1. Datums and Transformations inside GPS Software
WGS84
Joel Cusick
GIS Specialist
Anchorage Alaska
I’m in NAD83
There is much confusion on what datum is assumed and what datum transformations are used inside GPS processing software. This presentation addresses the confusion within the 3 main GPS enabled workflows in the National Park Service.
The reliable, trusted Garmin PND, usually the first location aware device on the scene.
The mapping grade submeter GPS suite. Represented by the Trimble mapping devices providing not only submeter solutions, but tieing mapping and surveying projects together in a common National Spatial Reference frame.
Lastly, the ArcPad software workflows, often found in a plane, or on an ATV providing instant GIS layer creation whereever you are.
We will explore shifters between WGS84 and NAD83 reference datums popular in the software today, and will hopefully guide you thru an often
Confusing, array of translators taking you back and forth. As this title slide shows, we hope you won’t get run over by the GPS crew this summer who
THINKS they are in one reference frame, while you as a GIS manager ASSUMES they are in another reference frame.
This confusion causes pain and suffering when metadata is not passed between the field and office.
Let’s begin!
Presented to Alaska Survey and Mapping Conference
February 2013

2. Audience Poll Who Uses? Garmin devices? – Answer about 15
Mapping Grade GPS (submeter)? – Answer about 15
ArcPad? Answer about 15
Real-time Coast Guard Beacon Signals (NDGPS)?
Answer – no one.

3. Common Datums in Alaska
NAD27 NAD83* WGS84 ITRF00/IGS08
- Pre-GPS; Digitized Topos
- GPS; Mandated for US(&AK!)
- Used by GPS satellites
- Global reference/ CORS
Explain a datum
In the US, GIS personnel are most likely to encounter three horizontal reference map Datums NAD27
NAD83
WGS84
ITRF00 – Global geodetic system used mainly in scientific studies. *

4. Over Time Datums Grew Apart
1986
Today
NAD83 = WGS84
NAD 83 ≠ WGS84
Anchorage
1.3 Meters (4.2 feet)
WGS84 = ITRF00
= IGS08
This slide is designed to show why there is a horizontal difference between the latest realizations of WGS84/ITRF00 and NAD83.
When NAD83 was rolled out in 1986, geodesists used every available observation to set the datum starting point. This included GPS observations. The geocentric (center of earth) position was known to a meter or so.
Then thru time, as measurements improved, GPS observations improved, more was learned about the center of the earth or geocentric center of earth.
Today, due to refinements, we now have measured the center of the earth within a cm precision level. Coupled with better knowledge of the earth center and better observations, the latest NAD83 is not equal to WGS84. In fact for Manassas Virginia, these differ horizontally about 0.9 Meters or 2.9 feet.
Not recognizing these differences can confound storage of high quality GPS data that are submeter accurate.

5. Today, You Have to Go Back
NAD83
NAD83 (CORS96) Or
NAD83 (2011)
ITRF00 or 1GS08
Anchorage
1.3 Meters (4.2 feet)
Reverse the sequence…

6. Latest Datum Frame Adjustments
refinements
reference frames
Latest Datum Frame Adjustments
WGS84 (G1640)
WGS Original
WGS84 (G730) 1992
WGS84 (G873) 1997
WGS84 (G1150) 2002
NAD83 (2011)
NAD83 (1986) - Original
NAD83 (1992) HARN
NAD83 (1994) CORS
NAD83 (2007)
flavors
epochs
realization
adjustments
Just when you thought all you had to learn was a Datum name like WGS84…
It’s a bit more complicated. As a datum is more precicesly defined or datums become adjusted over time as we learn more about the center of the earths mass. Satellite, gravity measurements all enhance the level of precision that we can measure things. Datums therefore are adjusted or realized using the term epochs. These datum reference frames are noted by a year (1986) or (1992).
Whats happened is that the most current realization of NAD83 and WGS84 differ by up to a meter. Errors can be introduced into a map by assuming these are the same

7. The Most Important NAD83’s for An Alaskan GIS
NAD83 (CORS96) epoch
NAD83 (2011) epoch
Datum Name Tag
Epoch

8. Difference between WGS84 (G1150) and NAD83(CORS96) at 2003.0
Horizontal difference
Conversion:
From 1.15Meters (NW) – 1.3Meters (SouthEast
Used with permission by Author: Michael Dennis

9. “NAD83 To WGS84” SHIFT
Originally WGS84 and NAD83 were essentially the same.
Both datums continually refined through time - today they differ by 1-2 Meters
There are specific times (epochs) where the “NAD83 and WGS84/ITRF00/IGS08” pairs are borne, hence specific transformations built (time dependent)
Finding information on what Shifter to use is tricky….
See Dave Doyle’s Datum Course this Friday

10. GPS Receiver Accuracies
Do datum shifts really matter?
Next two slides: multi-year accuracy evaluation

11. Garmin Accuracy with WAAS
Mean 1.1 Meters
STD: 0.6 Meters
*Horizontal diff (m) measured between OPUS controls and students Garmin Map76CSx(int. antenna) set to active track (1 point / sec) . Stats using NEAR Tool projected in NAD83 (CORS96) UTM Zone 6 CS.

