Datum Transformations: Nightmare on GIS Street

Speaker: Eric Gakstatter Contributing Editor – GPS World

Editor - Geospatial Solutions

Presented at: Minnesota Society of Professional Surveyors Bloomington, MN February 20, 2014

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Agenda

What’s the problem and why should I care? - The challenge of horizontal datums - The challenge of integrating high-accuracy

data into the GIS workflow. Vendor perspetive. - The challenge of integrating high-accuracy

data into the GIS workflow. User perspective. - Discussion.

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Challenges

- The cost of acquiring high-precision geospatial data is declining (eg. GNSS, airborne/terrestrial Lidar, UAVs, etc.)

- Legacy (and some current) geospatial “base” data is likely less accurate than what you are collecting today.

- How do you make good quality decisions when the data is so disparate?

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- Orthophotos: A picture is worth a thousand words…not.

- Understand the source of the data; how is was collected, which datum/epoch was is referenced to.

- The Truth: Checking against survey marks. - …but all survey marks aren’t created equal. Just

because it’s a disk on the ground, doesn’t make it a survey mark you should use.

How do I know which data is “good”?

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Survey Marks

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Audience Poll

Poll #1 Question How much of a problem is matching data with disparate datums in your organization? 1. It’s a big problem 2. It’s a problem. 3. It’s not a problem 4. I don’t know if it’s a problem.

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Datums

• Geometric (“horizontal”) datums – a.k.a. “geographic coordinate systems” – Basis for determining positions on the Earth – Modern ones are 4-D

• Latitude, longitude, ellipsoid height (or Earth-Centered, Earth-Fixed XYZ) • Time variation is 4th dimension

– Ellipsoid (“spheroid”) by itself is NOT a datum • Same ellipsoid for NAD 83 and WGS 84, but they differ by ~2 m

• Includes “local” and “global” datums – “Local” (regional) datums are for a specific part of the Earth

• e.g., North American Datum of 1983 (NAD 83) – “Global” datums are for the entire Earth

• e.g., World Geodetic System of 1984 (WGS 84)

• Vertical (geopotential) datums another topic for another time…

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Datums

• Datums must be “realized” – Connected to the Earth by observations (e.g., GNSS) – New realizations improve accuracy of coordinates – A single datum can have multiple realizations

• e.g., both NAD 83 and WGS 84 datums have 4-5 realizations

• Interrelationships given by “datum transformations” – Mathematical methods for converting between datums – Needed if combining data based on different datums – There are many different kinds and they can vary greatly

• From simple geometric shift to modeling tectonic motion

• Modern datum definitions are accurate but complex – Will focus on two here: NAD 83 and WGS 84.

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Datums

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• Original realization completed in 1986 – Almost entirely classical (optical) observations

• “High Accuracy Reference Network” (HARN) realizations (1990s) – Done essentially state-by-state – Based on GNSS but classical stations included

• National Re-Adjustment of 2007 – NAD 83(NSRS2007/CORS96) epoch 2002.00 – Nationwide adjustment (GNSS only)

• NAD 83 (2011/PA11/MA11) epoch 2010.00 – Also nationwide GNSS-only adjustment – Realization SAME for CORS and passive marks – This is NOT a new datum! (still NAD 83)

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Datums

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Brief History of WGS-84

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• Original realization completed in 1987 – Intent was to align with NAD 83(1986) – “Same” as NAD 83 (to within ±1-2 m)

• WGS 84 (G730) — adopted Jan 2, 1994 – “G730” is GPS week 730, began Jan 2, 1994 – Aligned with ITRF91

• WGS 84 (G873) — adopted Sep 29, 1996 – Aligned with ITRF94

• WGS 84 (G1150) — adopted Jan 20, 2002 – Aligned with ITRF2000/IGS00 (at epoch 2001.00)

• WGS 84 (G1674) — adopted Feb 5, 2012 – Aligned with ITRF2008/IGS08 (at epoch 2005.00) – Coordinates updated annually for tectonic motion – Note that current NAD 83 is epoch 2010.00

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Datums

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Datums

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Datums

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Datums

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Does this stuff really matter? • Significant for accuracies better than ~1-2 m – Can be problem for combining accurate datasets – Requires understanding of modern datums – Be careful when using “WGS 84”

• Which realization? At what epoch? For what level of accuracy? • Things are moving, and it can make a difference – e.g., Hawaii is moving…

• 7.2 cm/yr WNW in global reference frame (WGS 84/ITRF2008) • 8.0 cm/yr NW with respect to North America plate • But only 0.4 cm/yr E with respect to Pacific plate

• Modern GNSS becoming more “precise” – Autonomous positions soon better than 1-2 m – But accuracy another issue… with respect to what?

Predicted horizontal shifts for

new US geometric datum (2022)

Geography without geodesy

is a felony!

Good coordination

begins with good coordinates!

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Audience Poll

In managing its geospatial data, does your organization account for the epoch date of coordinates? 1. Yes. 2. No. 3. We are just starting to think about this. 4. I don’t know.

Esri Environment: Time-Dependent Geodetic Datum

• Not currently defined in Esri technology - Perhaps a “time-enabled” GEOGCS

• Pervasive among national geodetic datums • Maintain stability of reference frame axes by modeling

their movement over time • Physical locations move measurably with the crust

- Tectonic plate and local crustal motion

• So coordinates change measurably in two ways – - Due to variations in the reference frame - Due to crustal motion

• These coordinate changes cannot be ignored in GIS

Reference Frame Realizations

Coordinates Move • Between different reference frames

- 14 parameter Helmert transform

• Within the same reference frame

- Global Tectonic Plate Motion (Global Velocities) - Local continuous crustal motion (Local Velocities) - Abrupt Events – earthquakes

Some Challenges • Multiple transformation parameters between same or

similar reference frames - IGSxx-to-ITRFyy vs. ITRFxx-to-ITRFyy – Which one?

• Velocity model performance - Tectonic plate selection for each point - In GIS we deal with thousands, millions of points - How to handle discontinuous motion at plate boundaries

• Within frame movement referenced to source or target - Added burden of transforming velocity vectors

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Audience Questions?

Eric Gakstatter Contact Information: egakstatter@gpsworld.com

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