Datums
& Coordinate
Systems
Revenge of
the Shift
Objective
• Identify different datums
• Identify different coordinate systems
Setting The Stage
• GPS is rigid – collects one way
• GIS is flexible – designed to share
• #1 reason GPS data doesn’t line up in
GIS - coordinate system/datum mismatch
• How do we fix it? – Educate
Coordinate Systems
• Latitude and Longitude
• Universal Transverse Mercator (UTM)
Lat / Long Coordinate System
Latitude
90°N
Parallels of latitude
north
latitude
0° latitude
equator
south
latitude
Lat / Long Coordinate System
Longitude
Grenwich, England
0° longitude
Meridians of longitude
Prime Meridian
West
Longitude
East
longitude
UTM Coordinate Systems
A plane coordinate system to
relate the coordinates of points
on earth’s curved surface with
the coordinates of the same
points on a plane or flat surface
Allows projection of a
spherical surface onto a
flat surface
UTM Zones in North America
180 174 168 162 156 150 144 138 132 126 120 114 108 102
96
90
84
78
72
66
60
54
48
“Figure” of the Earth
Best-fit ellipsoid
(e.g., GRS-80, WGS-84)
Datum and Ellipsoids
• Datum - represented by ellipsoid
• Reference ellipsoid examples:
• Clarke 1866
• GRS 80
• WGS 84
Common Geographic Datums
•
•
•
•
NAD27
NAD83
WGS84
ITRF00
NAD27:
Clarke 1866 ellipsoid
origin in Kansas
Datum Origin
on Surface of
Earth
Ellipsoid Model
based on less
precise surveys
NAD27: Leaving Behind
• Invented Space Travel
• Increasing accuracy of
surveys
• Shift from optical surveys to
a mathematical model of the
Earths shape
• The result is a specific point
on the landscape can take
on multiple meanings
GPS Datum: WGS 84
 Origin is at the Earth’s center of mass (geocentric)
 This is the datum used for the NAVSTAR GPS satellites
WGS84:
WGS84 ellipsoid
origin center of earth
The origins of the
WGS84 and
NAD83 ellipsoids
are at the center of
the earth’s mass,
which makes them
ideal for a GPS
datum
Datum Adjustments
• Known as Datum Adjustments or Epochs
• WGS84 (G1150) – most current version
• “Original” NAD83 = NAD83 (1986)
– NAD83 (1992)
– NAD83 (2002)
– ……
• Most Current NAD83
– NAD83 (CORS96) (Epoch 2003.00)
Over Time Datums Have Grown
Apart
NAD83
NAD83 = WGS84
WGS84
Differences between Datums
NAD27 ≠ NAD83 (1986)
33 to 700
feet
Shift Happens - Redmond OR
2005 Example
710 Feet
Differences between Datums
• WGS84 = ITRF00
about 2cm
• NAD83 (CORS96) ≠ WGS84
about 4.3 feet
Reference Frames
• Differentially corrected GPS data are
always in terms of the corrections
source’s reference frame.
• In Arcata we are using CORS station
data and therefore must apply the
correct datum transform between the
GPS data and your GIS
Mapping / Coordinate System
• For Arcata BLM Office in Pathfinder
Office
Coordinate System: UTM Zone 10N
Datum: NAD 83 (CONUS) CORS96
Reason Were Using NAD83
(CONUS) CORS96
GIS In NAD83
CORS
Using CORS96
ITRF00
When Do Datums/Coordinate
Systems Matter?
• PFO Map View
– Coordinate system
• Terrasync
– Coordinate System
• Export Utility
– Shape Export
Coordinate System
• ArcGIS Catalog
– Define Projection
• ArcGIS
– Data Frame Properties
– Coordinate System
Tab
GPS Datum Tips
• Summary:
– Check, check, check
– ASK your GIS Specialist
• Good handouts in <cd>/references
Conclusion
• Now that you’ve learned there are
differences it will be important that
you learn how to…
• Make it Match
– Pre-Field Day 2
• Datum Transforms
– Friday’s Test against Truth
You are
here
27
Matching
Datums
I use GPS for
all my
mapping. I’m
using
NAD83(1986)
I’m using
WGS84
Revenge of
the Shift
31
Objective
• Overview of
Matching Datums
and Coordinate
Systems in
Pathfinder Office
and Data Transfer
32
Taking “GIS” out in the Field
• Rasters
• Vectors
• All require a projection (coordinate
system) defined ahead of time
33
Matching Data Steps
• Know what it is – Metadata helps
• Tell Pathfinder Office the correct
Datum and Coordinate System
34
When Do Datums/Coordinate
Systems Matter?
