Posted by Jerry Stuckle on 10/11/05 00:44

Nel wrote:
> "Jerry Stuckle" <jstucklex@attglobal.net> wrote in message
> news:kuadnX9eZvJdKNfeRVn-ug@comcast.com...
>
>>
>>Nel,
>>
>>First of all, you have a problem with your equation. It assumes 1 degree
>>if longitude has the same distance as 1 degree of latitude - which only
>>occurs at the equator. Everywhere else, 1 degree of longitude is less
>>than 1 degree of latitude. I don't have the correct equation handy, but I
>>found it one time quite easily with a google search.
>
>
> The figures are in metres, not degrees so hopefully this will not be a
> problem.
>
>

Metres from where? Some central point (i.e. Greenwich Observatory)?
For a (relatively) small country like GB, it probably wouldn't be that
much of a problem. But for Russia or Canada it could be a problem :-)

>
>>For instance, say you want everything within 25 miles. In your program,
>>define a 50x50 mi. square with the "from" point in the center. That is,
>>find the longitude 25 mi. east and west with no change in latitude. Then
>>find the maximum and minimum latitude with no change in longitude.
>
>
> Using a square would make it easier. I had visioned calculating the
> hypotinuse for each member.
>

You will have to do that eventually (not all locations in the 25km.
square will be within a 25 km. circle).

>
>
>>This now gives you a range that all the target post codes must be in. Now
>>let the database do the work for you. Search the database for all
>>postcodes where the longitude is between the min and max above, and the
>>latitude is within the min and max.
>
>
> Genius. So I can narrow down the processor work to a limited few.
>

Definitely. Calculations such as this are very CPU intensive. You want
to perform the calculation on as few items as necessary.

>
>>Once you have this (much smaller) set of postcodes, you can run your
>>equation again against every one to determine if it is within the 25 mile
>>radius circle.
>
>
> One final question, once I have narrowed down the sqlresult to those 25, and
> I calculate the distance, how do I sort the sql results in the order of the
> distance calculation?
>

You really can't, because you can't determine the actual distance until
you perform the final calculations. I'd suggest placing them in an
array of postalcode=>distance and use asort to perform the sort.

> Thanks,
>
> Nel.
>
>


--
==================
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Jerry Stuckle
JDS Computer Training Corp.
jstucklex@attglobal.net
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