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Datum Evolution
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A Horizontal Datum is reference surface; a
datum realization is an estimate of the location of
a set of points on the surface
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A Vertical Datum is reference surface; a
datum realization is an estimate of the height of a set of points above the
surface
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To Recent Present: Horizontal and Vertical Survey Benchmarks
4
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Now: Accurate, anytime, anywhere positioning
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Establishing a Datum
Horizontal and vertical datums are realized through large survey networks
1. Define the shape of the Earth (the ellipsoid) 2. Accurately measure the position of a set of
control points on the ellipsoid 3. Calculate/adjust the reference surface and
network, document these so they may be use as a reference against which all other points will be measured
6
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U.S. Datums through Time
We’ve had several major datums, soon to have another (by 2022)
NAD27, with NGVD29, no geoid
NAD83(86) with NAVD88, geoid 03
NAD83(HARN/HPGN), with NAVD83, geoid 03
NAD83(1996), with NAVD88, geoid 03
NAD83(NSRS2007), with NAVD88, geoid 09
NAD83(2011), with NAVD88, geoid 12B
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Why change datums/Realizations
• NAD27 based on old observations and old system • NAD83(86) based on old observations and new system • NAD83(92) based on old and new observations, same system • NAD83(96) based on newer observations, same system • NAD83(NSRS2007) new observations, same system, removed
regional distortions and made consistent with CORS • NAD83(2011) based on new observations and same system.
Kept consistent with CORS • 2022 will be based on new observations and new (Earth
centered) system
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North American Datum of 1927, 26,000 measured points, Clarke 1866 spheroid, fixed starting point in Kansas -‐ not Earth centered
NAD27 survey network 10
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Fixed location in Kansas, Propagated Error from There
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Vertical Datum NGVD29 based on physical/optical leveling, tied to several MSL gauges
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By 1970s, more and better measurements, better ellipsoid (GRS80) hence new datum, NAD 83 (1986)
• Original realization completed in 1986 – Consisted (almost) entirely of classical (optical)
observations • “High Precision Geodetic Network” (HPGN) and
“High Accuracy Reference Network” (HARN) realizations, GPS was sooooo much better – Most done in 1990s, essentially state-‐by-‐state – GNSS based, with classical obs. incl. in adjustments – Did NOT use CORS as constraints
• National Re-‐Adjustment of 2007 – NAD 83(CORS96) and (NSRS2007) – Simultaneous nationwide adjustment (GNSS only)
• Latest realization: NAD 83(2011) epoch 2010.00 • NAVD88 realized in 1989,’93, ’03, 2009, 2012
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We calculate new positions from better, more, and more recent measurements, and things “shift”
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NETWORK TIME NETWORK LOCAL SHIFT ACCURACY ACCURACY
NAD 27 1927-1986 10 meter s (1:100,000) 10-200 m
NAD83(86) 1986-1990 1 meter (1:100,000) 0.3-1.0 m
NAD83(199x)* 1990-2007 0.1 meter (1:1 million) 0.05 m “HARN”, “FBN” (1:10 million)
NAD83(NSRS2007) 2007-2011 0.01 meter 0.01 meter 0.03 m
NAD83(NSRS2011) 2011 0.01 meter 0.01 meter 0.01 m
National Spatial Reference System (NSRS) Changes over time
ShiftLocalAccuracy
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Practical guide to datum transformations in ESRI’s World
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So I need to convert data between datums. What is the best choice?
NAD27 (or legacy datums) to NAD83(86) or to NAD83(HARN) -‐ Use NADCON, a grid-‐based model, or local shia from benchmark
NAD83(86) to NAD83(96/2007/2011/HARN) -‐ consider them equivalent (1 m error), or local shia from BM. ARC DT DOES NOTHING!
NAD83(96) to NAD83(2007/2011/HARN) -‐ consider them equivalent (10cm), or local shia from BM. ARC DT DOES NOTHING!
