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| Subject: | RE: [gislist] Lat/Long to snail mail address? |
| Date: |
10/29/2003 10:30:02 AM |
| From: |
Dimitri Rotow |
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> I've found numerous sites which allow you to enter a snail mail
> address for
> any location in the United States and acquire the corresponding Lat/Long.
> However, I can't seem to find one that goes the other way. Surely, if you
> can go one way, then you can the other.
>
Not really. The "one way" nature of default geocoding arises because street
address databases in the US are not a collection of point locations for each
actual address. Instead, addresses are published in an estimated fashion by
giving the range of addresses over each individual street segment. So, for
example, a section of Main Street in Smallville might be marked as having
the range of addresses from 500 to 600 but there is no information as to
what the actual addresses are on that street segment.
A typical geocoder exploits this information to find the location of 525
Main St by simply interpolating 25% of the distance over the "500 to 600"
segment of Main Street and placing a dot there. It doesn't actually know
where 525 Main St is, but it is taking on faith that if you ask it to
geocode a "525" address then there really is an address like that.
Note that the reverse process doesn't work. Suppose you place a dot about
27% of the way up that Main Street segment. What addresses are "near" to
that dot? You don't really know what the addresses are in that segment, you
only know that any addresses that might be there will fall between 500 and
600. But, you don't know if there are two addresses, 500 and 600, at each
end of the street segment, or if there are two addresses, 501 and 575 Main
Street, or whatever. If you return an address like "527 Main Street" under
the assumption that there are even integer addresses between 500 and 600 on
that segment you will almost always be wrong.
Since there are many possible ways of deciding how to report an address in
this situation, most people who need to do it write a script. In Manifold,
it's a nearly trivial script since all you do is write a spatial SQL query
that finds the nearest street segment to a given point and reports the
address range for that segment. However, what you then do with that address
range information depends on how you want to handle the ambiguity of the
data.
Cheers,
Dimitri
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