3. 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
5. Lat / Long Coordinate System
Latitude
Parallels of latitude
0° latitude
north
latitude
south
latitude
90°N
equator
6. Lat / Long Coordinate System
Longitude
West
Longitude
Meridians of longitude 0° longitude
Prime Meridian
East
longitude
Grenwich, England
7. UTM Coordinate Systems
Allows projection of a
spherical surface onto a
flat surface
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
8. UTM Zones in North America
174
180 168 162 156 150 144 138 132 126 120 114 108 102 96 90 78 72 54
60
66
84 48
9. “Figure” of the Earth
Best-fit ellipsoid
(e.g., GRS-80, WGS-84)
10. Datum and Ellipsoids
• Datum - represented by ellipsoid
• Reference ellipsoid examples:
• Clarke 1866
• GRS 80
• WGS 84
12. NAD27: Clarke 1866 ellipsoid
origin in Kansas
Datum Origin
on Surface of
Earth
Ellipsoid Model
based on less
precise surveys
13. 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
14. GPS Datum: WGS 84
Origin is at the Earth’s center of mass (geocentric)
This is the datum used for the NAVSTAR GPS satellites
15. 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
16. 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)
21. 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
22. Mapping / Coordinate System
• For Arcata BLM Office in Pathfinder
Office
Coordinate System: UTM Zone 10N
Datum: NAD 83 (CONUS) CORS96
23. Reason Were Using NAD83
(CONUS) CORS96
GIS In NAD83
Using CORS96
CORS
ITRF00
24. 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
25. GPS Datum Tips
• Summary:
–Check, check, check
–ASK your GIS Specialist
• Good handouts in <cd>/references
26. 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
33. 33
Taking “GIS” out in the Field
• Rasters
• Vectors
• All require a projection (coordinate
system) defined ahead of time
34. 34
Matching Data Steps
• Know what it is – Metadata helps
• Tell Pathfinder Office the correct
Datum and Coordinate System
35. 35
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
36. 36
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
44. 44
Objective
• Identify the proper reference frame
for different differential sources
• Learn how to apply the correct
datum transformation inside PFO
45. 45
Prior to this Presentation
• You occupied “Truth”
• Collected at least one
Point feature
• Now we can compare!
46. 46
Mapping Grade GPS Accuracy
• Verification of accuracy is essential
to gain confidence
• We have to ensure we transform
the data correctly to GIS
49. 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
51. 51
Datum Adjustments
• Known as Datum Adjustments or
Epochs
• WGS84 (G1150) – most current
version
• Most Current NAD83
–NAD83 (CORS96) (Epoch 2003.00)
54. 54
NAD 83 (CORS96) to WGS-84 (G1150)
WGS-84 (G1150)
= ITRF 00 (2001.0)
“TRUTH”
NAD 83 (2003.0)
Anchorage
This is essentially the
CORS or WAAS
Reference Frame
55. 55
Reference
• Locate this support doc and follow along
– 2nd to last reference in Notebook
• SprtNote_PFO-GPSA_NAD83Datum.pdf
57. 57
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
58. 58
Export Out as NAD83 (CONUS)
CORS96
• The reason behind our class
standard
59. 59
To Keep Shifts at Bay
• Verify reference frame of differential
source
60. 60
To Keep Shifts at Bay
• Apply the correct datum
transform before the
Export depending on
your GIS needs
61. 61
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
62. 62
If You Use WAAS?
• Since WAAS is ITRF use NAD
1983 (Conus) CORS96
63. 63
If You Use Coast Guard Beacon?
• Since NDGPS is already in NAD83
(CORS96) use NAD 1983 (Conus)
• No need to apply a
transform.