ITRF/WGS84 to NAD83(all flavors) -‐ find the right serial set of datum transformahons
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NAD27 to NAD83(86/HARN/96/2007/2011)
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https://desktop.arcgis.com/en/arcmap/latest/map/projections/pdf/geographic_transformations.pdf
ESRI Geographic Transformation Documentation
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NAD27 to NAD83 - Calculated shift at each benchmark, fit a minimum curvature model (local least squares)
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Grid-based Model also fit for Longitude NAD27 - NAD83
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Not All Grid-based Datum Transformations
Supported
e.g.,
NGZD1949 to NGZD2000
You’re outta luck, at least for now
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e.g., when NAD83(CORS96) data to a NAD83 (86) data frame
results in a warning
Converting Between Flavors of NAD83 (86/HARN/96/2007/2011)
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To Transform between NAD83 versions
Use either a local shift, or consider them equal
ArcGIS applies the transformation below, but with 0 values for T, all R values
+
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+
Effectively, ArcGIS assumes the NAD83 continental versions are all equal, that is
NAD83(86) equals NAD83(96) equals NAD83(2007) equals NAD83(2011)
So in Arc, this continental Datum Transformation makes no difference
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Within a single state, there is a HARN grid-based method, so you should use those when converting, but only within a single state
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To Transform between “Early” Earth-Centered Datums, or, Ignoring Continental Drift, Improved Measurements
Use this when transforming between fixed-date WGS84/ITRF and NAD83
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Using published ITRF00 to NAD83(96) R values ITRF_2000_To_NAD_1983_CORS96 ITRF_2000_To_NAD_1983_HARN ITRF_2000_To_NAD_1983_2011 WGS_1984_(ITRF00)_To_NAD_1983_HARN WGS_1984_(ITRF00)_To_NAD_1983_CORS96 WGS_1984_(ITRF00)_To_NAD_1983_2011 WGS_1984_(ITRF00)_To_NAD_1983
Using published ITRF08 to NAD83(2011) R values ITRF_2008_To_NAD_1983_2011 WGS84_(ITRF08)_To_NAD_1983_2011
geodesy.noaa.gov/CORS/coord_info/ coordtrans_no_support_tables.shtml www.ngs.noaa.gov/CORS/coords.shtml
ESRI Transformahons with Idenhcal Parameters
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https://desktop.arcgis.com/en/arcmap/latest/map/projections/pdf/geographic_transformations.pdf
+
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ITRF (WGS84) to NAD83ITRF(20xx) to NAD83(86/HARN/96/20072011) -‐ find the right serial chain of datum transformahons -‐
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ITRF (WGS84) to NAD83ITRF(20xx) to NAD83(86/HARN/96/20072011) -‐ find the right serial chain of datum transformahons -‐
If ITRF/WGS84 in 2008, use single published Datum Transfrom, ITRF_2008_To_NAD_1983_2011
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ITRF (WGS84) to NAD83ITRF(20xx) to NAD83(86/HARN/96/20072011) -‐ find the right serial chain of datum transformahons -‐
If ITRF/WGS84 in 200x, use serial published DT, e.g. ITRF_2000_to_ITRF_2005, ITRF_2000_to_ITRF_2008 ITRF_2008_To_NAD_1983_2011
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ITRF (WGS84) to NAD83ITRF(20xx) to NAD83(86/HARN/96/20072011) -‐ find the right serial chain of datum transformahons -‐
If ITRF/WGS84 in 200x, use serial published DT, e.g. ITRF_2000_to_ITRF_2005, ITRF_2000_to_ITRF_2005 ITRF_2008_To_NAD_1983_2011
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NAD83(86) to NAD83(CORS96)
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NAD83 to ITRF, and Future Datums, We Must Add Time Component to Coordinates for Centimeter-‐level Accuracies
• your positional metadata should include: – datum – epoch – source
• Needed to transform from current to new datum • maintaining your original survey data, including date collected (epoch) will provide more accurate results
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We use our usual equations, but the coefficients of the R matrix are time dependent
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Datum Transformation Parameters (R, T) fit for ITRF96 to NAD83(CORS96)
T, v values depend on time since 1997
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Four Major “Continental” Horizontal Datums
U.S. & Canada Europe Australia South America
Soler & Marshall
Use Time-adjusted datum transformations between them
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What About the Vertical Height?
e.g., I have data in a 2003 geoid, how do I combine it with 2012B geoid data?
I have heights collected with GPS in an ITRF2008/WGS84 ellipsoid, how do I combine these with NAVD88 data?
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Orthometric height measurements always at right angles to each equipotential surface
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Heights are relative to the geoid, XYZ/Lat-Lon-R measured and datum transformed on the ellipsoid…..between
different reference frames and times, both the geoid and the ellipsoid can differ, affecting height
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How Much Does Geoid Differ? Use VDatum
6.5 cm difference
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How Much Does Geoid Differ? Use VDatum
2009 - 1.7 cm 2003 - 7.4 cm 1999 - 6.0 cm 1996 - 6.5 cm
In St Paul, Difference between Geoid 12B and previous datums:
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2009 - 1.7 cm 2003 - 7.4 cm 1999 - 6.0 cm 1996 - 6.5 cm
2012B to:
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From VDatumHeight from NAD83(96) ellipsoid surface to geoid 12B is -27.54 m
Height from WGS84/ITRF08 ellipsoid surface to geoid12B is -28.48 m
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From VDatum, and NOAA Geoid12B Website, Height from NAD83 Ellipsoid and WGS84/ITRF08 Ellipsoid is
0.925 m (WGS84 is above)
So difference between ellipsoids is greater than between geoid versions
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2.2 meters
hNAD83(2011)
hITRF08
ITRF08
NAD83(2011)
surface
What to do? In U.S., biggest difference due to difference in ellipsoid - use Datum
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NAD 83(2011) to IGS08 at epoch 2022.0
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NAD 83(2011) to IGS08 at epoch 2022.0