Datums and Transformations
inside GPS Software
I’m in
NAD83
Joel Cusick
GIS Specialist
907 644-3549 joel_cusick@nps.gov
Anchorage Alaska
Presented to Alaska Survey and Mapping
Conference
February 2013
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.
2
Common Datums in Alaska
NAD27
- Pre-GPS; Digitized Topos
NAD83*
- GPS; Mandated for US(&AK!)
WGS84
- Used by GPS satellites
ITRF00/IGS08 - Global reference/ CORS
* http://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc95-19408.pdf
3
Over Time Datums Grew Apart
Today
NAD 83 ≠ WGS84
1986
NAD83 = WGS84
Anchorage
1.3 Meters
(4.2 feet)
WGS84 = ITRF00
= IGS08
4
Today, You Have to Go Back
NAD83
ITRF00 or 1GS08
NAD83 (CORS96)
Or
NAD83 (2011)
Anchorage
1.3 Meters
(4.2 feet)
5
Latest Datum Frame Adjustments
WGS84 (G1640)
WGS84 1986 - Original
WGS84 (G730) 1992
WGS84 (G873) 1997
WGS84 (G1150) 2002
NAD83 (2011)
NAD83 (1986) - Original
NAD83 (1992) HARN
NAD83 (1994) CORS
NAD83 (2007)
6
The Most Important NAD83’s
for An Alaskan GIS
 NAD83 (1986)
 NAD83 (CORS96) epoch 2003.0
 NAD83 (2011) epoch 2010.0
Datum Name Tag
Epoch
7
Difference
between WGS84
(G1150) and
NAD83(CORS96)
at 2003.0
Horizontal difference
8
Used with permission by Author: Michael Dennis
“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
9
GPS Receiver Accuracies
 Do datum shifts really matter?
 Next two slides: multi-year accuracy
evaluation
10
Garmin Accuracy with WAAS
 N=2930
 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.
11
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
12
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
13
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….
* http://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc95-19408.pdf
14
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
15
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!
16
Geographic Transformations:
Choice to Do Nothing or
Something
 Depends on your realtime correction source,
CORS reference frames and your GIS
17
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
NAD83 (Alaska)
ESRI’s 0,0,0 Shifter
NAD83_To_WGS84_1
18
Datum Shifting
 Garmin / DNRGPS Software
 Trimble GPS Pathfinder Office Software
 ArcPad GPS Correct/ GPS Analyst
19
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
20
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.
21
Garmins: What Do you Do?
GPS
NAD83 To
WGS84_5
NAD83 To
WGS84_5
GIS
22
Garmins: Shift Using “ _5”
 Set Projection
 Parameters match ESRI’s NAD83 _ To_WGS84 _5
 Officially: ITRF96 to NAD83 at 1997.0*
*Paper: http://www.ngs.noaa.gov/CORS/Articles/SolerSnayASCE.pdf ,Table 1,
23
page, 51, Last column
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!)
24
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
25
Datum Shifting
 Garmin / DNRGPS Software
 Trimble GPS Pathfinder Office Software
 ArcPad GPS Correct/ GPS Analyst
26
Our CORS Network –
Part of the National
Spatial Reference Frame
27
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????
28
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
29
Which Datum Are You In?
WGS84 = IGS08
(G1640) (2005)
NAD83 (2011) epoch
2010.0
- CORS Post Processed
- CORS Post Processed
- Autonomous GPS
- NDGPS (Beacon)
- WAAS
- OMNISTAR (Satellite)
30
Datum of Base Station Becomes
Datum of Corrected File
Your GPS data MOVES
It can be in ITRF00 (1997.0) or NAD83(CORS96)
(2003.0)
31
Datum of Base Station Becomes
Datum of Corrected File
Your GPS data MOVES
It can be in IGS08 (2005.0) or NAD83(2011) (2010.0)
32
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 2010.0 ARP
– Check the New Coordinates page for the CORS station
to verify the position
33
PostProcess Differential Correction
 When selecting “Use reference
position from base provider” the
WGS84 (ITRF00- Derived from
IGS08) is used
34
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
35
SHIFT DEMO
ESRI ArcMap 10.1
*OPUS Control compliments of Eric Oppegard (JOA Associates)
36
ArcMap DEMO– Uncorrected data
About 1.5 m from the 83 (CORS96) EPOCH: 2003 monument
37
Diff. Correcting w/ CORS
ITRF00 (1997)
38
Results – 12 cm from 83 (CORS96)
39
Results - Zoom In
40
Diff. Correcting w/ CORS
NAD83 (CORS96)*
 Evaluate another pathway
*Must create own Base Station with Older NAD83 (CORS96) Coordinates
41
Results – 11 cm from 83(CORS96)
42
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
43
Results
– 14 cm inches from 83(CORS96)
44
What 8 years can do to your
coordinates - ~ 13 cm
*HDTP to transform from ITRF00(1997) to IGS08(2005)
45
Trimble: What Do you Do?
