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I want to convert TIGER data

GEOGCS["GCS_North_American_1983",
    DATUM["North_American_Datum_1983",
        SPHEROID["GRS_1980",6378137,298.257222101]],
    PRIMEM["Greenwich",0],
    UNIT["Degree",0.017453292519943295]]

to WGS84. How does ogr2ogr determine which transformation to use and how big would the introduced error be?

Study area is the county of Kauai in Hawaii, USA.

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1 Answer 1

Actually, ogr2ogr does not decide, it is up to the user to choose a suitable proj string. For NAD1983, the datum shift to WGS84 is 0, that is both use the same ellipsoid in the same position. Proj string would be:

+proj=longlat +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +no_defs

or s_srs EPSG:4269 t_srs EPSG:4326

Your data is in degrees, so no transformation is needed.

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NAD 83 = WGS84 for the original realizations only. That's no longer true. The TIGER data's (in)accuracy may mean that treating them as equivalent is a valid solution. For more accurate data, or that's on later realizations/re-adjustments, you may need to look for a NAD83 Pacific Plate / ITRF (WGS84) transformation. Try ngs.noaa.gov/CORS/coords.shtml for more information. –  mkennedy Feb 12 '13 at 18:32
    
OK, but as far as I know ogr2ogr does not take care of any time frames yet. This presentation gives some hints for NAD83 in Hawaii: higicc.camp8.org/Resources/Documents/… –  Andre Joost Feb 13 '13 at 11:10
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