Magnetic Declination

[Major Lord]

Not being able to memorize the entire magnetic declination for the entire USA, I wonder if it is safe to just use 13 Degrees positive across all of CA and NV, which pretty much splits the Western States. Do you ground team people out there phone in your magnetic bearings and let the IC plot them for you, or transmit corrected bearings?

Major Lord

[Pingree1492]

I tried that once after I first got my GBD qualification, and I got some truly screwed up bearings.

It's always best to request information from the field in the format that they collect/receive it.  Leave the calculations and such to the folks at mission base, saves your GTL's (& Pilots) headaches, and yourself from inaccurate information.  So in your example, I always have the bearing transmitted in Magnetic Bearings from Ground Teams.

I do the same thing with coordinates- I'll specify (or clarify) which Datum and format they are wanted in, then make sure everyone has their GPS's set up properly before leaving for the sortie.  Saves mucho headaches later on.

[es_g0d]

It generally a good idea to be like President Ronald Reagan, who said (of the Soviets): "Trust, but verify."

In other words, I'll radio bearings to mission base and specify the format of the bearing, magnetic or true.  Personally, I prefer to have them work the whole problem on their own, so I'll typically send it in magnetic.  I'll then do the math and change them to true for plotting on my own chart.  I trust mission base, but I'm going to come up with an independent solution to verify their results.

Magnetic variation does change over time, but generally not very quickly.  The best easily-obtained source of magnetic variation comes from a Sectional chart.  If you want to cover a larger area, a WAC chart would work also.  They're not very expensive if you obtain the recently-outdated ones from a chart reseller or even a pilot you know.

I don't worry too much about datum for SAR; WGS 84 is the ONLY datum as far as I'm concerned, and its the one all aviation products use.  Set a GPS to that and forget it!  There's only a few exceptions I'd consider doing otherwise, but are beyond the scope of this post.

Pingree1492 makes a good point with coordinate format; this can cause severe headaches, and I've seen it screwed up more often than I care to admit.  Fortunately, the most-oft confused formats are DD MM.mmm (Degrees, Minutes, decimal minutes) and DD MM SS (Degrees, Minutes, Seconds).  In these cases, the minutes will be the same and the decimal or the seconds cause the problem.  Fortunately, the maximum error at this point is now 1 nautical mile square.  Of course, that's not a lot of fun when you're on foot.  It IS best to specify beforehand.  It seems to me, through unscientific survey, that most aviation GPSs are set up in the DD MM.mmm format.  AFRCC provides coordinates in this format as well.  Handheld GPSs can vary widely, and are generally easily changed by a skilled user.

Of course, if there is misunderstanding between a mission base and a team in the field as to the coordinate format, it can create gross errors.  Compound this with magnetic-to-true errors, and your triangulation solution may not work so well.  Of course we haven't even considered operator error, or the accuracy of the DF unit in question.

These are common errors, and its unlikely that we'll ever stamp them out.  Trust, but verify!

[Eclipse]

Just make sure your operations are in a state where the declination is functionally zero - we discuss it here as a matter of information during training, but don't usually even address it during ops.   

[Pingree1492]

I don't worry too much about datum for SAR; WGS 84 is the ONLY datum as far as I'm concerned, and its the one all aviation products use.  Set a GPS to that and forget it!  There's only a few exceptions I'd consider doing otherwise, but are beyond the scope of this post.

I used to not worry about the Datum too, until I went out on a missing person search in coordination with a local SAR agency.  They use UTM coordinates, and a Datum different than WGS 84 (don't remember which one off the top of my head though).  Fortunately, I'm not that comfortable working in the UTM system, so I checked my GPS against one of their teams before leaving the ICP and found the error.  This would have dramatically screwed up the coordinates I radioed in that day had I not done that.

After that, I always verify Datums 

As far as coordinate formats, I prefer the DD MM.mmmm format- it seems to be a lot more accurate when plotting positions as a GBD.  Plus it's not too hard to convert in the field if I want to plot something on a paper map.

[ol'fido]

When I was in the Army, I was stationed at Schofield barracks.  The land nav course there is one of the toughest in the army.  We learned a very simple method of converting from magnetic to grid and back.  The declination in Hawaii at the time was 11 degrees. To go from magnetic to grid, think going from major to general so you would add 11 because you are going up. To go from grid to magnetic you would subtract bacause you were going from general to major.  If you are east of the zero line, just reverse this. This was something us grunts could wrap our brains around.

Now here in Southern Illinois, I'm so close to the zero line I don't really worry about it becauses the deviation is less than my fudge factor.

[sardak]

Not being able to memorize the entire magnetic declination for the entire USA, I wonder if it is safe to just use 13 Degrees positive across all of CA and NV, which pretty much splits the Western States. Do you ground team people out there phone in your magnetic bearings and let the IC plot them for you, or transmit corrected bearings?
Major Lord

Whether you can use 13 degrees east declination across all of CA and NV depends on how accurate you want to be, and what you're doing.
The NW corner of California is about 16 degrees (15° 47')
The NE corner of Utah is about 13 degrees (13° 24')
San Diego is about 12 degrees (12° 19')
Yuma is about 12 degrees (11° 46')
Reno is about 14 degrees (14° 23')

I teach all my land nav students (which includes more than just CAP) to send bearings (really azimuths) to base in degrees magnetic and let base convert. However, everyone must know how to convert in the field, and I make them learn how, even if they do send bearings in as degrees magnetic. It's much, much easier to plot true bearings on a map. The north end of the map can be pointing in any direction and you don't need to worry about nearby iron and magnetic objects.

I also tell my students not to set the declination into their compasses (some allow the user to set the declination so that the compass reads in degrees true). You also have to check whether your GPS unit is set to true or magnetic north.