12. Mapping Grade GPS Accuracies*
Fairbanks
Feb 2010; March 2011
N=136 points (680 positions)
Mean 0.61 Meters; STD: 0.69 Meters
Copper Center
June 08’;May 09’;June 12’
N=95 points (465 positions)
Mean 0.57 Meters; STD: 0.85 Meters
Anchorage
June 08’;May 09’;June 12’
N=108 points (540 positions)
Mean 0.45 Meters; STD: 0.35 Meters
Skagway / Gustavus
May 10’; June 11’
N=32 points (160 positions)
Mean 0.41 Meters; STD: 0.55 Meters
*Horizontal diff (m) measured between OPUS controls and students antenna. Stats using NEAR Tool projected in NAD83 (CORS96) UTM Zone 6 & 8 CS

13. GPS Accuracy vs Datum Shifts
A meter + of shift is a big deal for both recreational and mapping grade GPS
At decimeter level accuracies 1 cm of shift is starting to become worrisome
You will need to shift, so knowledge is key

14. Datums Would Not Be So Bad if It wasn’t for…..
Maps and digital forms of maps….GIS!!!
All Feds are mandated to “sic transition to NAD83”
If we always kept our GPS data on the receiver…. *

15. Basic Premise with GPS Software
Most assume you are in “WGS84”
Most offer transformations to go back and forth (sign switching)
Many offer a “do nothing” shift. Parameters are zero! ESRI calls these “bookkeeping transforms”
Most software assumes when sending data to a GPS, a shift back to WGS84 occurs.
This is unlike the GIS paradigm, where WGS84 is just one of many datums to choose from

16. What is a geographic transformation?
Geographic Transformations –
Mathematical calculation that is used to convert coordinates referenced from one datum to coordinates referenced to another datum
NGS designs transformations
Designed for specific epochs
See Dave Doyle’s presentation on Friday!

17. Geographic Transformations: Choice to Do Nothing or Something
Depends on your realtime correction source, CORS reference frames and your GIS

18. The Problem with Shifters
Different software companies and organizations use different libraries
The verbose titles of the shifters can be different, yet rely on the same math….
For Example
Trimble’s 0,0,0 Shifter ESRI’s 0,0,0 Shifter
NAD83 (Alaska) NAD83_To_WGS84_1

19. Datum Shifting Garmin / DNRGPS Software
Trimble GPS Pathfinder Office Software
ArcPad GPS Correct/ GPS Analyst

20. Personal Navigation Device (PND)
Garmins are the most popular GPS in Alaska NPS (500+ units)
1-3 meter accuracy w/ WAAS differential
High support, reliable, rugged, cheap
Free GIS software (DNRGPS – formerly DNRGarmin)
Ideal for Navigating to sample sites, control monumentation

21. Garmin Datum Primer Garmins receivers and software assume WGS84=NAD83
You will shift a meter (1+) relative to properly shifted NAD83 if you write down a coordinate
If you write, you may lose!*
Go Digital and Always use DNRGPS for data transfer!
* Shifts of up to 9 meters possible when setting a Garmin to Alaska NAD27 – Never, never use NAD27.

22. Garmins: What Do you Do?
GPS
NAD83 To WGS84_5
GIS
DNRGPS

23. Garmins: Shift Using “ _5”
Set Projection
Parameters match ESRI’s NAD83 _ To_WGS84 _5
Officially: ITRF96 to NAD83 at *
*Paper: ,Table 1, page, 51, Last column

24. Garmins: What Do you Do?
Stay consistent and use “_5” for all Garmin data management
Tag Garmin data as Plain “NAD83” (PRJ file)
Higher quality shifters have little effect : 1cm or less (Don’t worry!)
1 cm difference
between methods

25. Garmin Datum Summary
Garmins don’t care about NAD83 (You will lose a meter + when writing down a “NAD83” coordinate from the screen)
Always use DNRGPS and set Projection
Tag Garmin data as “NAD83” and use metadata to store your workflow
Use of Garmin software like MapSource, Basecamp are not recommended for GIS purposes

26. Datum Shifting Garmin / DNRGPS Software
Trimble GPS Pathfinder Office Software
ArcPad GPS Correct/ GPS Analyst

27. Our CORS Network – Part of the National Spatial Reference Frame

28. Mapping Grade GPS Correction Sources
Post Process Differential Correction
AFTER the fieldwork is done, correct against a reliable Base Station – A CORS site
Real-time Differential
Coast Guard NDGPS Beacon sites
WAAS
VRS????

