[Geotiff] Are false eastings and northings in meters *required*?
David Burken
dburken at comcast.net
Wed Nov 10 11:08:14 PST 2010
James,
This issue has been raised before and I believe if the linear units is
set to U.S. Survey Foot the false easting/northing should be in those
units. I just verified our ossim tiff reader does a convert from us_ft
to meters if the linear units are such. I don't think all tools see it
that way though. Typically for state plane projections you just set the
pcs code and the projection parameters are implied by the code. So that
kind of gets around the issue. I know that doesn't answer you question
though...
Dave
On 11/10/2010 12:05 PM, Jim Lacy wrote:
> All-
>
> So here is a deceptively simple question: does the GeoTIFF standard *require* false eastings and northings to be in meters, regardless of how the linear units are defined?
>
> If the answer is "yes", stop reading now. If it's "no" or "maybe" read on...
>
> I'm trying to troubleshoot a user problem involving GeoTIFF images projected to Lambert Conformal Conic (1SP). The linear units should be the U.S. Survey Foot. When viewed in ArcGIS 10, the false eastings/northings are offset by a factor 3.28083333. That happens to be the conversion factor of meters to US Survey Foot. More on this later.
>
> I started to dig into the issue by examining the header, but I'm confused by the conflicting results I'm getting from listgeo and gdalinfo. First, I should mention the correct false eastings/northings for this particular coordinate system are 187147.5744 meters (614000 feet) and 107746.7522 meters (353499.136 feet), respectively.
>
> Listgeo reports false eastings/northings as being defined in meters (even though ProjLinearUnitsGeoKey says Linear_Foot_US_Survey), but you see the actual values listed are for feet:
>
>
> PCSCitationGeoKey (Ascii,89): "Projection: Lambert Conformal Conic; Datum: North American 1983 - HARN; Ellipsoid: GRS80"
> ProjectionGeoKey (Short,1): User-Defined
> ProjCoordTransGeoKey (Short,1): CT_LambertConfConic_1SP
> ProjLinearUnitsGeoKey (Short,1): Linear_Foot_US_Survey
> ProjNatOriginLongGeoKey (Double,1): -90.4833333
> ProjNatOriginLatGeoKey (Double,1): 45.1778221
> ProjFalseEastingGeoKey (Double,1): 614000
> ProjFalseNorthingGeoKey (Double,1): 353499.136
> ProjScaleAtNatOriginGeoKey (Double,1): 1.00005976
> End_Of_Keys.
> End_Of_Geotiff.
>
> Projection Method: CT_LambertConfConic_1SP
> ProjNatOriginLatGeoKey: 45.177822 ( 45d10'40.16"N)
> ProjNatOriginLongGeoKey: -90.483333 ( 90d29' 0.00"W)
> ProjScaleAtNatOriginGeoKey: 1.000060
> ProjFalseEastingGeoKey: 614000.000344 m
> ProjFalseNorthingGeoKey: 353499.136000 m
> GCS: 4152/(unknown)
> Datum: 6152/(unknown)
> Ellipsoid: 7019/GRS 1980 (6378137.00,6356752.31) Projection Linear Units: 9003/(unknown) (1.000000m)
>
>
>
> Gdalinfo on the same file correctly reports the units as feet, as I would expect, BUT the false easting/northing values are incorrect. they are multiplied by 3.28+.
>
>
> PROJCS["Projection: Lambert Conformal Conic; Datum: North American 1983 - HARN; Ellipsoid: GRS80",
> GEOGCS[,
> DATUM["unknown",
> SPHEROID["GRS 1980",6378137,298.2572221010002,
> AUTHORITY["EPSG","7019"]],
> AUTHORITY["EPSG","6152"]],
> PRIMEM["Greenwich",0],
> UNIT[,0.0174532925199433],
> AUTHORITY["EPSG","4152"]],
> PROJECTION["Lambert_Conformal_Conic_1SP"],
> PARAMETER["latitude_of_origin",45.17782208583334],
> PARAMETER["central_meridian",-90.48333333333335],
> PARAMETER["scale_factor",1.0000597566],
> PARAMETER["false_easting",2014431.667795273],
> PARAMETER["false_northing",1159771.748693332],
> UNIT["US survey foot",0.3048006096012192,
> AUTHORITY["EPSG","9003"]]]
> Origin = (626452.779999999680000,316640.203999999790000)
> Pixel Size = (1.499999999999999,-1.499999999999999)
>
>
>
> Lastly, here is what QGIS has to say:
> +proj=lcc +lat_1=45.17782208583334 +lat_0=45.17782208583334
> ++lon_0=-90.48333333333335 +k_0=1.0000597566 +x_0=614000.0003439998
> ++y_0=353499.1359999996 +ellps=GRS80 +to_meter=0.3048006096012192
> ++no_defs
>
>
> What I suspect is happening is the false eastings and northings values MUST be in meters in the header. In my case, it appears 614,000 and 353,499 are stored in the header, but GDAL and ArcGIS look at the units and multiply by the conversion factor to report the easting/northings in the desired linear units. However, if the easting/northings are *already* in the desired linear unit, everything is offset by the conversion factor.
>
> Are you with me? :)
>
> In fact, if I redefine the header by manually editing the false easting/northings values and changing to 187147.5744 meters and 107746.7522 meters, but leave units set to U.S. Survey Foot, everything lines up just dandy in ArcGIS. Likewise, gdalinfo reoports values I would expect:
>
> PROJCS["Lambert_Conformal_Conic_1SP",
> GEOGCS["GCS_North_American_1983_HARN",
> DATUM["unknown",
> SPHEROID["GRS_1980",6378137,298.257222101,
> AUTHORITY["EPSG","0"]],
> AUTHORITY["EPSG","6152"]],
> PRIMEM["Greenwich",0],
> UNIT["degree",0.0174532925199433]],
> PROJECTION["Lambert_Conformal_Conic_1SP"],
> PARAMETER["latitude_of_origin",45.17782208583334],
> PARAMETER["central_meridian",-90.48333333333335],
> PARAMETER["scale_factor",1.0000597566],
> PARAMETER["false_easting",614000],
> PARAMETER["false_northing",353499.136],
> UNIT["US survey foot",0.3048006096012192,
> AUTHORITY["EPSG","9003"]]]
> Origin = (626452.779999999680000,316640.203999999790000)
> Pixel Size = (1.499999999999999,-1.499999999999999)
>
>
>
> Thus my question: should we always be using meters in the TIFF headers for eastings/northings?
>
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> James P. Lacy, GISP
> Associate State Cartographer
> Wisconsin State Cartographer's Office
> 384 Science Hall, 550 N. Park St.
> Madison, WI 53706-1491
>
> (608) 262-6850
> Lacy at wisc.edu
> www.sco.wisc.edu
>
>
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