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

[Ben Madin]

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




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Ben Madin
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