29. How Does Corrections Affect Your Data?
Corrections are in the terms of the correction source reference frame (Datum)
The key is knowing the datum referenced for the correction source
WHY? When you differentially correct, you MOVE your data to the corrected data’s datum
You might have to apply a geographic transformation if your differential correction source datum is different than your database’s datum

30. Which Datum Are You In? WGS84 = IGS08 NAD83 (2011) epoch 2010.0
- CORS Post Processed
- Autonomous GPS
- WAAS
- CORS Post Processed
- NDGPS (Beacon)
- OMNISTAR (Satellite)

31. Datum of Base Station Becomes Datum of Corrected File
OLD Coordinates
Your GPS data MOVES It can be in ITRF00 (1997.0) or NAD83(CORS96) (2003.0)

32. Datum of Base Station Becomes Datum of Corrected File
NEW Coordinates
Your GPS data MOVES It can be in IGS08 (2005.0) or NAD83(2011) (2010.0)

33. PostProcess Differential Correction
When selecting “Use reference position from base files” the coordinate system depends on what the owner of the base station stored in RINEX header
For CORS the corrected data file references NAD83(2011) epoch ARP
Check the New Coordinates page for the CORS station to verify the position
Recommended
Approach for CORS if you want your
solution in the latest NAD83(2011)

34. PostProcess Differential Correction
When selecting “Use reference position from base provider” the WGS84 (ITRF00- Derived from IGS08) is used
Not Recommended
At this Time Since the Older
ITRF00 (1997.0) reference is used

35. Your GNSS data MOVES when you differentially correct
What does this mean?
Let me state this again –
Your GNSS data MOVES when you differentially correct
SHIFT DEMO – differential correction
Trimble GPS Pathfinder Office
ESRI ArcGIS 10.1

36. SHIFT DEMO
ESRI ArcMap 10.1
*OPUS Control compliments of Eric Oppegard (JOA Associates)

37. ArcMap DEMO– Uncorrected data
About 1.5 m from the 83 (CORS96) EPOCH: 2003 monument
WAAS Disabled

38. Diff. Correcting w/ CORS ITRF00 (1997)

39. Results – 12 cm from 83 (CORS96)
~1.38 METER
DIFFERENCE

40. Results - Zoom In

41. Diff. Correcting w/ CORS NAD83 (CORS96)*
Evaluate another pathway
*Must create own Base Station with Older NAD83 (CORS96) Coordinates

42. Results – 11 cm from 83(CORS96)
1 cm difference
between methods

43. Diff. Correcting w/ Latest NAD83 (2011)*
Use Base File option populated with the latest NAD83 (2011)
* NAD83 (2011) Coordinates are embedded in the RINEX header of base file

44. Results – 14 cm inches from 83(CORS96)
3 cm difference
between methods

45. What 8 years can do to your coordinates - ~ 13 cm
*HDTP to transform from ITRF00(1997) to IGS08(2005)

46. Trimble: What Do you Do? CORS NEW NAD83 (2011) GPS NAD83 (2011)
Use Base File
NAD83 (Alaska)
Export using
GIS Defined as NAD83 (2011)
CORS
NEW
NAD83 (2011)

47. GIS Defined as NAD83 (CORS96)
Trimble: What Do you Do?
GPS
NAD83 (CORS96) 2003
Seed
NAD83 (Alaska)
Export using
GIS Defined as NAD83 (CORS96)
CORS
OLD
NAD83 (CORS96)

48. Trimble: What Do you Do? NDGPS NAD83 (2011) GPS Already NAD83 (2011)
NAD83 (Alaska)
Export using
GIS Defined as NAD83 (2011)
NDGPS
NAD83
(2011)

49. HOT TIP: How to create your own base station
GoTo ngs.noaa.gov/CORS
Select Coordinates for a CORS Station
Select the Position and Velocity link
Copy an Existing CORS coordinates
Seed Coordinates into Base Provider Reference Position

50. HOT Tip – Batch Processing
Automate Base Station Coordinates and Export utilities together
Ensures fewer mistakes are made.
Insert Movie file JCusick_DatumsandTransformationsinGPSSoftware_MovieofBatchProcessing.avi

