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» Online Classroom   »   » Public Discussion of Inland and Coastal Navigation   » Where the fugawi?? is Los Alamitos Airfield

   
Author Topic: Where the fugawi?? is Los Alamitos Airfield
lmirwin


 - posted December 29, 2009 11:34 PM      Profile for lmirwin           Edit/Delete Post 
In trying to solve a barometer calibration problem in the Marine Weather online course I ran into these apparent anomalies:

http://adds.aviationweather.gov/metars/stations.txt provides the location of Los Alamitos Airfield as 33 46'N and 118 02'W

Plotting this location on Google Earth shows the location to be at a bearing of 147 degrees and 1.66 NM displaced from the airfield !!!

Checking a different website http://en.allmetsat.com/metar-taf/california-los-angeles-san-diego.php?icao=KSLI for the KSLI Metar office it provides the location of Los Alamitos Airfield as 33 47'N and 118 03'W. Clearly much better but this location is still in the farmland SE of the airport proper as plotted on Google Earth.

Checking the FAA location for the airfield yields the following
33.7900322W / 118.0514292N which on Google Earth puts you right smack between the parallel runways 4-22L and 4-22R.

Further complicating this anomaly is that the barometer is on a vessel in Alimitos Bay, Long Beach CA with coordinates provided as 33D 44'N 118D 07'W. This location is NOT Alimitos Bay but rather is outside of the breakwaters to Alimitos Bay and just off of Seal Beach, Long Beach CA.

Finally plotting the coordinates of OHBC1 (weather buoy) on the same Google Earth map exactly where the National Buoy Center location shows it to be.... just off a pier

Can anyone explain what the root cause of the discrepancies are??

Look forward to the feedback

Leslie Irwin

David Burch


 - posted December 31, 2009 03:06 PM      Profile for David Burch           Edit/Delete Post 
We have three issues to resolve or address:

(1) location of the airport

(2) location of the boat

(3) location of a weather station

First, it is right and always valuable to question such things. No detail is too small to not be at least considered. When navigating, if you see a light on the horizon you cannot identify, it is not right to just let it go. If it should come to the point that your crew, your associates, or your family start heckling you because of your care for the details, then simply track down the details quietly without broadcasting what you are doing. But good navigation, like so many things, is in the details.

Let us take each one of these individually as the time allows.

From: Starpath, Seattle, WA
David Burch


 - posted December 31, 2009 03:20 PM      Profile for David Burch           Edit/Delete Post 
(2) where is the boat.

"We want to calibrate a barometer on a vessel in Alimitos Bay, Long Beach, CA (33° 44'N, 118° 07'W)."

But as you note, this is actually outside of the Bay. The hint here is there are no decimals given at all. the position has been rounded off. The boat was actually at 33° 44.947'N, 118° 06.842'W. That precision, however, does not affect the problem at hand in any way, so it was not given.

*** Notice too, there was a typo in the problem and it should have read 33° 45'N, not 44'N. This has now been fixed. Thanks for pointing this out.

From: Starpath, Seattle, WA
David Burch


 - posted December 31, 2009 03:55 PM      Profile for David Burch           Edit/Delete Post 
(3) location of a weather station OHBC1 is listed as 33.720 N 118.272 W.

This is correct. And although it has the word "weather buoy" on its web page, and it is on the link called national data buoy center, this one is not a buoy. This site NDBC covers buoys, lighthouses, and other stations, not all floating on the water.

This one is on a building roof at the end of a pier in LA.

Here are some pictures of the actual station
http://tidesandcurrents.noaa.gov/stationphotos.shtml?stnid=9410660&photo=J

From: Starpath, Seattle, WA
David Burch


 - posted December 31, 2009 04:48 PM      Profile for David Burch           Edit/Delete Post 
(1) location of the airport. These days it is easy to see where an airport really is from Google Earth. There are also official maps, such as this one.

The problem again is rounding off. The FAA does not round off but are giving location of the runways, often the location of the highest point on the main runway, or some central value such as in this case.

The location of the weather station itself is almost impossible to pinpoint. These are ASOS automated stations usually, setting off in a nearby field. Since the metar coordinates are rounded off again, we do not know where they are. I had an ongoing challenge with this when working on the Barometer Handbook section dealing with converting elevated station pressures to equivalent sea level pressures. In some cases, even prominent air ports, there would be as many as 3 different elevations listed in various places for the official barometer. Also, strangely enough, this information is treated as private by the FAA and NWS agencies that have control over it.

Each of the ASOS stations feeds data into a particular software program and only those privy to the actual code of the program knows the precise elevation that is used in the pressure computations. Please see the appendix to the Barometer Handbook for more discussion of this.

In short, the lat-lon issue is only the tip of the iceberg when it comes to tracking down key weather related data from these reporting stations.
-------------

However, the purpose of this exercise is the use of these official reports for home made pressure calibrations. The location data will typically be rounded to the nearest whole minute, which is about half a mile. This is in practice plenty accurate enough to do this. You will get variations in data as time goes by due to timing issues and even uncertainties in the official sources. But the longer you do it, the better your barometer calibration becomes.

A good site for evaluating the pressure data of any of the metar sites is from the Gladstone Family, which as I understand it is all volunteer work. See http://weather.gladstonefamily.net/site/KSLI and then check the link where they monitor the pressure reports compared to nearby stations.

From: Starpath, Seattle, WA
David Burch


 - posted December 31, 2009 04:59 PM      Profile for David Burch           Edit/Delete Post 
Related thought on the lat-lon rounding off issue.

Again let me stress the value of pursuing things like this, even in such dead ends. When you use Tidal Current tables, for example, they specify the location of the station in lat lon rounded to the nearest whole minute, just like this was done above. If you do not fully realize what they have done, you can go astray and get into trouble.

The actual location of the station is not at these rounded locations, but at some other more precise location that rounds off to what is shown. Tidal currents can be dramatically different just a hundred yards from the reference station, so it is important to know this and be aware.

Usually the text description of the station location is much more precise, such as "1.3 nmi SW of Point Doda."

(Tide height, on the other hand, is another matter. Tide heights do not matter much at all on precise location, so you do not get into trouble using their station locations right out of the book.)

From: Starpath, Seattle, WA
lmirwin


 - posted December 31, 2009 07:46 PM      Profile for lmirwin           Edit/Delete Post 
Thank you for the erudite investigation and insight.

The devil is in the details with navigation and my personal perspective is "constant vigilance"

David Burch


 - posted December 31, 2009 08:06 PM      Profile for David Burch           Edit/Delete Post 
Thanks for your reply.

Just noticed that this discussion about a marine weather course quiz question is in the wrong discussion group... though in fact these topics are fairly generic. So, i will move this to the marine weather student discussion group so others working on the quiz will see it.

Thanks again for raising these interesting points.

From: Starpath, Seattle, WA


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