Line layers from OGR Virtual Spatial Data?

[Frank Warmerdam]

On 7/6/05, jim at ccsservicesofva.com <jim at ccsservicesofva.com> wrote:
> Hi Frank
> 
> Thanks for the super-fast info and recommendation - much appreciated
> 
> I'd just like to clarify my understanding of your suggestion (might help
> some others here too)...
> 
> Say I have 4 data points - an origin, two intermediate points and a
> destination - and the lat/long coords are:
> 35, -70
> 30, -75
> 25, -81
> 20, -85
> 
> Would I then specify THREE linestrings: one for each "leg"?  Which would
> result in the following:
> 
> LINESTRING(35.0 -70.0, 30.0 -75.0)
> LINESTRING(30.0 -75.0, 25.0 -81.0)
> LINESTRING(25.0 -81.0, 20.0 -85.0)

Jim,

No, this would be:

LINESTRING( 35.0 -70.0, 30.0 -75.0, 25.0 -81.0, 20.0 -85.0)

Hopefully still fairly easily translated. 

> If I'm right, then I have a little scripting work to do to assemble the
> LINESTRING strings from adjacent pairs of rows in my table, but that's a
> small price to pay  :)
> 
> If my understanding here is correct, I reckon this should be documented in
> simple terms in the OGR Virtual Spatial Data documentation - from my
> searches of this list, WIKI and even Google today, it seems I'm not the only
> one who's struggled with line layers ex virtual sources.  (I'd volunteer,
> but I'm just a humble newbie round these parts!)

The wiki is currently frozen, so we can't fix that up. But if you propose
specific additions to the VRT format doc, I'll look at adding them.  
However, the driver docs are intended to be a reference specifically
for the driver, not a general tutorial on their use, nor an introduction to
OGC WKT format. 

I think what we really need to do is get the OGR/VRT wiki document
updated and onto the new web site. 

I would add that this is an excellent work item for a new user!  You can
ensure it is talked about at an appropriate level for a new user, and
it takes a load off the core developers.  I would be happy to review 
any material.

Best regards,
-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent