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Old 10-20-2014, 06:45 AM   #1
Soldier311 OP
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Do you prefer Lat/Lon or UTM's ?

This past weekend while trekking through the Croatan National Forest here in eastern N.C. I was using my older handheld GPS unit to locate myself on paper USGS topo maps and was having a lot of trouble figuring out where I was. I was somewhat familiar with the area, but just couldn't pinpoint my location to where I thought I should be. Then I realized that my GPS unit and the paper maps were in different datums. There's several popular UTM datums and I never seem to be linked up with the right one on my travels.
I believe I'm going to just stick with Lat/Lon from now on to avoid such confusion.
What do you prefer?
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Old 10-20-2014, 10:23 AM   #2
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Another consideration is if you need to give position to 911. They also have much confusion between degrees & minutes or degrees minutes & seconds but I think clueless on UTM.
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Old 10-20-2014, 09:02 PM   #3
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Lat/Long also have different datums! Its always important to make sure your GPS is using the same datum as the map your using regardless of whether your using UTMs or Lat/Long.

For more on datum: http://geography.about.com/od/geogra...n/a/datums.htm
This may not be the best source to learn about datum, but has some decent info.

I've been working with our local Search & Rescue team for over 8 years and we have learned to always check what datum people are using when they try to give us UTMs or Lat/Long. We have also experienced issues getting coordinates (usually relayed through dispatch) and whether they are deg min sec or decimal degrees. Seems like people like to say "point" between numbers even when it is deg min secs. Keeps things confusing that way. We are totally fine using UTMs, but seems like most lost/injured people around here prefer Lat/Long.

Also keep in mind that when you give 911 your coordinates, you should also give them the datum your using and realize that these coordinate travel through several people before getting to the actual SAR Team that is physically looking for you. It also MIGHT help by actually saying the words ## DEGREES, ## MINUTES, ## SECONDS. Realize that dispatchers are not always that knowledgeable about these things.

I have found the Lat/Long is usually easier to use and more familiar to most people, but you can be a bit more precise with UTMs - when it comes to pinpointing locations on a map.

For backcountry travel, I still prefer using the tried and true map & compass! Just shoot some bearings and triangulate your position - just make sure to set declanation (or "variance" if your a pilot).
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Old 10-21-2014, 04:10 AM   #4
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Lat/Lon ?

How can Lat/Lon have a different datum? I thought it was the "ground truth" so-to-speak.
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Old 10-21-2014, 06:50 AM   #5
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The link in post 3 explains it quite nicely.
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Old 10-21-2014, 07:29 AM   #6
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Definitely check the link, or just google it. There is a lot of good information out there on datums.

As for Lat/long vs. UTMs, just think about them as units... Like the difference between inches and centimeters.


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Old 10-21-2014, 01:08 PM   #7
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Quote:
Originally Posted by Countdown View Post
Another consideration is if you need to give position to 911. They also have much confusion between degrees & minutes or degrees minutes & seconds but I think clueless on UTM.
If you are using a SPOT device, knowing what datum it uses (WGS84 I think) as well as what datum your map or mapping software is in could be the key to finding a lost/injured party.

Here is a great post that demonstrates this issue: http://advrider.com/forums/showthread.php?t=613416
In the first few pages, it seems that everyone is thinking the solo rider is way off coarse. A few pages in it looks like someone figured out that the SPOT GPS coordinates were using a different datum than the mapping software(s) (and paper maps) that everyone was using to try to pinpoint his location. Looks like the difference in datums came out to about a half mile difference in his actual location. When searching in the backcountry, a half mile difference can be huge. I only skimmed through the thread, but it seems to emphasize the importance of understanding datums when using GPS coordinates. This is the case for both Lat/Long and UTMs.

Anyone who uses GPS in a backcountry environment should really take a little time to learn about datums and be familiar with what datum you are using. It could mean the difference of being found (in a timely manor) in an emergency situation.

WGS84 seems to be one of the more popular datums these days on GPS units, but keep in mind that a lot of USGS maps (and others) haven't been updated since the 60's (or earlier) and are generally using an older datum.
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Old 10-21-2014, 01:36 PM   #8
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Originally Posted by alpin View Post
Looks like the difference in datums came out to about a half mile difference in his actual location. When searching in the backcountry, a half mile difference can be huge.
My two experiences were with choppers.

One from Vegas ran out of fuel just as he finally spotted rider with broken neck near Tonopah. He left but directed local SAR 4x4 in. They were verbal lon/lat to 911 via sat phone confirmed correct by 2nd observer. Don't know the cause.

