Variation in lat/long distances

[Kelly Koehn]

I'm working with Level III data pulled from their FTP site (in the process 
of setting up a NOAAPORT system though).  It all started with a desire to 
learn how to read it and then it just built into something more.  I don't 
use anything to process the files and read them in with PHP and decode 
everything bit by bit :)

I've been doing some reading and I guess calculating the distance over 
longitudes are somewhat more complicated because the distances change as the 
latitude changes.  Anyone know how to calculate that?

Kelly

----- Original Message ----- 
From: "Gerry Creager" <gerry.creager at tamu.edu>
To: "Kelly Koehn" <kkoehn at ASPIREDILLUSION.COM>
Sent: Sunday, May 21, 2006 12:36 AM
Subject: [SPAM] Re: [UMN_MAPSERVER-USERS] Variation in lat/long distances


>I suspect you and I need to talk.  Are you using LEvel II or Level III 
>data?  We're doing a fair bit w/ Nexrad data and using Gempak to create the 
>imagery for display.
>
> gerry
>
> Kelly Koehn wrote:
>> I'm working with some NEXRAD radar data and reading the binary data and 
>> drawing it on a map.  I'm using Mapserver on the backend to produce the 
>> map.  What i'm basically doing is calculating 124 nautical miles (142 
>> statute miles) from a center point (the radar) and then drawing my radar 
>> data on top of that.  I found this nifty little formula on the net that 
>> supposedly converts nautical miles to statute miles but it doesn't seem 
>> to really be working as expected.
>>  The formula is as follows:
>> (((sm * 0.0254) * 63360) / 1852) / 60 = nm
>> where sm = statute miles and nm = nautical miles
>>  At that point I take the value that I receive above and do the 
>> appropriate + or - calclations from the radar's location (lat/long) and 
>> get the extent for the map which is pushed through the URL.  What I've 
>> noticed is that the latitude seems to be correct but the longitude always 
>> falls short 20 or 30 miles from what it should be (almost cutting off a 
>> county on either side).  Has anyone else experienced this?  I've checked 
>> the x and y distances and they are both equal.  I'm producing a 460 x 460 
>> GIF image that uses the nationalatlas.gov SHP files.  I'm using the 
>> county and states sets.
>>  I can post my .map files and anything else that might be helpful 
>> (including sample images).
>>  Thanks
>> Kelly Koehn
>>
>
> -- 
> Gerry Creager -- gerry.creager at tamu.edu
> Texas Mesonet -- AATLT, Texas A&M University Cell: 979.229.5301 Office: 
> 979.458.4020  FAX 979.862.3983
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>