States.• These changes have come as a result of maturing GPS and other

geodetic technologies that have allowed for more precise measurements of the earth’s center of mass, and other elements that define a reference system

• The original NAD 83 (referred to as NAD 83 (1986) by the National Geodetic Survey) was developed using the best technology available at the time. However, the geocentric, Cartesian axes, and scale were all somewhat imprecise. ( using non-GPS)

• Starting in 1989, each state - in conjunction with NGS and various other state institutions used GPS to establish regional reference frames that were to be consistent with NAD83.

• These networks of GPS control points were originally called High Precision Geodetic Networks (HPGN), and currently are referred to as High Accuracy Reference Networks (HARN).

The Story of NAD83• This collection of regional realizations referred to as NAD 83 (HPGN)

or NAD 83 (HARN) is the second realization of NAD 83. (GPS only controls)

• Beginning in late 1994, NGS introduced further realizations of NAD 83 when the agency organized a network of Continuously Operating Reference Stations (CORS).

• These realizations are the result of transforming then-current ITRFxx positional coordinates of a subset of CORS stations in the United States, and in its last realization, in the US and Canada, to NAD 83.

• These realizations are referred to as:– NAD 83 (CORS93)– NAD 83 (CORS94)– NAD 83 (CORS96)

• In 2007, NGS re-computed NAD83 (CORS96) at epoch 2002 and introduced new realization of NAD83 called NAD83 (NSRS2007)

• In 2011, NGS re-computed NAD83 (CORS96) at epoch 2010 and introduced new (latest) realization of NAD83 called NAD83 (NSRS2011)

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Orthometric Height (H)

• The distance along the plumb line from the geoid up to the point of interest

H

Earth’sSurface

The Geoid

NAVD 88 reference levelH (NAVD 88)

Errors in NAVD 88 : ~50 cm bias, ~ 100 cm CONUS tilt, 1-2 meters ave in Alaska NO tracking

World Tectonic Plates (15 plates)

Pacific, African, Amur, Antarctica, Arabia, Australia, Caribbean, Eurasia, India, North America, Nazca, Okhotsk, South America, Somalia, Yangtze

Non-Geocentric Datum: Why Should We Be Concerned? - The earth's surface is not fixed and rigid like an egg shell. - Earth consists of many sections, or plates, which move slowly over time in various directions and rates in a process called crustal motion.

Plate Angular Velocities Estimation

• Scientists have been studying this movement to know where land masses are with respect to one another and where they will be in the future.

Why Should We Be Concerned?

Accurately Measured Geoid Heights From Goco Satellite

Gravity Modeling

The birth of the International Terrestrial Reference System (ITRS) and ITRF

• Scientific community rigorously establishes International Standard for Terrestrial Reference System

• The International Earth Rotation and Reference Systems Service (IERS)** maintains the standard

• The Standard shaped the International Terrestrial Reference System (ITRS) which is realized (defined) through the International Terrestrial Reference

Frame (ITRF) through periodical computations of the positions and velocities of over 550 sites around the world for a given date.

** The International Earth Rotation and Reference Systems Service (IERS), formerly the International Earth Rotation Service, is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Parameter (EOP) and International Celestial Reference System (ICRS) groups.

WHAT IS ITRF Anyway?It is co-ordinates in an International Terrestrial Reference System (ITRS) which are computed at different epochs and the solutions are called InternationalTerrestrial Reference Frame (ITRF).

It is also:• An internationally accepted standard;

• The most accurate geocentric reference system currently available.

• Reliable – maintained by internationally well known scientific organizations;

• Internally consistent over time;

• Unambiguously specified datum definition (origin, scale, orientation and their respective time evolution)

• The longer the sites operate, the better the positions and velocities can be determined and the more accurate the ITRF will be.

• It is continuously evolving with our maturing understanding of Geodesy, gravity, and other advances in technologies and physical sciences

Identifying ITRF• ITRF defined (realized) to be

– Earth-Based System: Geocentric coordinate system based on GRS1980 ellipsoid – Constructed from a set of reference points on topographic surface of the Earth

– Accurate: Based on multiple data sources:• Very Long Baseline Interferometry (VLBI)• Satellite Laser Ranging (SLR)• GPS• DORIS (Doppler satellite tracking system developed for precise orbit determination and precise ground location)

– Continuously Evolving: Reference station coordinate solutions and velocities define frame at specific time

– ISO compliant: Solutions based on consistent set of conventions, constants and models

Benefits of an ITRF based geocentric datum• A single standard for collecting, storing and using geographic or survey related data and

maps;

• Provide direct compatibility with GNSS measurements and mapping or geographic information system (GIS) which are also normally based on an ITRF based geodetic datum;

• Minimize the need for casual users to understand datum transformations; • Allow more efficient use of an organizations’ spatial data resources by reducing need for duplication and unnecessary translations; • Help promote wider use of spatial data through one user friendly data environment; • Reduce the risk of confusion as GNSS, GIS and navigation systems become more widely

used and integrated into business and recreational activities.

The Story of WGS84• In the early 1980s the need for a new world geodetic system was generally recognized by the

geodetic community and the Department of Defense (DoD) as the then-current WGS 72 system is no longer serves the needs;

• The DoD intended for WGS84 to be used as “the single standard of reference within the Department of Defense.”

