[postgis-users] Distances off in the Southern US

[Mike Hostetler]

I'll try to answer everyone's suggestions:

I didn't know that Yahoo's service was in 4326.  That did change things up a
bit.  And, yes, I did copy the value from Birmingham to Miami twice. Sorry
about that. . .

My customer was comparing my results to a different source but it seems
close to Google Maps.  I know that I was using the road miles in Birmingham
to Miami (
and other southern cites) but my other mileage to the northern cities was
>5% off from the Google Maps was.  So that I was kinda close.

Now I combined a lot of the suggestions, so now my Geometry looks like:
select AddGeometryColumn('cities','geom',2163,'POINT',2);
UPDATE cities SET  geom=transform(setsrid(makepoint(longitude,
latitude),4326),2163

Birmingham to Miami is:
leader=# select distance( (select geom from cities where id=104), (select
geom from cities where id=41) )*0.000621371192 as miles;
      miles
------------------
 656.738138375537
(1 row)


Which is around 13% off in the other direction.

But  McHenry to Dallas is now off from what I had:
leader=# select distance( (select geom from cities where id=26251), (select
geom from cities where id=67) )*0.000621371192 as miles;
     miles
---------------
 808.949257448
(1 row)

it was 997, which was close to Google Maps. Now it's further off.

But I understand what everyone is saying, and makes sense that flying
distance would be shorter than the road distance.  I think I'm comparing
apples to oranges and it was by chance that some random samples I tested
with were close.  I think I need to find some interstate shapefiles, put
them in, and have PostGIS calculate the distance between those points on
that route.

Thanks everyone.

On Mon, Aug 15, 2011 at 11:04 AM, Paul Ramsey <pramsey at opengeo.org> wrote:

> You're using polar stereograpic as your projection?
>
> http://nsidc.org/data/atlas/epsg_32661.html
>
> Well, yes, the farther south you go, the more over-determined your
> distance will be.
>
> Use the geography type, as suggested below, or the
> st_distance_spheroid() function, or just use a projection that is more
> conformal for the area of interest (continental USA?
> http://prj2epsg.org/epsg/2163)
>
> P.
>
> On Mon, Aug 15, 2011 at 3:50 AM, Mike Hostetler
> <mike at squarepegsystems.com> wrote:
> > Hello,
> > I'm somewhat new to GIS and I have a problem that I thought appeared to
> be
> > simply using a wrong projection or datum, but it seems to be a bit more
> > subtle than that.
> > I have a table of cities in the US and I'm trying to find distances
> between
> > them. When I use a city that is in the northern US, it works fine.  When
> I
> > try to find the distance between two cities in the Southern US, the
> distance
> > becomes way off.
> > I setup a Geometry in my cities table and populated it like the
> following:
> > select AddGeometryColumn('cities','geom',32661,'POINT',2);
> > UPDATE cities SET  geom=transform(setsrid(makepoint(longitude,
> > latitude),4269), 32661)
> > (I find the latitude and longitude from the Yahoo Geocode service)
> > A distance calc from McHenry, IL to Dallas, TX is calculated as:
> > select distance( (select geom from cities where id=26251), (select geom
> from
> > cities where id=67) )*0.000621371192 as miles;
> >      miles
> > ------------------
> >  996.717850542391
> > (Google Maps reads as 972, off by 25 miles or off around 4%)
> >
> > But Birmingham, AL, to Miami, FL is calculated as:
> > leader=# select distance( (select geom from cities where id=26251),
> (select
> > geom from cities where id=67) )*0.000621371192 as miles;
> >       miles
> > ------------------
> >  996.717850542391
> > (Google Maps reads as 767, off by 120 files, or 13%).
> > I can handle a little error, as long as it's somewhat small (<5%).  But
> this
> > is way off.
> > Again, it smells to be to be a datum or projection issue to me, but I'm
> not
> > sure how to find the sweet spot to be accurate everywhere.
> > Your input is appreciated.
> >
> > --
> > Mike Hostetler
> > SquarePeg Systems
> > http://www.squarepegsystems.com
> >
> > _______________________________________________
> > postgis-users mailing list
> > postgis-users at postgis.refractions.net
> > http://postgis.refractions.net/mailman/listinfo/postgis-users
> >
> >
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>



-- 
Mike Hostetler
SquarePeg Systems
http://www.squarepegsystems.com
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