# [postgis-users] Traverse set distance along a multiline?

Aren Cambre aren at arencambre.com
Tue Dec 28 08:00:17 PST 2010

Just want to again say thanks for all the help on this.

After projecting from 4269 to 3081, I reanalyzed everything and checked my
points sample again. All my points at major intersections and state borders
were <= 1 mile off! That's good enough for me.

Now my next two problems (which I need to research) are:

1. Forcing my algorithm to start mile 0 from a route's the southern or
western extreme. I am guessing I would get the points at 0% and 100% of a
route and the compare them. Whichever "wins" then becomes the starting point
for my traversal. At least for most mainline IH and US routes, the algorithm
to determine the "winner" may takes the route number as input since even =
W-E and odd = N-S.
2. Connecting some routes that are inconsistent or noncontiguous.
Examples I've found in the Interstate system:
- Where I-35W and I-35E split south of Dallas/Ft. Worth, I-35W starts
over at mile marker 0, but I-35E maintains mileposts as if it is the
contiguous I-35 route. So I need to connect I-35 south of
Dallas, I-35E, and
I-35 north Dallas as one route. I think the PostGIS functions exist to
combine these three routes into one.
- I-10 for some reason doesn't follow I-35 near downtown San Antonio,
so I have two I-10s separated by that short I-35 multiplex gap.
Mile markers
on a contiguous route would count up to well over 800 by the Sabine River
(eastern Texas border).

Aren

On Sun, Dec 26, 2010 at 6:35 PM, Aren Cambre <aren at arencambre.com> wrote:

> I am trying to determine mile markers along Texas highways. My starting
> point is the ShapeFile TxDOT Roadways 2010 at
> shp2pgsql to get it into a PostGIS 1.52-enabled Postgres 9.01 database.
>
> I naively thought I could just figure out the number of miles per unit of
> latitude and then traverse each roadway, one mile at a time, using *
> ST_Line_Interpolate_Point*. However, predictably, the more "longitudinal"
> a route, the more error it shows when I compare my calculated mile markers
> to what Google Maps shows.
>
> Again, this is because I was using a consistent ratio of degrees to miles,
> so any route E-W component introduces errors.
>
> So here's the question--does PostGIS allow any way to traverse a route a
> set distance at a time? Specifically, is there a way I can traverse a route
> a mile at a time and then record the points at the end of each mile?
>
> I reviewed the functions available at
> http://postgis.refractions.net/documentation/manual-1.5/reference.html and
> am not seeing anything clear.
>
> In case it matters, the SHP's PRJ file says NAD83.
>
> Aren Cambre
>
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