Re: [Geotools-devel] Shapefile and coordinate system

[Paul R. <pr...@re...>]

Don't give up completely :)
  The traditional "shapefile" (shp/shx/dbf) has no information about 
units or coordinate system. I suppose it would be relatively easy to add 
a hack to a viewer which tests the coordinate extents and says "this 
looks alot like lat/lon".
  Since ArcGIS8, ESRI has added support for Yet Another Shapefile File, 
the .prj file, which includes a well-known-text projection string. The 
dialect of the ESRI WKT has a few quirks but nothing really substantial. 
I attribute the quirks to the looseness of the WKT spec to start with.
  I wonder if in general it would be good for data source objects to try 
and interrogate their sources for information regarding coordinate 
system. The ArcGIS way is not bad. It tries to figure out the coordinate 
system from the data source, and if it fails just adds the source to the 
view in unprojected units. So if your view is set up to be UTM10, and 
you load a data set which is UTM10 and does not know its projection it 
will still work. And loading data which is Lat/Lon would automatically 
get projected to the view coordinate system UTM10.


Andrea Aime wrote:
> Andrea Aime wrote:
> 
>> Hi everybody,
>> I have a question for you. As far as I can tell the Shapefile
>> specification provides no information about units, geodetic
>> datum and coordinate system. Yet opening shapefiles with
>> ArcExplorer it seems it knows that a file is in geographic
>> coordinates, knows how to express the scale in meters and
>> how to measure distances... I suspect that the format specification
>> omits some information in the "unused" bytes... does
>> anyone have some more information about it?
>> Best regards
>> Andrea Aime
>>
> 
> Sorry... I must sleep some more at night... obviously
> the program only uses user provided information: you can
> specify the units in a menu, it only happened that the
> default ones did match my shapefiles...
> Sorry for the stupid question
> Best regards
> Andrea Aime
> 
> 
> 
> 
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