[Gdal-dev] Shapfiles and Projections

[Ed McNierney]

Kent -

Unfortunately, the concept of projections and coordinate systems in general was late in coming to the shapefile format, and many (if not most) shapefiles have no projection/coordinate information with them.

Your shapefile should have a .txt file with it (presumably roadtrl20.txt if you're using the roads layer from the National Atlas) and it will contain this information:

Spatial_Reference_Information: 
  Horizontal_Coordinate_System_Definition: 
    Geographic: 
      Latitude_Resolution: 0.000278 
      Longitude_Resolution: 0.000278 
      Geographic_Coordinate_Units: Decimal degrees 
    Geodetic_Model: 
      Horizontal_Datum_Name: North American Datum of 1983 
      Ellipsoid_Name: GRS1980 
      Semi-major_Axis: 6378137 
      Denominator_of_Flattening_Ratio: 298.257222

As far as I am aware, all of the vector data available through the National Atlas site is in geographic coordinates (unprojected decimal degrees) on the NAD83 datum.

	- Ed

Ed McNierney
Chief Mapmaker
Demand Media / TopoZone.com
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
Phone: 978-251-4242, Fax: 978-251-1396
ed at topozone.com



-----Original Message-----
From: gdal-dev-bounces at lists.maptools.org [mailto:gdal-dev-bounces at lists.maptools.org] On Behalf Of Kent Eschenberg
Sent: Thursday, October 04, 2007 5:04 PM
To: GDAL
Subject: [Gdal-dev] Shapfiles and Projections

gdrinfo doesn't seem to find a projection in a shapefile describiing roads 
(from the National Atlas). I was expecting something like what gdalinfo finds 
in a geotiff file. Could someone point me towards the answer? Thanks!

Kent
Pittsburgh Supercomputing Center
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