• PFO Map View
– Coordinate system
• Terrasync
– Coordinate System
• Export Utility
– Shape Export
Coordinate System
• ArcGIS Catalog
– Define Projection
• ArcGIS
– Data Frame Properties
– Coordinate System
Tab
35
Today’s Background Image
• Humboldt Campus Map
– PDF obtained from Website
– Converted to TIFF
– Co-registered 12 control points
between Ortho
– ~ Depending on accuracy of Ortho,
maybe +/- 20 meters in horizontal
accuracy
36
Coordinate System
and Datum
Coordinate System: UTM Zone 10N
Datum: NAD 83 (CONUS)
37
Next Step
• Data Transfer background imagery
to the GeoCE device
38
You are
here
39
Datum
Transforms
Getting it Right in
Pathfinder Office
43
Objective
• Identify the proper reference frame
for different differential sources
• Learn how to apply the correct
datum transformation inside PFO
44
Prior to this Presentation
• You occupied “Truth”
• Collected at least one
Point feature
• Now we can compare!
45
Mapping Grade GPS Accuracy
• Verification of accuracy is essential
to gain confidence
• We have to ensure we transform
the data correctly to GIS
46
Accuracy Slides
Using Real Data.
Today is Your Chance to Test
• First Some
Background
48
NAD83 Developed Using WGS84
The origins of the
WGS84 and
NAD83 ellipsoids
are at the center of
the earth’s mass,
which makes them
ideal for a GPS
datum
Over Time Datums Have Grown
Apart
NAD83
NAD83 = WGS84
WGS84
50
Datum Adjustments
• Known as Datum Adjustments or
Epochs
• WGS84 (G1150) – most current
version
• Most Current NAD83
– NAD83 (CORS96) (Epoch 2003.00)
51
Differences between Datums
• WGS84 = ITRF00
about 2cm
• NAD83 (CORS96) ≠ WGS84
about 4.3 feet
52
Reference Frames
• CORS Post Process = ITRF00
• Autonomous GPS = WGS84 = ITRF00
• WAAS = ITRF00
• NDGPS (Radio DGPS) = NAD83
(CORS96)
• OMNISTAR (Satellite DGPS) = NAD83
(CORS96)
53
NAD 83 (CORS96) to WGS-84 (G1150)
This is essentially the
CORS or WAAS
Reference Frame
WGS-84 (G1150)
= ITRF 00 (2001.0)

“TRUTH”
NAD 83 (2003.0)
Anchorage
54
Reference
• Locate this support doc and follow along
– 2nd to last reference in Notebook
• SprtNote_PFO-GPSA_NAD83Datum.pdf
55
This Training Used Scenario 2
56
Since our Reference Frame is
CORS
• Our GPS data is corrected against
a source that is in WGS84 or
ITRF00
• And our GIS is in NAD83
• We therefore define an ITRF00 to
NAD83 Transform
57
Export Out as NAD83 (CONUS)
CORS96
• The reason behind our class
standard
58
To Keep Shifts at Bay
• Verify reference frame of differential
source
59
To Keep Shifts at Bay
• Apply the correct datum
transform before the
Export depending on
your GIS needs
60
Summary
• Identified the proper reference frame
for different differential sources
• Learned how to apply the correct
datum transformation inside PFO
• Keep Shifts at Bay by verifying and
using the right transform
61
If You Use WAAS?
• Since WAAS is ITRF use NAD
1983 (Conus) CORS96
62
If You Use Coast Guard Beacon?
• Since NDGPS is already in NAD83
(CORS96) use NAD 1983 (Conus)
• No need to apply a
transform.
63
You are
here
64