Hi Dan,<div><br></div><div>If you think your data is very accurate, then setting this value very low is the best. It can be millimeters or even less. It doesn't matter how small you make it if you don't want two points to snap. I think the problem often comes up when your data projection is in degrees and you set the value too high. Then it can happen that all your data snaps into one point.</div>
<div><br></div><div>Daniel</div><div><br></div><div><br><br><div class="gmail_quote">2010/8/27 Dan Putler <span dir="ltr"><<a href="mailto:putler@yahoo.com">putler@yahoo.com</a>></span><br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
Thanks Steve, but I do need a bit of a follow on I think, and probably provide a bit more explanation.<br>
<br>
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).<br>
<br>
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?<br>
<br>
Dan<br>
<br>
--- On Thu, 8/26/10, Stephen Woodbridge <<a href="mailto:woodbri@swoodbridge.com">woodbri@swoodbridge.com</a>> wrote:<br>
<br>
> From: Stephen Woodbridge <<a href="mailto:woodbri@swoodbridge.com">woodbri@swoodbridge.com</a>><br>
> Subject: Re: [pgrouting-users] Setting the tolerance argument in assign_vertex_id<br>
> To: "Dan Putler" <<a href="mailto:putler@yahoo.com">putler@yahoo.com</a>><br>
> Cc: "pgRouting list" <<a href="mailto:pgrouting-users@lists.osgeo.org">pgrouting-users@lists.osgeo.org</a>><br>
> Received: Thursday, August 26, 2010, 4:27 PM<br>
<div><div></div><div class="h5">> On 8/26/2010 2:51 PM, Dan Putler<br>
> wrote:<br>
> > Hi,<br>
> ><br>
> > I'm curious how to determine the value of the second<br>
> argument in the<br>
> > function assign_vertex_id(). Most of the examples I<br>
> can find look<br>
> > like:<br>
> ><br>
> > assign_vertex_id('<edge table>', 0.001,<br>
> 'the_geom', 'gid')<br>
> ><br>
> > Most of the examples are based on edge tables that<br>
> have SRIDs that<br>
> > correspond to either WGS84 geographic or some sort of<br>
> US State Plane<br>
> > feet. I'm using data that is using a UTM coordinate<br>
> SRID, so the map<br>
> > units are in meters. My question is whether the map<br>
> units should<br>
> > influence the choice of the second argument to the<br>
> function (which I<br>
> > assume is a tolerance between edges), or should I just<br>
> keep with the<br>
> > value 0.001? Implicitly, I'm asking if the parameter<br>
> is in the map<br>
> > units of the SRID.<br>
><br>
> Dan,<br>
><br>
> Right units is important.<br>
><br>
> When I use data in WGS84 that has at lease 6 decimal places<br>
> then I use 0.000001 as the tolerance. What this means if<br>
> that if the distance between two points are less than or<br>
> equal 0.000001 units then they should be considered the same<br>
> point. 0.000001 degrees == ~5 inches if I did the math<br>
> right.<br>
><br>
> I think there are two issues you need to be aware of:<br>
><br>
> 1) units<br>
> 2) your data and what precision it was built at<br>
><br>
> HTH,<br>
> -Steve<br>
><br>
><br>
<br>
<br>
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