[postgis-users] Basic question #2 Shapefile (meta?)data columns - to keep or discard

[Paul Ramsey]

Those values are not shapefile values, they are actually arccoverage
values, passed through the conversion process. They might not even be
valid, depending on if the coverage was built and cleaned before being
exported. However, if they are built, they might be handy. You could
use the from/to nodes to do traversals, and you could use the l/rpoly
values to pull edges of contained area and build polygons with
ST_Buildarea.

There's not a huge benefit to dropping data "just because".

P.

On Wed, Feb 18, 2009 at 4:30 PM, Ben Madin <ben at remoteinformation.com.au> wrote:
> Following on from my last basic query:
>
> (I'm updating my data based on new sets of shapefiles - but I'm not from an
> ESRI Arc background, so I don't know much about how it might work.)
>
> I notice that Shapefiles often have in their associated .dbf file
> information about the GIS objects (not just the real world item it is meant
> to represent), ie a file of roads might have the name of the road, the
> category (ie A, B or C road), the number of lanes etc. And it also has a
> number of other columns :
>
> fnode, tnode?
>
> I gather these are for line direction - Isn't this stored in the line
> definition in PostGIS? (I note Surya's questions from yesterday - I need to
> be able to route in the system)
>
> rpoly, lpoly? I have no idea what these are for - How does a left or right
> polygon information help? - should I keep them ?
>
> length - Is it necessary, or am I better to calculate it on the fly using
> postgis functions?
> (area for some data)
>
> ultimately, would keeping them be helpful, or would discarding them be
> prudent to shrink the table, and the information that they provide can be
> calculated if / when I need it?
>
> cheers
>
> Ben
>
>
>
>
> --
>
> Ben Madin
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>
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>
> ben at remoteinformation.com.au
>
>
>
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