Datum Shifts Part II

1. Datum Shifts Part II The Little Shift

2. Matching Datums Revenge of the Shift I’m using WGS84 I use GPS for all my mapping. I’m using NAD83(1986) 4.3 ft

3. Objective – Little Shift • Test the accuracy of various GPS units • Identify the NAD83-ITRF00 Shift

4. Process • 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

5. Prior to this Presentation • You occupied “Truth” • Collected at least two Point Features (ReferPT) • Diff. Corrected against USFS Lassen Base Station • Open PFO Now

6. Export 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

7. 1: Export out as WGS84 • Bad This does NOT Transform the data • This is a zero transformation This Retains WGS84 or ITRF00

8. 2: Export Out as NAD83 (CONUS) CORS96 • Our class standard - Good Moves Data Back to NAD83 Using the Best Transform Possible

9. Datum Transform Occurred in PFO • 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

10. 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”

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

12. In 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”

13. In ArcMap • Symbolize the Bad data with a Large Red Symbol • Symbolize the Good data with a Large Green Symbol

14. Extra 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.

15. Datum Transforms thanks

16. You are here