Here's a link to an online declination finder:
http://www.ngdc.noaa.gov/geomagmodels/Declination.jsp

Here's the link to a standalone version to run on a PC. Skip to the bottom of the page if you don't want to register.
http://www.ngdc.noaa.gov/wmmsurvey/magneticSurvey.do?plang=g

I don't worry too much about datum for SAR; WGS 84 is the ONLY datum as far as I'm concerned, and its the one all aviation products use.

Yes, all aviation products (printed and in GPS units) use WGS 84, as do DoD products. Some FAA products are actually in NAD 83, which, unless you're surveying, is functionally equivalent to WGS 84. Sarsat hits and NTAP points are also in WGS 84.

However, many USGS products use NAD 27 (which is what Pingree's were in her example). At my location, the difference between NAD 27 and WGS is about 50 meters for lat/lon coordinates. But in UTM, the difference is just over 200 meters. Depending on what you're doing, you may or may not need to account for datum differences between the map and GPS. The differences between NAD 27 and WGS 84 vary across North America (NAD = North American Datum).

I don't even get into the whole degrees/decimal degrees, deg/min/decimal minute, deg/min/sec discussion.  Here is what the National Search and Rescue Committee (NSARC, the fed agencies responsible for SAR) published for use when the Federal Response Plan is invoked.
Latitude and Longitude should be in one standard format: DD-MM.mmm.  If required, use only 3 digits to the right of the decimal; 1 or 2 digits is acceptable.

NSARC says that the US National Grid should be the primary coordinate system for ground SAR and lat/lon for air SAR.

Which coordinate system and lat/lon format to use is a discussion for another thread, and could be quite entertaining.

Mike

[RiverAux]

How often are ground teams calling in bearings to mission base or the IC?  I'm sure it happens, but I've never done it.  I don't think that I've ever operated with a map on a CAP mission with a scale fine enough that it would make a difference whether you were taking declination into account or not.  I'm positive that no one in my Wing operates with them either on the ground or at base.  The only time we see an actual topo map is when we're doing map training. 

While I think it is a good general practice to set your compass for the declination, a case could be made that CAP ground teams SHOULD NOT do it since the compasses in our airplanes don't have that option and if we want to send an aircrew on the same exact heading as a ground team suggests, the declination would screw up the aircraft unless you remember to convert it before giving the bearing to them.

[es_g0d]

I think we've just identified an outstanding training opportunity in your wing, RiverAux!  I'm certain yours is not the only one... VERY certain, as it seems like it must be explained every mission...

With the advent of software such as Google Earth and other pay-for programs, plotting bearings to the ELT shouldn't be terribly difficult.  It will take practice.

I was involved in some training a few years back and attempted to demonstrate triangulation ... and found that the "cocked hat" solution was reasonable, but not great.  When I had my "DOH!" moment, I realized that I had forgotten to add the 10 degrees East variation for our area.  And suddenly, everything aligned...  So I ended up emailing everyone the new map with a screenshot of the accurate bearings, and stern warnings not to forget...

[Eclipse]

How often are ground teams calling in bearings to mission base or the IC?  I'm sure it happens, but I've never done it. 

Anytime I'm GBD - SOP is briefed that when they hear the ELT, and anytime they stop at a point which is a major change, they call into mission base with position and azimuth.

Especially with multiple teams it helps me coordinate their efforts, identify likely areas, and makes for a better, more efficient search.  It also helps identify teams that are either not using their equipment correctly, or hearing a second ELT.

Any Gazetteer, decent street map, and most PC-base computer programs are granular enough to provide good tracking.

I use Garmin Mapsource, dropping waypoints with different symbols / colors for each team as they report their position during the roll-call checks, and the routing tool to plot the azimuths.  That generates images like the below:



Which can then be printed and sent upstream to planning, posted on the status boards for reference, and included with the paperwork when things are closed out.

I also request locations of aircraft that are doing coordinated searches from AOBD and plot them as well.

The above was the first one I could find, and for whatever reason on that day I chose not to display the streets, so I'm not sure of the scale.

[RiverAux]

Perhaps my experience isn't typical, but with ELTs you are either going to first acquire the signal when you're so close to the target that mapping is entirely unnecessary (i.e., "Go down to the airport and turn off that darned ELT") or it is within a mile or two of where you're picking up the signal, which is close enough that taking the declination into account isn't entirely useless, but isn't that helpful either since you're eventually going to home in on one of those signals and follow it to the source anyway.

I suppose that if we were able to pick up the signal from the ground when it was 5-15 miles away then you would definetely want to worry about it since triangulation at that range would be a big help.  But, even then most (but certainly not all) of the time, we would probably have an aircraft out there to narrow it down even more. 

In any case, thats a neat tool Eclipse. 

[Eclipse]

Danke.  I also use it for flight tracks when I'm aircrew.  The waypoints in this case are photos or photo runs.


It really depends on where it is and how you're looking for it.

In an urban, on-airport situation, that's mostly correct, however in a "stash it in the weeds" situation with an aircraft involved, not so much. 

The aircraft may pick it up miles out, and if the area is heavily wooded enough, it may take a long time to find it. 

One way to negate declination is for the IC, GBD, etc., to orientate a map to North using a compass, tape it to the table, and then use that same compass to plot all the azimuths reported.

[jimmydeanno]

Depending on the map you're using it should tell you what the declination is.  The USGS 7.5 minute maps definitely have it on the bottom.

But here in my neck of the woods we follow the rule "compass to map subtract 15."  Could be 14.8 or 15.2 but, my pencil isn't that accurate anyway.

NOAA also has a calculator that is pretty handy at times: http://www.ngdc.noaa.gov/geomagmodels/Declination.jsp