Do you give a Datum?

[Siskiyou]

The following is based on a true incident.

At approximately 2245 hrs. the Emergency Command Center(ECC) receives a 911 call from an Amateur Radio Operator.  The caller relays a message regarding a group camped in a remote area.  The area is approximately 3 miles up a nasty 4-wheel drive road.  The caller relays GPS UTM coordinates, the nature of the emergency, and a general  location.

The ECC calls back the relaying Amateur Radio Operator asking if he can re-contact the other party for the following information.

Confirm if they are North or South of a major highway.(The problem is that their is a commerical operation South of the Freeway with the same name as lakes North of the Freeway.)

The response is that they are located South of the Highway.(Oops they were actually North of the Freeway.)

Request that the gps coordinates be checked and repeated.

Before any more communication are possible the mountain top repeater the Amateur Radio Operators are using automatically shuts off at 2300 hrs.  And will not come up until the following morning.

The UTM coordinates are flawed!  The ecc has two computer programs that take the imputed gps coordinates and places them on a map.  Neither program works with the flawed coordinates.

A second member of the party requesting assistance makes it to a location where his cellphone works.  He provides a better location for the incident.  He also says that the incident is not critical but they would like a contact in the morning.  He also provides a more detailed description of the location.  About a mile from the description given earlier.

Based on this information and the difficulty of getting people into the location it is decided to wait until daylight to send a unit into the area.  Sending a unit into an area with ice, snow, and rocks in a 4-wheel drive vehicle is risky in the dark.  It has been determined that the incident is not life threatening, and only requires checking.
No additional gps information is provided.

The ECC receives a call from another Amateur Radio Operator.  He is also a gps owner.  The new caller states that the UTM information was incorrect.  He has gone to the internet and did a little research.  He believes the original caller was using the wrong Datum. ( A datum is the model that was used to match the location of features on the ground to coordinates and locations on a map.)  The caller tried to use the coordinates he heard the requesting party give to the original Amateur Radio Operator.  When he tried them using his Garmin Map Source Topo software it would not work, he then tried entering it in his EtrexVista and again it would not work.

After researching the Internet he felt he could place the location of the requesting party.  He was not far from being right.  Good work.  

After hearing about this incident I thought I better do a little more research.  I knew that my gps unit contain the option of many Datums, but am using the correct one.  The factory default for Datum in Garmin units is WGS 84 (World Geodetics Survey 1984)  The manual recommends that you change the Datum to match the map you are using.  I grabbed up a USGS TOPO MAP and checked the Datum listed on it.  This is found in the left had corner.  The map states that it is based on the NAD27 Datum.  A check of a Forest Service recreation map shows that it is based on USGS maps.  I have been collecting and using topo maps for many years.  What do I do?  Do I use NAD27 or do I use WGS 84?  In the Datum options of a gps NAD27 is normally stated as NAD27 CONUS.  For Dave up in Alaska it is listed as NAD27 ALASKA.

I understand the difference between NAD 27 AND WGS 84  can be up to 650 feet in the lower 48.  Most aviation charts use WGS 84.  

I cannot explain the incorrect coordinates provided by the original requester.  Was his foreign accent a clue.  Was he set on a Datum for another part of the world?  For those hunters out there in steep country a 650 foot error can make a major difference.  The difference could place you on the wrong side of a ridge line.

Then another question comes up.  What Datum are the maps in a modern computer dispatch center based on?  I will be asking some friends in the business if the know or who I should be asking.

I mess around with all the functions on my gps trying to learn how it works.  It would not be difficult to place a gps on the wrong datum.  

I toss this whole issue out to you guys.  What datum are you using?  I have not had any problems using WGS 84.  But if I am out in the woods with a broken leg.  I want to be found.

Seems like a lot of guys fall out of tree stands now days.  If their cellphone is still working, the gps reading could be important in speeding up the rescue.

Thanks Amatuer Radio

[victorcharlie]

I've been using the default WGS 84.  It would be interesting to know how a gps cell phone works, and what datum the phone companies are using.  Very good questions, I've heard the same complaint before, that if you call in a coordinate to, in this particular case, life flight, that they have problems using it.  I would think garmin and some of the other manufactures would know something about the problem.

[Siskiyou]

I suspect the next time the folks in the ECC see me coming they will lock the electronic door.  Kidding aside because of the number of formats that they deal with,(1) UTM, (2)Degrees Decimal Degrees, (3)Degrees Decimal aka(Degrees Minutes Decimal Minutess Degrees Minutes Tenths) and (4)Degrees Minutes Seconds one starts suffering brain damage.  Normally the computer will display the given coordinates on a map and will automatically show the other formates along with the Township, Range, and Section.  The problem is that these formats will place the location in three or four different places so it is critical that everybody is on the same page.  

According to a document I was given in Region 5(CA, Hawaii, and Pacific Islands) any dispatches, fire locations, aerial ignitions plans, or anything that might need be located from an aircraft, degrees decimal minutes formate will be used.  As always there is an exception when writing the coordinates for a Temporary Flight Restrictions.  In the case of TFR's the Degrees Decimal minutes formate will be used without the signs. Confused!  The document fails to mention what MAP DATUM to use.  

My grandson and I stopped at the ECC today.  It was on our way out to go plinking.  I asked the NAD question of a couple of knowledgeable people but they could not provide an answer.  But they gave me the name of the person who did the computer mapping working.  I will be contacting him in the next week or so after the holiday.

I am starting to feel like a little boy asking mommy, "Why."

[daddywpb]

I have also been using the default setting, but I can see how it would be a problem. If you call to be rescued from some lonely woods somewhere, I would guess that the rescuers would be using USGS topo maps, so that would be the datum you would need to use to give coordinates. How do the manufacturers decide what the default setting will be? I've given coordinates to FWC officers a couple times for various reasons, and they didn't question it at the time, but it's also possible that they had no idea what I was talking about. It's all very confusing to me - I think I need to do some research on this subject. I think Rhett said it best when he said, "Frankly Scarlet, I don't give a datum". :grin:

[Siskiyou]

I am currently using the default WGS 84 Datum.  I am also using Garmin's MapSource Topo software for my gps mapping.  A gps user who is "aware" should be able to provide 911 with the correct Lat/Long and tell the 911 Center what format they are using.

The problem is that many people do not know what format they are using.  The default format works for them and the other issues are not of immediate concern.  Hopefully I can get some answers in the next few weeks.

In my case I provide the following basic information in writing to my wife when I go out hunting or beating around the woods.  The Township, Range, 1/4 Section, and Section I plan on being in.  And the geographic location name.  There was a time that I thought I was incident proof.  But one day with my wife and two kids we took a ride in our 4x4 into the high country.  When we came down below the snowline I pulled into a old logging landing for target practice.  We eat some lunch and enjoyed our selfs as the March sun warmed the area up above freezing.  When we started out the vehicle started sinking into the thawed ground.  We were stuck.  We had told nobody were we were going.  We had food and a couple of blankets.  We built a warming fire planning on spending the night and hoping for the ground to freeze.

Luck was with us when a pickup with a wench stopped.  Poor planning on my part had put the family at risk.