51. Final Word on Mapping Grade Datum Shifts
Michael Dennis (A real Geodesist from NGS) says it best….
“If you want your results in NAD 83, then use the published NAD 83 values as seed coordinates.  Relying on all this tricky and confusing transformation/velocity business is just asking for trouble.
Joel Cusick (A biologist from NPS) says it this way….
“Doing the right thing” use only CORS stations and PFO software, “Use reference position from base file” option with the NEW Position in NAD83 (2011). Finally, during export use the NAD83 (Alaska) shifter and assign the data as NAD83 (2011) (e.g. NAD 83 (2011) UTM Zone 6N).

52. Mapping Grade Software Summary
More choices exist in mapping grade software to hold, transform and export to a CORS reference frame
Always verify the coordinates for CORS and seed the published NAD83 values if you have too
Define all exports precisely to match the CORS published values

53. Datum Shifting Garmin / DNRGPS Software
Trimble GPS Pathfinder Office Software
ArcPad GPS Correct/ GPS Analyst

54. REVIEW Which Datum Are You In? WGS84 = IGS08 NAD83 (2011) epoch 2010.0
- CORS Post Processed
- Autonomous GPS
- WAAS
- CORS Post Processed
- NDGPS (Beacon)
- OMNISTAR (Satellite)

55. ArcPad VS TerraSync :Opposites
Change the GPS Datum to the correction source’s datum
This shifts your data on the device from correction source to your map (output datum)
TerraSync
Keep the datum at WGS 84 – This will shift your data to the correction source’s datum
If you change the datum to the correction source’s datum, your data will shift to the WGS 84 that the satellites use

56. 3 ArcPad Workflows ArcPad autonomous or w/ WAAS
ArcPad w/ GPScorrect w/Real-time network or local base station
ArcPad w/ GPScorrect, post-processed in GPS Analyst

57. Understanding ArcPad w/ Autonomous GPS or WAAS
ArcPad MUST have 3 pieces of information:
Input GPS datum
Output datum
Transformation parameters

58. ArcPad with WAAS
Input GPS Datum = WGS 84

59. ArcPad with WAAS Set Output datum
defined by .prj from shapefile or feature class

60. ArcPad with WAAS
Transformation – Check the transformation

61. ArcPad with WAAS Parameters match ESRI’s NAD83_To_WGS84_5
Officially ITRF96 to NAD83 at
*Paper: ,Table 1, page, 51, Last column

62. ArcPad Problem with NAD 83 CORS 96 or NAD83(2011)
Proper Datum Transformation not available

63. ArcPad: Create own Transform on device if using CORS
Must use the Datum Configuration tool
Just realize if you are using default ArcPad shifters, all your data got to NAD83 from WGS84 using
ITRF96 to NAD83 at shifter
Got it!

64. ArcPad Summary Don’t forget the 3 pieces: Input Output Transformation
Anytime you use other correction sources, must know when NOT to shift or shift with a custom datum transformation tool.

65. Metadata: Don’t Wait! Promotes Accurate Data Sharing
Any GPS project should specify the following
Device Used
Correction Applied (if any)
Field and PC software used
CORS station held
CORS Realized Datum Reference and Epoch held
Important to conduct a survey check (more on this later)

66. Metadata for a Garmin Bad Good
Waypoints collected with a Garmin. Lat/Long set to NAD83 Datum from display screen. Accuracy is 5-10 meters
Good
Field data collected using Garmin Map76CSx with WAAS enabled. All points averaged for minimum of 10 seconds. All data downloaded and saved to shapefile using DNRGPS with Set Projection of Alaska Albers 83 (POSC 3338). During course of project, survey check data collected on nearby PID (TT7584) using active tracklog for 180 seconds (1pps). CEP calculated at 1.2 Meters

67. Metadata for a Mapping Grade
Bad
All data collected by biologists (we all have Ph.D.’s) with $10K GPS and saved in Excel spreadsheets. No idea what Datum the data is in….
Good
Field data collected by field crews with Trimble GeoExplorer 6000 receivers running TerraSync 5.2 field software. All points collected with internal antenna at 1 second interval for 5 seconds each. Data processed with Pathfinder Office(v5.2) with CORS Kenai (KEN5) holding the NAD83 (2011) at epoch 2010 realization. Data exported to geodatabase using NAD83(Alaska) datum in UTM Zone 6N. Assigned as NAD83(2011) datum. Survey check conducted on PID TT % CI at 33cm.

