[pgrouting-users] Setting the tolerance argument in assign_vertex_id

[Dan Putler]

Thanks Steve, but I do need a bit of a follow on I think, and probably provide a bit more explanation.

I'm using a DMTI CanMap routing layer for BC that started out in NAD83 geographic, which I converted to NAD83 UTM Zone 10N via ogr2ogr, I also shrank it down using a "with" statement in ogr2ogr to cover only part of the province. After I sent my email, I did a bit more Google searching and based on a question in the pgRouting forum, a user using NAVTEQ layers was using a tolerance that worked out to between 1 and 1.5 meters. Based on this, I set the tolerance to 2 meters, it sounds like I should set the tolerance in assign_vertex_id to be much smaller (say 0.15 meters, which is just over 5 inches).

In general, is it better to err on the side of too small or too large a tolerance, or is that a "it depends" question?

Dan

--- On Thu, 8/26/10, Stephen Woodbridge <woodbri at swoodbridge.com> wrote:

> From: Stephen Woodbridge <woodbri at swoodbridge.com>
> Subject: Re: [pgrouting-users] Setting the tolerance argument in assign_vertex_id
> To: "Dan Putler" <putler at yahoo.com>
> Cc: "pgRouting list" <pgrouting-users at lists.osgeo.org>
> Received: Thursday, August 26, 2010, 4:27 PM
> On 8/26/2010 2:51 PM, Dan Putler
> wrote:
> > Hi,
> > 
> > I'm curious how to determine the value of the second
> argument in the
> > function assign_vertex_id(). Most of the examples I
> can find look
> > like:
> > 
> > assign_vertex_id('<edge table>', 0.001,
> 'the_geom', 'gid')
> > 
> > Most of the examples are based on edge tables that
> have SRIDs that
> > correspond to either WGS84 geographic or some sort of
> US State Plane
> > feet. I'm using data that is using a UTM coordinate
> SRID, so the map
> > units are in meters. My question is whether the map
> units should
> > influence the choice of the second argument to the
> function (which I
> > assume is a tolerance between edges), or should I just
> keep with the
> > value 0.001? Implicitly, I'm asking if the parameter
> is in the map
> > units of the SRID.
> 
> Dan,
> 
> Right units is important.
> 
> When I use data in WGS84 that has at lease 6 decimal places
> then I use 0.000001 as the tolerance. What this means if
> that if the distance between two points are less than or
> equal 0.000001 units then they should be considered the same
> point. 0.000001 degrees == ~5 inches if I did the math
> right.
> 
> I think there are two issues you need to be aware of:
> 
> 1) units
> 2) your data and what precision it was built at
> 
> HTH,
>   -Steve
> 
>