Transcript Datums and Coordinate Systems

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
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“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)
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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)
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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
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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
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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.
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You are
here
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