68. Metadata for ArcPad Projects
Bad
Data collected using a GPS and ArcPad with a custom application.
Good
GIS data collected by field crews using Toshiba laptops and bluetooth GENEX Sirf Chip GPS receivers. Custom ArcPad 10.0 application developed to enhance data collection. All points collected using WAAS. Data stored as NAD83(CORS96) at epoch using the default _5 transformation in projected NAD83 UTM Zone 6N. Several survey checks conducted before and after survey on PID TT % CI at 5.2 Meters.

69. Metadata Light If Dave Doyle (Geodesist – Retired NGS) had a choice.
GPS data collected in NAD83 (CORS96) Survey check on PID NSSDA 95% CI 0.33 Meters
That’s IT!

70. Export Datum Settings data in GPS Pathfinder Office software
NAD 1983 (Conus) or NAD 1983 (Alaska)
0,0,0 bookkeeping shifters.
NAD 1983(Conus)CORS 96
shifts data to CORS 96
Based on NAD83_To_WGS84(CORS96)
BE CAREFUL – you can double shift your data. If your corrected data is already NAD83 (2011), then transform doing NOTHING.

71. Geographic transformations in ArcGIS 10.0 – For Alaskans
NAD_1983_To_WGS_1984_1 - North America
This is a ZERO transformation
NAD_1983_To_WGS_1984_2 - Aleutians
NAD_1983_To_WGS_1984_3 - Hawaii NAD_1983_To_WGS_1984_ contiguous states (superseded by _5)
NAD_1983_to_WGS_1984_5* Officially ITRF96 To NAD83 at
NAD_1983_To_WGS_1984_6, _7, and _8 are NTv2 transformations for Quebec, Saskatchewan, and Alberta, respectively.
WGS_1984_(ITRF00)_To_NAD_1983 – applies a shift from ITRF00 to NAD 83(CORS96) at **
* Table 1, page, 51, Last column ; **Table 2, page 52

72. TOP Three Shifters for Modern GPS in ArcGIS 10.1
NEW
NO SHIFT
OLD
Choose the one that has zero translations or matches the reference frame of your data.

73. When geographic transformations help
When you have data from two different coordinate systems. Here’s an example with ArcMap data in WGS84 (ITRF ) and MUNI imagery is assigned UTM NAD 83

74. No transformation – See the Shift!

75. Zoomed in

76. Geographic transformation applied

77. 1.2 Meter Shift Resolved WGS84 NAD83

78. This Could Work A lot Better!
If the Ortho was defined as NAD83 CORS96 (epoch )
Don’t settle for modern GPS-based products (like orthos) to be assigned “NAD83”
It’s time to get more precise

79. NAD83 Quiz
Which NAD83 for NAD27 data NADCON’d to NAD83 (source from TOPOS)
A. NAD83 (Plain)
Which NAD83 for Garmin data you collected
A. NAD83 (Plain)
Which NAD83 for mapping grade data from 3 years ago (holding a CORS position)
A. NAD83 (CORS96) epoch
Which NAD83 for an ORTHO you are flying this year?
A. NAD83 (2011) epoch

80. NAD83 (2011) – Same Datum, New Reference Epoch
OPUS reports out NAD83 (2011) EPOCH 2010.
7 Parameters to go between IGS08 and NAD83 (2011) are published*
Same 7 parameters are in most software (ESRI), Trimble (CSM)
Define your data as NAD83(2011) IF you are really based on that new reference EPOCH.
It’s here to stay…. Until 2022….** *
** See Handout from NGS on Coordinate and Height Changes

81. Always Check! – How To Conduct A Survey Check
Find a GPS quality survey marker (PIDs)
Pay attention to the survey control reference frame (Datum name and epoch)
Differentially correct holding same CORS reference frame
Export using appropriate shifter (or NOT!)
Compare coordinates from the data sheet to your coordinates in Esri ArcMap
Apply appropriate shifter (or NOT!)
Use Measure Tool and Statistics

82. Conclusion Know where your are going and how you got there !
If your wrong, stay consistent. Stay wrong until you shift everything at once
Write Accurate Metadata
All data must be tagged with Epoch to ensure you can get back (NAD83(CORS96)
Always Check Yourself to Gain Confidence in your GPS Accuracy and Using the Right Datum

83. Acknowledgements Daisy Rettaliata (Duncan Parnell)
Michael Dennis (NGS)
Giovanni Sella (NGS)
Julian Idle (Trimble)
Alison Walker (Electronic Data Solutions)
Dave Doyle (NGS)
Neil Winn (NPS – Assateague NS)

84. Thanks for attending
Questions?