[Gdal-dev] Shapfiles and Projections

[Dean Mikkelsen]

Hi Kent,
 
The Shapefile technical definition can also be found here:
 
http://www.esri.com/library/whitepapers/pdfs/shapefile.pdf

and 
 
http://facweb.knowlton.ohio-state.edu/selhami/crpinfo/teaching/CRP608/Shape%20file%20structure.doc
 
Note that the .prj is optional and was a late addition to the Shapefile definition (not sure myself when it came along, but it was not present in the Original White paper).
 
Cheers,
Dean

----- Original Message -----
From: Kent Eschenberg <eschenbe at psc.edu>
Date: Tuesday, October 9, 2007 11:08 am
Subject: Re: [Gdal-dev] Shapfiles and Projections
To: gdal-dev at lists.maptools.org

> Thank you all for your assistance. The gdal/ogr library can read 
> the shapefile 
> if the directory also contains the dbf and shx files that came 
> along with the 
> shp file. Specific responses below.
> 
>   -----Original Message-----
>  > 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?
> 
> Martin Chapman wrote:
>  > Make sure there is a .prj file with the same name as the 
> .shp file.  NTAD
>  > data doesn't always come with these projection 
> files.  If you read the .txt
>  > file with the shape file it will tell you what datum it 
> is using.  Attached
>  > is a wgs84 prj file that should work if you rename it to 
> be the same as your
>  > .shp file.  NTAD is generally in WGS84.
> 
> Hmmm ... I didn't do that; there is no prj file but it now seems 
> to work.
> 
> Ed McNierney wrote:
>  > 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:
> <clip>
> 
> Yes, a txt file was supplied and it has information similar to 
> your example. 
> However it doesn't seem to be used. I'm still developing my 
> application and so 
> I'll find out soon enough if it is needed by hiding or not 
> hiding the txt file.
> 
> Frank Warmerdam wrote:
>  > I assume you were using ogrinfo?
>  >
>  > It will report coordinate system info if it finds 
> it.  Does your shapefile
>  > dataset include a .prj file?   If not, there 
> is not declared coordinate
>  > system for the file.
> 
> Yes, my mistake, I am using ogrinfo. No prj file was supplied 
> but it seems that 
> information was provided in the txt file.
> 
> Kent
> Pittsburgh Supercomputing Center
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> 

Dean C. Mikkelsen, B.Sc., P.Eng.
Terra ETL Ltd.
Victoria, B.C., Canada
Phone: +1 (250) 361-6672
http://www.terraetl.com

Consulting in Geodesy & GIS  for the Natural Resources
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