[OpenLayers-Users] Geocoding

[Paul Spencer]

I've used a couple:

* http://geocoder.us - based on US Tiger/Line, for fee service or you  
can get the source and build your own

* http://imaptools.com/geocode-us.html - based on US Tiger/Line, buy a  
license

We are using navteq street data now for our maps and geocodes against  
Tiger/Line don't align well so we currently have a home-grown solution  
using imaptools address parser and our own postgis database.

There is some indication that PAGC (http://www.pagcgeo.org/) will  
become more generic, supporting multiple back ends and datasets in a  
cleaner way.  I would probably go with PAGC if that happened.

Cheers

Paul

On 2009-10-14, at 7:09 PM, David Lozzi wrote:

> Hi Paul,
>
> WOW. Thanks for the insight, I've been wondering... What geocode  
> service do you use?
>
> David Lozzi
> Product Development Manager
> Delphi Technology Solutions, Inc.
> (978) 988-8007 x204 - www.delphi-ts.com
>
> Blog  -  LinkedIn  - Twitter
>
>
> -----Original Message-----
> From: Paul Spencer [mailto:pagameba at gmail.com]
> Sent: Wednesday, October 14, 2009 5:54 PM
> To: David Lozzi
> Cc: users at openlayers.org
> Subject: Re: [OpenLayers-Users] Geocoding
>
> This isn't a direct answer, rather I'm answering why services may be
> different and providing some guidance on geocoding based on my
> experience.  Don't read on if you don't care about that.
>
> The short answer is that differences in location returned by different
> geocoders probably depends more on the data source being used by the
> geocoder than the geocoder itself.
>
> Address parsing aside, geocoding is actually a fairly straight forward
> process that probably uses one of two techniques:
>
> technique 1: linear interpolation
>
> * find the record that matches the requested street (a lot goes into
> identifying the correct street, let's leave that out)
> * find a segment of the street with a start and end address that
> contain the requested street number
> * find a point on the segment equivalent to where the requested number
> is between the start and end of the line
>
> What is typically different here between geocoders (assuming they find
> the same street) is the quality of the street number data (is it
> there, is it correct, is it the right way around, is it associated
> with the correct side of the street for odd/even etc) and the quality
> of the geographic line segment that represents the street.
>
> Some services might also use some heuristic data for adjusting the
> interpolated location from being straight linear interpolation to
> something (that may be) more accurate.
>
> technique 2: exact location
>
> if you have tax lot/parcel/cadastral data then you can find an exact
> match on a street address to a parcel associated with that address and
> return the centroid of the parcel (perhaps snapped to the appropriate
> street segment).
>
>
> The other component of a typical geocoder is its ability to parse an
> address and match it to a street segment.  This can involve a lot of
> complicated steps and has to accommodate a seemingly infinite number
> of ways that addresses can be written, missing address components,  
> mis-
> spellings of all parts, incorrect information such as incorrect zip
> code or wrong street type, and more.
>
> You need to evaluate geocoders on a couple of parameters:
>
> * what is the source data they are using and how 'accurate' is it?
>
> * is it accurate enough for you vs what you are paying for it? (a
> commercial data set might be arguably more accurate than a free one
> for instance)
>
> * given how your addresses are expected to supplied, how well does
> each service parse the address, accommodate missing, incorrect or mis-
> spelled data and ultimately map that to the right address
> (disregarding actual accuracy of the address)
>
> Typically, if you are mapping results of geocoding, you will also want
> to ensure the geocoder is using the same source data as for generating
> the maps to minimize the visual impact of differences in the quality
> of the actual street vector data.  In the end, this is usually more
> important than the physical accuracy for many use cases.
>
> Cheers
>
> Paul
>
> On 2009-10-14, at 5:21 PM, David Lozzi wrote:
>
>> Hello,
>>
>> What service are you using to geocode your addresses? I've looked at
>> a couple and I've been getting different results per set.
>>
>> Thanks,
>>
>> David Lozzi
>> Product Development Manager
>> Delphi Technology Solutions, Inc.
>> (978) 988-8007 x204 - www.delphi-ts.com
>>
>> Blog  -  LinkedIn  - Twitter
>>
>> _______________________________________________
>> Users mailing list
>> Users at openlayers.org
>> http://openlayers.org/mailman/listinfo/users
>
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