[pgrouting-users] Setting the tolerance argument in assign_vertex_id

[Stephen Woodbridge]

Dan,

One more thing that you might want to consider is if the DMTI data has 
zlevels at intersections, the you might need to mimic assign_vertex_id() 
in the form of assign_vertex_id_3d() where you update your edge 
endpoints with a Z value in the vertices_tmp table and you set the Z 
value to zlevel*FACTOR where FACTOR greater than TOLERANCE

So if you have intersection points like at an overpass where the under 
pass is A-B(0)-C and the overpass is J-B(1)-K

B has and location (x,y) and the zlevel for A-B and B-C is 0 and the for 
edges J-B and B-K the zlevel is 1, so while these are at the same x,y 
they do not intersect. In the assign_vertex_id(), these will get 
assigned the same node, but you need to write assign_vertex_id_3d() so 
they would get different nodes assign based on the zlevel separation.

-Steve

On 8/27/2010 11:31 AM, Stephen Woodbridge wrote:
> I think DMTI data is pretty high quality data. I have used it in the
> past, granted not for routing but, I never noticed any quality problems.
>
> When I did some of the analysis on other data sets, I created extra
> columns on the edges table an did things like mark the edges for all
> that had the same source and target nodes and display them in using
> mapserver and openlayers and displayed the vertices_tmp table with
> deadends as red dots and other nodes as green dots.
>
> Then you can get a quick visualization of the data. Mostly what you want
> to look for is to see if deadends fall on connected lines, this
> indicates a problem. Look for segments with matching source and target
> numbers (color them so they stick out on the map.
>
> If you set the tolerance wrong you will see lots of errors like these.
> I also then run pgRouting and overlay that on the maps and can turn on
> the reference layers for debugging weird routes.
>
> Look at this:
> http://gis.imaptools.com/routing/leaddog/?zoom=10&lat=33.86222&lon=35.51589&layers=B0TTTF&start=35.504139%2033.833331&stop=35.546364%2033.883493&method=STS&lang=eng
>
>
> Open the layer switcher, zoom in and select "Just the Streets" and "Dead
> Ends" and you can see what I mean. "Just the Streets" labels the streets
> with the gid of the edge table so you can go investigate a problem.
>
> -Steve
>
> On 8/27/2010 3:44 AM, Dan Putler wrote:
>> Thanks a lot Steve, this looks really helpful. I can see that routing
>> involves much of the same "art" that geocoding does.
>>
>> Based on past experience, I'm not sure that the road network I am
>> working with is as clean as you and Daniel appear to assume. I'm
>> going to look at the dead-end and other issues tomorrow. I'm also
>> wondering if there is any potential benefit of reading the original
>> shapefile into GRASS which will attempt to clean it, dumping it back
>> into a shapefile from GRASS, and then importing it into PostGIS, or
>> does assign_vertex_id basically do the same cleaning that GRASS
>> does?
>>
>> 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, 8:55 PM Dan,
>>>
>>> I think that picking a number around 0.5 to 1 meter should be fine
>>> for that data. In general it is unlikely that you will have nodes
>>> that are NOT connected but that close together.
>>>
>>> You can also do some analysis of the results like this, after you
>>> run assign_vertex_id():
>>>
>>> alter table vertices_tmp add column cnt int; update vertices_tmp
>>> set cnt = (select count(*) from "<edge_table>" where
>>> vertices_tmp.id=target or vertices_tmp.id=source); select cnt as
>>> connections, count(*) from vertices_tmp group by cnt order by cnt;
>>>
>>> This will analyze the connectedness of you network.
>>>
>>> connections 1 - dead ends 2 - segments connected but only an
>>> intersection if different names 3+ - intersections
>>>
>>> If you run assign_vertex_id() with different tolerance values and
>>> then run this analysis as your tolerance gets too big you will see
>>> a shift of these numbers to the smaller end which is bad.
>>>
>>> Another important analysis you check is:
>>>
>>> select count(*) from "<edge_table>" where target=source;
>>>
>>> this count should be zero unless you have zero length segments in
>>> your data and you can check that with:
>>>
>>> select gid, st_length(the_geom) from "<edge_table>" where
>>> target=source;
>>>
>>> These are a good way to learn about your data.
>>>
>>> -Steve
>>>
>>>
>>> On 8/26/2010 8:37 PM, Dan Putler wrote:
>>>> 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
>>>>>
>>>>>
>>>>
>>>>
>>>
>>>
>>
>>
>
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