datum evolution - gis courses · 2016. 11. 1. · and datum transformed on the...

Datum Evolution

A Horizontal Datum is reference surface; a

datum realization is an estimate of the location of

a set of points on the surface

A Vertical Datum is reference surface; a

datum realization is an estimate of the height of a set of points above the

surface

To Recent Present: Horizontal and Vertical Survey Benchmarks

4

Now: Accurate, anytime, anywhere positioning

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

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

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

North American Datum of 1927, 26,000 measured points, Clarke 1866 spheroid, fixed starting point in Kansas -‐ not Earth centered

NAD27 survey network 10

Fixed location in Kansas, Propagated Error from There

Vertical Datum NGVD29 based on physical/optical leveling, tied to several MSL gauges

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

We calculate new positions from better, more, and more recent measurements, and things “shift”

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

Practical guide to datum transformations in ESRI’s World

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

NAD27 to NAD83(86/HARN/96/2007/2011)

https://desktop.arcgis.com/en/arcmap/latest/map/projections/pdf/geographic_transformations.pdf

ESRI Geographic Transformation Documentation


NAD27 to NAD83 - Calculated shift at each benchmark, fit a minimum curvature model (local least squares)

Grid-based Model also fit for Longitude NAD27 - NAD83

Not All Grid-based Datum Transformations

Supported

e.g.,

NGZD1949 to NGZD2000

You’re outta luck, at least for now

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)

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

+

+

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

Within a single state, there is a HARN grid-based method, so you should use those when converting, but only within a single state

To Transform between “Early” Earth-Centered Datums, or, Ignoring Continental Drift, Improved Measurements

Use this when transforming between fixed-date WGS84/ITRF and NAD83

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

http://www.ngs.noaa.gov/CORS/coords.shtml


+


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


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


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

NAD83(86) to NAD83(CORS96)

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

We use our usual equations, but the coefficients of the R matrix are time dependent

Datum Transformation Parameters (R, T) fit for ITRF96 to NAD83(CORS96)

T, v values depend on time since 1997

Four Major “Continental” Horizontal Datums

U.S. & Canada Europe Australia South America

Soler & Marshall

Use Time-adjusted datum transformations between them

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?

Orthometric height measurements always at right angles to each equipotential surface

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

How Much Does Geoid Differ? Use VDatum

6.5 cm difference

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:

2009 - 1.7 cm 2003 - 7.4 cm 1999 - 6.0 cm 1996 - 6.5 cm

2012B to:

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

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

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

NAD 83(2011) to IGS08 at epoch 2022.0

datum evolution - gis courses · 2016. 11. 1. · and datum transformed on the...

Documents