Datum Transformations: Nightmare on GIS StreetEsri Environment: Time-Dependent Geodetic Datum • Not currently defined in Esri technology -Perhaps a “time-enabled” GEOGCS• Pervasive
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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
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.
• 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.
• 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)
• 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
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?
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