Date post: | 18-Jan-2016 |
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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?