• The WGS 84 originally used the GRS 80 reference ellipsoid (refined later) and was designed to be close or equal to NAD83/86

• The latest major revision of WGS 84 included an Earth Gravitational Model first published in 1996, called EGM96, with revisions as recent as 2004.

• EGM96 model has the same reference ellipsoid as WGS 84, but has a higher-fidelity geoid (roughly 100 km resolution versus 200 km for the original WGS 84).

• The latest Earth Gravitational Model EGM2008 has been publicly released by the National Geospatial-Intelligence Agency (NGA) uses a 5 x 5-Minute Geoid undulation Grid (about 9 km grid). Accuracy world wide 3-5 cm / km (for baseline length of <30 km)

Adopted from NGA presentation to the International Committee on GNSS - Working Group DSaint Petersburg, Russia 16 September 2009

The Story of WGS84• In the early 1980s the need for a new world geodetic system was generally recognized by the geodetic community and the Department of Defense (DoD) as the then-current WGS 72 system is no longer serves the needs;

• The DoD intended for WGS84 to be used as “the single standard of reference within the Department of Defense.”

• The WGS 84 originally used the GRS 80 reference ellipsoid (refined later) and was designed to be close or equal to NAD83/86

• The latest major revision of WGS 84 included an Earth Gravitational Model first published in 1996, called EGM96, with revisions as recent as 2004.

• EGM96 model has the same reference ellipsoid as WGS 84, but has a higher-fidelity geoid (roughly 100 km resolution versus 200 km for the original WGS 84).

• The latest Earth Gravitational Model EGM2008 has been publicly released by the National Geospatial-Intelligence Agency (NGA) uses a 5 x 5-Minute Geoid undulation Grid (about 9 km grid). Accuracy world wide 3-5 cm / km (for baseline length of <30 km) Adopted from NGA presentation to the International Committee on GNSS - Working Group D

Saint Petersburg, Russia 16 September 2009

Adopted from NGA presentation to the International Committee on GNSS - Working Group DSaint Petersburg, Russia 16 September 2009

What Happened to WGS84?• Since the DoD decided to align WGS84 with ITRF in 1994, for

all practical purposes, an ITRF based geodetic datum, and WGS84 are the same. The difference is of the order of centimeters.

• The WGS 84 (G1150) is equivalent to ITRF2000

• The latest adjustment is referenced as WGS 84 (G1674) is equivalent to ITRF2008 (agrees to the centimeter level)

• The origin of the WGS84 framework is also the earth’s center of mass.

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Adopted from NGA presentation to the International Committee on GNSS - Working Group DSaint Petersburg, Russia 16 September 2009

What all his means to my work and how?• Answer: A lot!!• How?

– GCP coordinates in NAD83(86) ≈ WGS84(original) = WGS84(transit) differs from WGS84 (1150) coordinates by about 1.6 m (1+ m in horizontal and 1+ m vertical);

– GCP coordinates in WGS84(original) differs from WGS84 (1150) by 1.627 m (1+ m in horizontal and 1+ m vertical);

– NAD83(NSRS2007) = NAD83(CORS96)@2002 ≈ NAD83(HARN)) differs from WGS84 (G1150) by 1.628 m

– NAVD88 has a bias of 50 cm, therefore, it is not true orthometric height. This means transforming NAVD88 to ellipsoidal height based on GPS is wrong by a bias of 50 cm!!• Best strategy is to use EGM08 for orthometric heights conversion for new projects

when possible (unless client wants NAVD88)

– Use ITRF in production if you are using GPS-based controls and ABGPS to the end when possible and then export your data in the client specified datum!!

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NGS Plans on Replacing NAD83

New Geometric Datum by 2022

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PID AI0952 Anchorage, AKITRF00/WGS84(G1150).... 61 11 14.38134 N 149 53 41.90314 WNAD 83.............................. 61 11 14.37495 N 149 53 41.81980 WDifference......................... 1.261 meters

PID DF9223 Los Angeles, CAITRF00/WGS84(G1150).... 34 36 12.66234 N 118 05 02.00539 WNAD 83.............................. 34 36 12.64749 N 118 05 01.96477 WDifference......................... 1.132 meters

PID DF9219 Denver, COITRF00/WGS84(G1150).... 40 11 14.29552 N 105 07 37.99705 WNAD 83.............................. 40 11 14.27224 N 105 07 37.96036 WDifference......................... 1.126 meters

PID DF5765 Chicago, ILITRF00/WGS84(G1150).... 41 43 47.44273 N 087 32 18.37413 W NAD 83.............................. 41 43 47.41453 N 087 32 18.35359 WDifference......................... 0.991 meters

PID DF4064 New York, NYITRF00/WGS84(G1150).... 40 47 03.58007 N 073 05 49.78556 WNAD 83.............................. 40 47 03.55019 N 073 05 49.78137 WDifference......................... 0.927 meters

PID DF9225 Miami, FLITRF00/WGS84(G1150).... 25 49 28.60270 N 080 19 09.07673 WNAD 83.............................. 25 49 28.58580 N 080 19 09.06695 WDifference......................... 0.587 meters

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What Should I Use Now?

Best Conversion Tools

• HTDP for NAD83(NSRS2007) to different WGS84 and ITRF

• Blue Marble (with HTDP insert) for almost all conversions

• ESRI ArcGIS for limited conversions

• CORPSCON for limited conversions

• If non of those does what you want, please call Blue Marble!

Thank You!

Qassim.abdullah@woolpert.commapping_matters@asprs.org