[postgis-users] Pgrouting directions
Stephen Woodbridge
woodbri at swoodbridge.com
Mon Jan 5 06:45:32 PST 2009
In the example I used below, the road segment would have had 4 points,
but regardless of the number of points, you would use:
pnt[1], pnt[2] for the calculations at the start of the segment.
pnt[numpoints-1], pnt[numpoints] for the calculations at the end of the
segment.
So in postgis you could use:
st_x(st_pointn(the_geom, 1)) or st_x(st_startpoint(the_geom))
st_y(st_pointn(the_geom, 1)) or st_y(st_startpoint(the_geom))
st_x(st_pointn(the_geom, 2))
st_y(st_pointn(the_geom, 2))
st_x(st_pointn(the_geom, st_numpoints(the_geom)-1))
st_y(st_pointn(the_geom, st_numpoints(the_geom)-1))
st_x(st_pointn(the_geom, st_numpoints(the_geom))) or
st_x(st_endpoint(the_geom))
st_y(st_pointn(the_geom, st_numpoints(the_geom))) or
st_y(st_endpoint(the_geom))
If you need to compute the bearing or Azimuth of the segment ends look
at this:
http://postgis.refractions.net/pipermail/postgis-users/2007-November/017768.html
http://postgis.refractions.net/documentation/manual-svn/ST_Azimuth.html
-Steve
Ahmad Bdair wrote:
> Thanx Steve, your information were helpful, but when I think of it it
> confuses me a lot, lets first start with my fields in DB.
>
> Columns are:
> gid serial NOT NULL,
> rod_name_a character varying(50),
> label character varying(50),
> rod_length integer,
> rod_no integer,
> rod_type character varying(1),
> rod_class character varying(1),
> rod_code character varying(10),
> shape_leng numeric,
> the_geom geometry,
> source integer,
> target integer,
> length double precision,
> x1 double precision,
> y1 double precision,
> x2 double precision,
> y2 double precision,
> to_cost double precision,
> "rule" text,
> cost_factor integer,
> main_cost double precision,
> reverse_cost double precision,
> roadclass text DEFAULT 'OTHER'::text,
> cost_reverse_factor integer DEFAULT 5,
>
> Now, I am confused, because I cannot calculate the heading in the first
> place, If I have
> B+---\
> x2,y2
> |
> x1,y1
> A+---/
>
> Now I can calculate the heading simply by comparing x1 and y1 to x2 to
> y2 and which is larger (based on lonlat) so I can judge it, but some
> times I am having this:
> B+---\
> x1,y1
> |
> x2,y2
> A+---/
>
> Now it is misleading, maybe thats why you told me not to use start and
> end point, but wait, what else to use ? According to my DB I cannot
> think of any other way to calculate headings and angles.
>
> Regards
>
> On Wed, Dec 31, 2008 at 7:45 AM, Stephen Woodbridge
> <woodbri at swoodbridge.com <mailto:woodbri at swoodbridge.com>> wrote:
>
> Ahmad Bdair wrote:
>
> Yes, that is something very similar to what I need to do, I
> already wrote a php code to check angels and slopes, but it was
> not accurate, it was more life "if else" code, I check the
> longlat for current segment
>
>
> When you check the angles do not use the end points because they are
> mis-leading.
>
> B+---\
> |
> A+---/
>
> If you have a segment from A to B like the above and you calculate
> the heading or angles based on the vector AB then it will point
> north and is should be pointing west at the end of the vector or
> east at the start of the vector.
>
> You also need to look at weather or not you need to flip segments
> because you route might traverse them from start to end OR from end
> to start.
>
> When I compute angles I grid the angles into 8 pie pieces of 45
> degrees each.
>
> continue straight
> turn slight right/left
> turn right/left
> turn sharp right/left
> make a u-turn
>
>
> with longlat for the previous one, I guess you didn't use the
> database for direction calculation(except for routing) am I
> right? Isn't steps 3 and 4 depends on your angel calculations?
>
>
> I have done this in C in a prototype router I wrote years ago. I
> have implemented it in PHP and in perl in various incarnations in
> the past. The demo page has it written in pgpsql, but it is based on
> my own heavily modified versions of the pgRouting stored procedures.
>
> Hope the above helps with your problems. I find it easiest to work
> out the math problems of computing the angles and headings first and
> make sure they are correct. Then move on to the other issues. If
> your math is wrong computing the headings and angles it will cloud
> all the other results.
>
> Best regards,
> -Steve
>
> I have the roads with two different languages, a road map like
> anyother road map I guess.
>
> On Tue, Dec 30, 2008 at 3:17 AM, Stephen Woodbridge
> <woodbri at swoodbridge.com <mailto:woodbri at swoodbridge.com>
> <mailto:woodbri at swoodbridge.com
> <mailto:woodbri at swoodbridge.com>>> wrote:
>
> bdair2002 wrote:
>
> Hello,
> I am developing an application using Pgrouting with Postgis
> data, everything
> is fine, but now I am looking to do something like Google
> directions guide,
> where it tells you turn left or right, I am wondering if this
> feature is a
> built-in feature in Pgrouting or Postgis.
>
> Regards
>
>
> No this is not a feature that is part of either.
>
> I have implemented a directions explicator, but it is somewhat
> specific to the data that you have, as you need to know what
> column(s) contain the road names, if you have signage information
> then you need to know how to link to that. If you want to be
> able to
> explicate in multiple languages, then you probably need to
> consider
> supporting multiple languages each in a separate table the
> you can
> select from when you generate the languages.
>
> The algorithm is pretty straight forward as the you currently
> get a
> list of segments that make up the route. Then you need to:
>
> analyze each segment and
> 1) check to see if you want to join it with the previous segment
> because the road name matches
> 2) check the angle that it makes with the last segment to
> determine
> right, left, straightness of turn
> 3) compute the compass heading for the segment
> 4) determine if you have signage
> 5) based on these, determine if you are read to explicate and
> which
> predefined instruction you need to explicate an instruction
> 6) loop through all segments
>
> For a demo of this you can try:
> http://imaptools.com/leaddog/routing/dd.html
>
> Zoom into a city in one of the yellow areas, set a start and end
> point and click [calculate route] button. The driving directions
> should show below the button if it was successful, be
> patience the
> routing is running on a 333MHz box.
>
> -Steve
>
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