1. U.S. Department of the Interior U.S. Geological Survey Horizontal Datum Shifts in NHD History and Action Plans in Place

2. NHD Create Process  2002  USGS, USFS, and others begin work to produce 24k (High Resolution) NHD  Different file types used to create initial point, line and polygon vector datasets  Digital Line Graph (DLG)  Cartographic Feature File (CFF)  Tagged Vector Hydro (TVH)  Technicians assigned to do a group of quadrangles within a subbasin  Most quadrangles were in North American Datum 1927 (NAD27) and had to be converted to NAD 1983 by transformation  2007  Conversions and conflation of Nationwide 24K (High Resolution) geodatabase completed

3. What was used in the NHD Create Process? (AK 98% TVH or other data; HI, PR, Guam mostly DLG)

4. Discovery of Horizontal Datum Shifts  November 2008 - Dean Tyler and a colleague from USGS Earth Resources Observation and Science (EROS) developed method to compare NHD streamline end-points with DLG end-points  Examined 1271 7.5’ quadrangles  36 quads were determined to have a horizontal datum shift (>28M shift)  4 quads were in the “possible” category  Two contiguous groups of quadrangles - 15 in VA, 18 in NM  Total USGS 7.5 Minute Quadrangles – 65,532  20,806 quads converted from only DLG, most other quads were converted from CFF (6668), CFF+DLG (3697), or TVH (34361)  Dean estimated that approximately 3% of quadrangles where DLG were used could be affected based on these findings – approximately 735 quadrangles

5. Linear Feature End-Point Comparison

6. Water Body Centroid Comparison

7. How did this happen?  Seems to be limited to areas of grouped quadrangles  Incorrect projection assigned upon conversion of source data  Projection deleted or no transformation applied  Source data incorrectly assigned projection

8. Horizontal Shift Across Quadrangle Boundary (NAD83 – NAD27)

9. Shifts are mostly apparent

10. Source Comparison Data Accuracy

11. Correcting Horizontal Datum Shifts  Phase I – Detection and verification, establish areas of interest (AOI)  Phase II – Extraction (clipping), conversion (pgdb2shp), transformation applied, visual check with source data  Phase III – Erase, load feature changes (ArcCatalog Load), update status table (Load of ComID and Modify Geometry status)  Phase IV – Edge matching and merge features  Phase V – Verification of correction, Run QA/QC process, upload to repository

12. Questions or Comments?