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Datums Datums & Coordinate & Coordinate
SystemsSystems
Revenge of the Shift
NAD27 D
atum
ObjectiveObjective
• Identify different datums
• Identify different coordinate systems
Setting The StageSetting 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 SystemsCoordinate Systems
• Latitude and Longitude
• Universal Transverse Mercator (UTM)
Lat / Long Coordinate SystemLat / Long Coordinate System
LatitudeLatitude
Parallels of latitude
0° latitude
northlatitude
southlatitude
90°N
equator
Lat / Long Coordinate SystemLat / Long Coordinate System
LongitudeLongitude
West Longitude
Meridians of longitude 0° longitudePrime Meridian
Eastlongitude
Grenwich, England
04/18/23
UTM Coordinate SystemsUTM 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
04/18/23
UTM Zones in North AmericaUTM Zones in North America174180 168 162 156 150 144 138 132 126 120 114 108 102 96 90 78 72 54606684 48
““Figure” of the EarthFigure” of the EarthBest-fit ellipsoidBest-fit ellipsoid
(e.g., GRS-80, WGS-84)(e.g., GRS-80, WGS-84)
Datum and EllipsoidsDatum and Ellipsoids
• Datum - represented by ellipsoid
• Reference ellipsoid examples: • Clarke 1866• GRS 80• WGS 84
Common Geographic DatumsCommon Geographic Datums
• NAD27• NAD83• WGS84• ITRF00
04/18/23
NAD27: NAD27: Clarke 1866 ellipsoidClarke 1866 ellipsoidorigin in Kansasorigin in Kansas
Datum Origin
on Surface of
Earth
Ellipsoid Model based on less
precise surveys
04/18/23
NAD27: Leaving Behind 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
04/18/23
GPS Datum: WGS 84GPS Datum: WGS 84
Origin is at the Earth’s center of mass (geocentric)
This is the datum used for the NAVSTAR GPS satellites
04/18/23
WGS84:WGS84: WGS84 ellipsoidWGS84 ellipsoidorigin center of earthorigin 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 AdjustmentsDatum 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 Over Time Datums Have Grown ApartApart
NAD83 = WGS84 WGS84
NAD83
~4+ feet
Differences between DatumsDifferences between Datums
NAD27 ≠ NAD83 (1986)
NAD27 D
atum
33 to 700 feet
Shift Happens - Redmond OR Shift Happens - Redmond OR 2005 Example2005 Example
710 Feet
Differences between DatumsDifferences between Datums• WGS84 = ITRF00
about 2cm
• NAD83 (CORS96) ≠ WGS84about 4.3 feet
Reference FramesReference 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 SystemMapping / Coordinate System
• For Arcata BLM Office in Pathfinder Office
Coordinate System: UTM Zone 10N
Datum: NAD 83 (CONUS) CORS96
Reason Were Using NAD83 Reason Were Using NAD83 (CONUS) CORS96(CONUS) CORS96
GIS In NAD83
Using CORS96
CORS
ITRF00~4+ feet
When Do Datums/Coordinate When Do Datums/Coordinate Systems Matter?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 TipsGPS Datum Tips
• Summary:– Check, check, check
– ASK your GIS Specialist
• Good handouts in <cd>/references
ConclusionConclusion
• 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
27
You are here
31
Matching Matching DatumsDatums
Revenge of the Shift
I use GPS for all my
mapping. I’m using
NAD83(1986)
I’m using WGS84
4.3 ft
32
ObjectiveObjective
• Overview of Matching Datums and Coordinate Systems in Pathfinder Office and Data Transfer
33
Taking “GIS” out in the FieldTaking “GIS” out in the Field
• Rasters
• Vectors
• All require a projection (coordinate system) defined ahead of time
34
Matching Data StepsMatching Data Steps
• Know what it is – Metadata helps
• Tell Pathfinder Office the correct Datum and Coordinate System
35
When Do Datums/Coordinate When Do Datums/Coordinate Systems Matter?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
Today’s Background ImageToday’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
37
Coordinate SystemCoordinate Systemand Datumand Datum
Coordinate System: UTM Zone 10N
Datum: NAD 83 (CONUS)
38
Next StepNext Step
• Data Transfer background imagery to the GeoCE device
39
You are here
43
Datum Datum TransformsTransforms
Getting it Right in Pathfinder Office
44
ObjectiveObjective
• Identify the proper reference frame for different differential sources
• Learn how to apply the correct datum transformation inside PFO
45
Prior to this PresentationPrior to this Presentation
• You occupied “Truth”
• Collected at least one Point feature
• Now we can compare!
46
Mapping Grade GPS AccuracyMapping Grade GPS Accuracy
• Verification of accuracy is essential to gain confidence
• We have to ensure we transform the data correctly to GIS
Accuracy SlidesAccuracy Slides
Using Real Data.
48
Today is Your Chance to TestToday is Your Chance to Test
• First Some Background
04/18/23
NAD83 Developed Using WGS84NAD83 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
50
Over Time Datums Have Grown Over Time Datums Have Grown ApartApart
NAD83 = WGS84 WGS84
NAD83
~4+ feet
51
Datum AdjustmentsDatum Adjustments• Known as Datum Adjustments or
Epochs
• WGS84 (G1150) – most current version
• Most Current NAD83– NAD83 (CORS96) (Epoch 2003.00)
52
Differences between DatumsDifferences between Datums• WGS84 = ITRF00
about 2cm
• NAD83 (CORS96) ≠ WGS84about 4.3 feet
53
Reference FramesReference Frames• CORS Post Process = ITRF00
• Autonomous GPS = WGS84 = ITRF00
• WAAS = ITRF00
• NDGPS (Radio DGPS) = NAD83 (CORS96)
• OMNISTAR (Satellite DGPS) = NAD83 (CORS96)
54
NAD 83 (CORS96) to WGS-84 (G1150)NAD 83 (CORS96) to WGS-84 (G1150)
4.2 ft
WGS-84 (G1150)= ITRF 00 (2001.0)
“TRUTH”NAD 83 (2003.0)
Anchorage
This is essentially the CORS or WAAS Reference Frame
55
ReferenceReference
• Locate this support doc and follow along– 2nd to last reference in Notebook
• SprtNote_PFO-GPSA_NAD83Datum.pdf
56
This Training Used Scenario 2This Training Used Scenario 2
57
Since our Reference Frame is Since our Reference Frame is CORSCORS
• 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
Export Out as NAD83 (CONUS) Export Out as NAD83 (CONUS) CORS96CORS96
• The reason behind our class standard
59
To Keep Shifts at BayTo Keep Shifts at Bay• Verify reference frame of differential
source
60
To Keep Shifts at BayTo Keep Shifts at Bay
• Apply the correct datum transform before the Export depending on your GIS needs
61
SummarySummary
• 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
If You Use WAAS?If You Use WAAS?
• Since WAAS is ITRF use NAD 1983 (Conus) CORS96
63
If You Use Coast Guard Beacon?If You Use Coast Guard Beacon?
• Since NDGPS is already in NAD83 (CORS96) use NAD 1983 (Conus)
• No need to apply a transform.
64
You are here