[Geotiff] Linear units problem,
feets or meters? (sorry for cross-posting)
Simone Giannecchini
simboss1 at gmail.com
Wed Jan 10 07:58:40 EST 2007
Hi list,
I am writing because I would like to have some feedback on an issue I
ran into yesterday.
I was supplied with a GeoTiff file whose tiff tags are dumped here below.
<TIFFField number="34735" name="GeoKeyDirectory">
<TIFFShorts>
<TIFFShort value="1"/>
<TIFFShort value="1"/>
<TIFFShort value="0"/>
<TIFFShort value="12"/>
<TIFFShort value="1024"/>
<TIFFShort value="0"/>
<TIFFShort value="1"/>
<TIFFShort value="1"/>
<TIFFShort value="1025"/>
<TIFFShort value="0"/>
<TIFFShort value="1"/>
<TIFFShort value="1"/>
<TIFFShort value="2050"/>
<TIFFShort value="0"/>
<TIFFShort value="1"/>
<TIFFShort value="6269"/>
<TIFFShort value="2056"/>
<TIFFShort value="0"/>
<TIFFShort value="1"/>
<TIFFShort value="7019"/>
<TIFFShort value="3072"/>
<TIFFShort value="0"/>
<TIFFShort value="1"/>
<TIFFShort value="32111"/>
<TIFFShort value="3073"/>
<TIFFShort value="34737"/>
<TIFFShort value="80"/>
<TIFFShort value="0"/>
<TIFFShort value="3076"/>
<TIFFShort value="0"/>
<TIFFShort value="1"/>
<TIFFShort value="9003"/>
<TIFFShort value="3080"/>
<TIFFShort value="34736"/>
<TIFFShort value="1"/>
<TIFFShort value="0"/>
<TIFFShort value="3081"/>
<TIFFShort value="34736"/>
<TIFFShort value="1"/>
<TIFFShort value="1"/>
<TIFFShort value="3082"/>
<TIFFShort value="34736"/>
<TIFFShort value="1"/>
<TIFFShort value="2"/>
<TIFFShort value="3083"/>
<TIFFShort value="34736"/>
<TIFFShort value="1"/>
<TIFFShort value="3"/>
<TIFFShort value="3092"/>
<TIFFShort value="34736"/>
<TIFFShort value="1"/>
<TIFFShort value="4"/>
</TIFFShorts>
</TIFFField>
<TIFFField number="34736" name="GeoDoubleParams">
<TIFFDoubles>
<TIFFDouble value="-74.5"/>
<TIFFDouble value="38.8333333"/>
<TIFFDouble value="1614580.1"/>
<TIFFDouble value="0.0"/>
<TIFFDouble value="0.9999"/>
</TIFFDoubles>
</TIFFField>
<TIFFField number="34737" name="GeoAsciiParams">
<TIFFAsciis>
<TIFFAscii value="State Plane Coordinate System 1983; North
American 1983; GRS80; Zone New Jersey|"/>
</TIFFAsciis>
</TIFFField>
What seems strange to me about these tags is that it is clearly
declared that this image is using EPSG:32111 whose standard
description is as follows:
PROJCS["NAD83 / New Jersey",
GEOGCS["NAD83",
DATUM["North American Datum 1983",
SPHEROID["GRS 1980", 6378137.0, 298.257222101, AUTHORITY["EPSG","7019"]],
TOWGS84[1.0, 1.0, -1.0, 0.0, 0.0, 0.0, 0.0],
AUTHORITY["EPSG","6269"]],
PRIMEM["Greenwich", 0.0, AUTHORITY["EPSG","8901"]],
UNIT["degree", 0.017453292519943295],
AXIS["Geodetic longitude", EAST],
AXIS["Geodetic latitude", NORTH],
AUTHORITY["EPSG","4269"]],
PROJECTION["Transverse Mercator", AUTHORITY["EPSG","9807"]],
PARAMETER["central_meridian", -74.5],
PARAMETER["latitude_of_origin", 38.833333333333336],
PARAMETER["scale_factor", 0.9999],
PARAMETER["false_easting", 150000.0],
PARAMETER["false_northing", 0.0],
UNIT["m", 1.0],
AXIS["Easting", EAST],
AXIS["Northing", NORTH],
AUTHORITY["EPSG","32111"]]
while if I check the tags in the geodir I see that a bunch of
information in addition to the CRS itself are provided, like as an
instance:
GeogGeodeticDatumGeoKey
<TIFFShort value="2050"/>
<TIFFShort value="0"/>
<TIFFShort value="1"/>
<TIFFShort value="6269"/>
GeogEllipsoidGeoKey
<TIFFShort value="2056"/>
<TIFFShort value="0"/>
<TIFFShort value="1"/>
<TIFFShort value="7019"/>
ProjLinearUnitsGeoKey
<TIFFShort value="3076"/>
<TIFFShort value="0"/>
<TIFFShort value="1"/>
<TIFFShort value="9003"/>
I checked the geotiff with gdal (the one shipped with fwtools 1.1.3)
and I got the following WKT
PROJCS["NAD83 / New Jersey",
GEOGCS["NAD83",
DATUM["North_American_Datum_1983",
SPHEROID["GRS 1980",6378137,298.2572221010002,
AUTHORITY["EPSG","7019"]],
AUTHORITY["EPSG","6269"]],
PRIMEM["Greenwich",0],
UNIT["degree",0.0174532925199433],
AUTHORITY["EPSG","4269"]],
PROJECTION["Transverse_Mercator"],
PARAMETER["latitude_of_origin",38.83333333333334],
PARAMETER["central_meridian",-74.5],
PARAMETER["scale_factor",0.9999],
PARAMETER["false_easting",492125],
PARAMETER["false_northing",0],
UNIT["US survey foot",0.3048006096012192,
AUTHORITY["EPSG","9003"]],
AUTHORITY["EPSG","32111"]]
while the EPSG file contains the following line for the proj params:
<32111> +proj=tmerc +lat_0=38.83333333333334 +lon_0=-74.5 +k=0.999900
+x_0=150000 +y_0=0 +ellps=GRS80 +datum=NAD83 +units=m +no_defs <>
Note that the proj params are equivalent to the EPSG WKT while the WKT
I get fom GDAL has a different unit of measure definition.
I am wondering what is the right way to manage such geodirectory. Here
you have some thoughts:
>I always thought that when a GCS or a PCS is explicitly defined (like
we do here for EPSG:32111) we should go with it
>I could interpret this case as "modify the EPSG:32111" PCS with the
provided params" hence change the unit of measure. Anyway, IMHO, if I
do this (like I guess GDAL is doing) I think I should not state
anymore that this CRS is EPSG:32111 but I should be creating a
user-define PCS.
>Is this issue related to the old thread from early 2006 in the
geotiff ml with regard to handling feet UoM?
Sorry again for cross-posting, I am hoping to get some information on
this issue.
Regards,
Simone.
--
-------------------------------------------------------
Eng. Simone Giannecchini
President /CEO GeoSolutions
http://www.geo-solutions.it
-------------------------------------------------------
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