GPS: Centroid vs. Precision Polygon Based data for Latitude & Longitude geopairs

[Rik Brown]

A general GPS question:

I'm looking at latitude and longitude pairs for street addresses that
are listed in 2 formats:

1) Centroid Based
2) Precision Polygon Based

The database lists both formats per street address (and they are NOT
EXACTLY the same numbers).

What is the difference in the 2 types and which would tend to be the
most accurate for a street address?

Does anyone know which method might be used by Google for street
addresses?

Thanks. -- Rik


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Rik Brown

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[Rik Brown]

Any GPS gurus out there that know the answer to my question in the
previous message? -- Rik

[claudegps]

On 19 Dic, 20:51, Rik Brown <Rik.Brown.550...@no-mx.forums.travel.com>
wrote:

Rik, I think it's not a specific question for "GPS gurus".
It seems more a data formats (used in mapping)...
Do you have references explaining
Precision Polygon Based and Centroid based that could help understand
better your question?

[Rik Brown]

claudegps;3386810 Wrote:
> Rik, I think it's not a specific question for "GPS gurus".
> It seems more a data formats (used in mapping)...
> Do you have references explaining
> Precision Polygon Based and Centroid based that could help understand
> better your question?

Thanks for your reply.

To answer your question: unfortunately not.

I have a lot of latitude/longitude pairs that for a single address are
fairly much the same, yet different.

latitude centroid: 61.1515
longitude centroid: -149.8227

latitude polygon: 61.180837
longitude polygon: -149.816495

The sample street address (just happens to be a Quiznos restaurant) is:

3421 E Tudor Rd
Anchorage, Alaska 99507 (USA)

I can see that the "polygon" method is more precise in a numerical sense
as it has more digits to the right of the decimal point. However, there
are discrepancies in the numbers besides the "precision" of the decimal
-- especially looking at the latitude numbers.

Both ways of creating the lat/long pairs where created from the street
address above. I just can't figure out which method I should use for
mapping purposes.

Thanks. -- Rik

[claudegps]

On 21 Dic, 09:10, Rik Brown <Rik.Brown.553...@no-mx.forums.travel.com>
wrote:

> claudegps;3386810 Wrote:
>
> > Rik, I think it's not a specific question for "GPS gurus".
> > It seems more a data formats (used in mapping)...
> > Do you have references explaining
> > Precision Polygon Based and Centroid based that could help understand
> > better your question?
>
> Thanks for your reply.
>
> To answer your question: unfortunately not.
>
> I have a lot of latitude/longitude pairs that for a single address are
> fairly much the same, yet different.
>
> latitude centroid: 61.1515
> longitude centroid: -149.8227
>
> latitude polygon: 61.180837
> longitude polygon: -149.816495
>
> The sample street address (just happens to be a Quiznos restaurant) is:
>
> 3421 E Tudor Rd
> Anchorage, Alaska 99507 (USA)
>
> I can see that the "polygon" method is more precise in a numerical sense
> as it has more digits to the right of the decimal point. However, there
> are discrepancies in the numbers besides the "precision" of the decimal
> -- especially looking at the latitude numbers.

You should figure out if the differences are related to the accuracy
used to retrieve the information (which database in more precise) or
if the two methods have a different reference.
You could try to figure it out looking at a certain area. If the
difference is (nearly) costant, then maybe they have the same
information but a different reference (even if I think it's not
likely... I think that everything is WGS84...)

> Both ways of creating the lat/long pairs where created from the street
> address above. I just can't figure out which method I should use for
> mapping purposes.

Does centroid and polygon data sets come from different providers?
If yes, I can suppose that there are diference coming from the
accuracy used to retrieve the information.
You should figure out which databes is better (maybe trying checking
the accuracy of several places in different areas)
Anyway be prepared to have deceptive results (one DB may be better in
one area and worst in another one)

Im not sure I have understood what you want to do with the data
anyway :)
Do you need the best accuracy possible, or the worst accuracy of the
may satisfy your requirements?

[Gene E. Bloch]

On 12/21/2011, claudegps posted:

Whatever his concern is, Googling for the two terms turns up posts by
Rik in two forums, so he is really concerned.

BTW, Google seems to show no real information about those terms other
than links to Rik's posts :-)

--
Gene E. Bloch (Stumbling Bloch)

[Gene E. Bloch]

On 12/21/2011, Gene E. Bloch posted:

OTOH, if you go here:

http://en.wikipedia.org/wiki/Geocoding

and search for the two terms "centroid" and "polygon" (one at a time,
please!), perhaps what they have to say will offer a clue.

[claudegps]

That's why I was asking Rik for more details :) Googling I really
had no clues.
Anyway your link on wiki has some.
My interpretation is that the "polygon based" bases the location on
the approximated position taking into account that a polygon (i.e. a
segment of a road) includes a certain numbers of locations (i.e. from
10 to 50).
So the approsimation takes the segment and puts n "10" at the start, n
"50" at the end, and the other street numbers are interpolated in
between.
This may clear clearly lead to big errors in certain cases (10 to 50
are not spaced equally in reality)

Centroid position instead should take the center of the specific
parcel. This should indicate the real position of the place, but not
it's position on the road. (if a parcels extends far from the road,
the lat/lon indicated by teh centroid may be far from the nearest
road). I think that navigation systems, are more likely to use the
"polygon" based because in that way all the street numbers are easily
linked to the road.

(You my above interpretation at your own risk! ) :)
Anyway if Rick has two DBs of a certian are, we could do some
comparisons and maybe check if my interpretation is correct.

[Rik Brown]

Wow! I guess I bit off more than I can chew. That's a lot of
information. For my needs, I guess I'll stay with the centroid pairs
since my database has more of those than the polygon pairs.

Thanks for all the information. -- Rik

[Rik Brown]

Well, here is what I just found out on the matter:

> The Centroid coordinates are central to the postal code, the polygon
> coordinates are the address accurate ones.

I guess that is what I needed to know.

Thanks for all the assistance. -- Rik