U.S. Department of the InteriorU.S. Geological Survey

Horizontal Datum Shifts in NHD

History and Action Plans in Place

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

What was used in the NHD Create Process?

(AK 98% TVH or other data; HI, PR, Guam mostly DLG)

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

Linear Feature End-Point Comparison

Water Body Centroid Comparison

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

Horizontal Shift Across Quadrangle Boundary (NAD83 – NAD27)

Shifts are mostly apparent

Source Comparison Data Accuracy

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

Questions or Comments?