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Datum Datum Shifts Part IIShifts Part II

The Little Shift

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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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Objective – Little ShiftObjective – Little Shift

• Test the accuracy of various GPS units

• Identify the NAD83-ITRF00 Shift

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ProcessProcess

• Export From PFO 2 “versions” of your data

• Assign the correct prj for ReferPT features

• Observe GPS Accuracy in an ArcMap dataframe relative to Survey Grade GPS

• Observe the on-ground relative differences between GPS in two different reference frames

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Prior to this PresentationPrior to this Presentation

• You occupied “Truth”

• Collected at least two Point Features (ReferPT)

• Diff. Corrected against USFS Lassen Base Station

• Open PFO Now

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Export From Pathfinder OfficeExport From Pathfinder Office

• Exported out from PFO two “versions” of the same data

• Once set will be BAD

• One set will be GOOD

• This was a transform, but inside Pathfinder Office

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1: Export out as WGS841: Export out as WGS84

• Bad This does NOT Transform the data

• This is a zero transformation

This Retains

WGS84 or ITRF00

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2: Export Out as NAD83 (CONUS) 2: Export Out as NAD83 (CONUS) CORS96CORS96

• Our class standard - Good

Moves Data Back to NAD83 Using the

Best Transform Possible

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Datum Transform Occurred in Datum Transform Occurred in PFOPFO

• The Datum selection was switched between the 1st and 2nd Exports

• 2nd Export was intentionally left in the reference frame of WGS84 or ITRF00 – Bad

• 1st Export was done properly to line up with our GIS in NAD83– Good

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Assign the Correct PRJ (cont)Assign the Correct PRJ (cont)

• 1st Export was intentionally done wrong – Bad– Assign as UTM WGS84, Zone 10

– Name as “WGS84ReferPt”

• 2nd Export was done properly – Good– Assign as UTM NAD83, Zone 10

– Name Shapefile as “NAD83CORSReferPt”

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In ArcMapIn ArcMap• Open… M:/../Data/ Exercise_Truth.mxd

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In ArcMapIn ArcMap

• Add the Poor or mis-assigned data– “WGS84ReferPt”– You get a Warning error that detects

a difference– Press OK

• Add the Good or properly transformed shapefiles– “NAD83CORSReferPt”

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In ArcMapIn ArcMap• Symbolize

the Bad data with a Large Red Symbol

• Symbolize the Good data with a Large Green Symbol

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Extra CreditExtra Credit

• The first set of data added to the data frame is being transformed into NAD83 (the datum of the data frame) using the default WGS_1984_NAD 1983_1 transform

• This is a “do nothing” transform

• Select _5 to transform the data back to NAD83

• Do note, that until ArcGIS 9.2 comes out, this transform is not exact.

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Datum Datum TransformsTransforms

thanks

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