Datums and Coordinate Systems

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

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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

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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

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NAD27: NAD27: Clarke 1866 ellipsoidClarke 1866 ellipsoidorigin in Kansasorigin in Kansas

Datum Origin on Surface of Earth

Ellipsoid Model based on less

precise surveys

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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

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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

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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 WGS84NAD83

~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 NAD83Using CORS96

CORSITRF00~4+ 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

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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

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ObjectiveObjective• Overview of

Matching Datums and Coordinate Systems in Pathfinder Office and Data Transfer

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Taking “GIS” out in the FieldTaking “GIS” out in the Field

• Rasters• Vectors

• All require a projection (coordinate system) defined ahead of time

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Matching Data StepsMatching Data Steps• Know what it is – Metadata helps

• Tell Pathfinder Office the correct Datum and Coordinate System

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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

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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

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Coordinate SystemCoordinate Systemand Datumand Datum

Coordinate System: UTM Zone 10NDatum: NAD 83 (CONUS)

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Next StepNext Step• Data Transfer background imagery

to the GeoCE device

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Datum Datum TransformsTransformsGetting it Right in Pathfinder Office

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ObjectiveObjective• Identify the proper reference frame

for different differential sources

• Learn how to apply the correct datum transformation inside PFO

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Prior to this PresentationPrior to this Presentation• You occupied “Truth”• Collected at least one

Point feature

• Now we can compare!

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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.

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Today is Your Chance to TestToday is Your Chance to Test• First Some

Background

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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

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Over Time Datums Have Grown Over Time Datums Have Grown ApartApart

NAD83 = WGS84 WGS84NAD83

~4+ feet

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Datum AdjustmentsDatum Adjustments• Known as Datum Adjustments or

Epochs

• WGS84 (G1150) – most current version

• Most Current NAD83– NAD83 (CORS96) (Epoch 2003.00)

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Differences between DatumsDifferences between Datums• WGS84 = ITRF00

about 2cm

• NAD83 (CORS96) ≠ WGS84about 4.3 feet

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Reference FramesReference 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)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

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ReferenceReference• Locate this support doc and follow along

– 2nd to last reference in Notebook• SprtNote_PFO-GPSA_NAD83Datum.pdf

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This Training Used Scenario 2This Training Used Scenario 2

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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

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Export Out as NAD83 (CONUS) Export Out as NAD83 (CONUS) CORS96CORS96

• The reason behind our class standard

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To Keep Shifts at BayTo Keep Shifts at Bay• Verify reference frame of differential

source

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To Keep Shifts at BayTo Keep Shifts at Bay• Apply the correct datum

transform before the Export depending on your GIS needs

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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

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If You Use WAAS?If You Use WAAS?

• Since WAAS is ITRF use NAD 1983 (Conus) CORS96

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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.

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