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" <[EMAIL PROTECTED]>
To: "Kelly Koehn" <[EMAIL PROTECTED]>
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 -- [EMAIL PROTECTED]
Texas Mesonet -- AATLT, Texas A&M University Cell: 979.229.5301 Office:
979.458.4020 FAX 979.862.3983
MAIL: AATLT, 3139 TAMU
Physical: 1700 Research Parkway, Suite 160,
College Station, TX 77843-3139