GPS
GIS Defined
as NAD83
(2011)
Use Base File
NAD83 (2011)
Export using
NAD83
(Alaska)
46
Trimble: What Do you Do?
GPS
Seed
NAD83
(CORS96) 2003
GIS Defined as
NAD83
(CORS96)
Export using
NAD83 (Alaska)
47
Trimble: What Do you Do?
GPS
GIS Defined
as NAD83
(2011)
Already
NAD83
(2011)
Export using
NAD83
(Alaska)
48
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
49
HOT Tip – Batch Processing
 Automate Base
Station Coordinates
and Export utilities
together
 Ensures fewer
mistakes are made.
Insert Movie file
JCusick_Datums
andTransformati
onsinGPSSoftwa
re_MovieofBatch
Processing.avi
50
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).
51
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
52
Datum Shifting
 Garmin / DNRGPS Software
 Trimble GPS Pathfinder Office Software
 ArcPad GPS Correct/ GPS Analyst
53
Which Datum Are You In?
WGS84 = IGS08
(G1640) (2005)
NAD83 (2011) epoch
2010.0
- CORS Post Processed
- CORS Post Processed
- Autonomous GPS
- NDGPS (Beacon)
- WAAS
- OMNISTAR (Satellite)
54
ArcPad VS TerraSync :Opposites
 ArcPad
– 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
55
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
56
Understanding ArcPad w/
Autonomous GPS or WAAS
 ArcPad MUST have 3 pieces of information:
– Input GPS datum
– Output datum
– Transformation parameters
57
ArcPad with WAAS
 Input GPS
Datum = WGS
84
58
ArcPad with WAAS
 Set Output
datum
–defined by .prj
from shapefile
or feature
class
59
ArcPad with WAAS
 Transformation – Check the
transformation
60
ArcPad with WAAS
 Parameters match ESRI’s NAD83_To_WGS84_5
 Officially ITRF96 to NAD83 at 1997.0
*Paper: http://www.ngs.noaa.gov/CORS/Articles/SolerSnayASCE.pdf ,Table 1, page,
51, Last column
61
ArcPad
Problem with NAD
83 CORS 96 or
NAD83(2011)
Proper Datum
Transformation
not available
62
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 1997.0 shifter
 Got it!
63
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.
64
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)
65
Metadata for a Garmin
 Bad
– Waypoints collected with a Garmin. Lat/Long set to
NAD83 Datum from display screen. Accuracy is 510 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
66
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 TT7658. 95% CI
67
at 33cm.
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 1997.0 epoch using the
default _5 transformation in projected NAD83 UTM Zone
6N. Several survey checks conducted before and after
survey on PID TT7658. 95% CI at 5.2 Meters.
68
Metadata Light
 If Dave Doyle (Geodesist – Retired NGS) had
a choice.
– GPS data collected in 2003. NAD83 (CORS96)
2003.0. Survey check on PID 59967. NSSDA 95%
CI 0.33 Meters
 That’s IT!
69
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.
70
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_4 - 48 contiguous
states (superseded by _5)
NAD_1983_to_WGS_1984_5* Officially
ITRF96 To NAD83 at 1997.0
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 1997.0**
* http://www.ngs.noaa.gov/CORS/Articles/SolerSnayASCE.pdf Table 1, page,
51, Last column ; **Table 2, page 52
71
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.
72
When geographic
transformations help
When you have data from two different coordinate systems. Here’s
an example with ArcMap data in WGS84 (ITRF00 1997.0) and
MUNI imagery is assigned UTM NAD 83
73
No transformation – See the Shift!
74
Zoomed in
75
Geographic transformation applied
76
1.2 Meter Shift Resolved
WGS84
NAD83
77
This Could Work A lot Better!
 If the Ortho was defined as NAD83 CORS96
(epoch 2003.0)
 Don’t settle for modern GPS-based products
(like orthos) to be assigned “NAD83”
 It’s time to get more precise
78
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 2003.0
 Which NAD83 for an ORTHO you are flying
this year?
– A. NAD83 (2011) epoch 2010.0
79
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….**
*http://www.ngs.noaa.gov/CORS/coords.shtml#KEYEPOCH
** See Handout from NGS on Coordinate and Height Changes
80
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
81
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) 2003.0
 Always Check Yourself to Gain Confidence in
your GPS Accuracy and Using the Right
Datum
82
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)
Thanks for attending
 Questions?
84