Second was simple flight from Barstow to near the Husky 25 miles or so using SPOT location notification. Chopper pilot told someone at the sight it was a units issue.

I was un-impressed by lack of standardization of SAR service.
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Old 10-21-2014, 06:15 PM   #9
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Quote:
Originally Posted by Countdown View Post
My two experiences were with choppers.

One from Vegas ran out of fuel just as he finally spotted rider with broken neck near Tonopah. He left but directed local SAR 4x4 in. They were verbal lon/lat to 911 via sat phone confirmed correct by 2nd observer. Don't know the cause.

Second was simple flight from Barstow to near the Husky 25 miles or so using SPOT location notification. Chopper pilot told someone at the sight it was a units issue.

I was un-impressed by lack of standardization of SAR service.
Could you elaborate more on what it was that "un-impressed" you? I'm just curious.

Living at 8000 feet with many 13000 to 14000 ft peaks all around, our helicopters generally have to sacrifice fuel for performance and refuel often. On one mission (started at 10pm = dark = dangerous in the mountains) the pilot of the helicopter we were using could only take 15 gallons of fuel and one person at a time. He would drop that person and return to airport for 15 more gallons of fuel and another person. He was having to land on a ridge using night vision. I'm not a big fan of helicopters in the mountain at high altitudes in general, but add in night condition... a little scary. It was slow getting the team inserted, but it was the fasted way to do it safely. The guy we were going after took a nasty fall climbing a peak and his foot was barely still attached. We started inserting the team at 10pm and got the patient to the hospital at 8am. It was a long night, but in the end, he got to keep his foot.

I can't comment on the 2 stories above because I don't know any of the details, but I find that a lot of people think if they call SAR, we will be there to save them within the hour. The reality is sometimes it takes quite a bit of time to do it safely.

I feel that if someone goes into the backcountry, whether it be climbing, kayaking, motorized travel or whatever, they need to accept the fact that there are risks and that help could be a long ways out. A lot of people do, but I have come across plenty that just think they can hit their SPOT or make a quick call and everything will be okay. It doesn't always work that way. This is not intended towards anyone specific - just a generalization.

Countdown, I would love to hear more about your experiences - if mistakes were made, I would prefer to learn from others than make my own.

I know we are a bit off topic here, sorry.
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Old 10-21-2014, 06:47 PM   #10
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The link in post 3 explains it quite nicely.
I've read the link and it still doesn't say anything about Lat/Lon being based on any sort of datum; nor do any Google results discuss it.
I work with this kind of stuff every day in my real job as a meteorologist and atmospheric dispersion modeler, so I'm not totally uneducated on the subject of cartography and geography.

The reason I've used UTM's in the past is twofold: 1) it's what I use at my work, so I'm familiar with it, and 2) I can draw lines across my USGS topos maps which allows me to more easily pinpoint locations since I can use a grid tool such as those available from maptools.com.
But if I'm always forgetting which map datum I've got up in my GPS unit, relying on UTM's is a recipe for confusion.

It is my understanding that Lat/Lon is the ground truth and that the UTM coordinate system is based on a map projection (transverse Mercator) of the earth. I'm not trying to pick a fight over this, nor am I from Missouri, but I'd like for someone to "show me" where Lat/Lon coordinates are discussed as being based on any sort of datum.
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Old 10-21-2014, 06:49 PM   #11
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Could you elaborate more on what it was that "un-impressed" you? I'm just curious.
This was not back country in either case, both were on quite flat desert, within 10 miles of a town and on a well defined roads just off a graded county roads.

However that has nothing to do with giving a position that was probably accurate within 50-100 ft and then SAR can't find a large group of people and vehicles.

I come from a world where we drop a package on Mars right where we want it and send a robot vehicle up to the Space Station and it docks itself. The big difference is one is machines and the other is human.

I am not impressed with all the human errors in what should be a simple problem for machines.
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Old 10-21-2014, 06:57 PM   #12
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Originally Posted by Countdown View Post
This was not back country in either case, both were on quite flat desert, within 10 miles of a town and on a well defined roads just off a graded county roads.

However that has nothing to do with giving a position that was probably accurate within 50-100 ft and then SAR can't find a large group of people and vehicles.

I come from a world where we drop a package on Mars right where we want it and send a robot vehicle up to the Space Station and it docks itself. The big difference is one is machines and the other is human.

I am not impressed with all the human errors in what should be a simple problem for machines.
I definitely hear what your saying there! I would hope that there are circumstances unknown to me that caused (or explains) this, but from the limited information I have on it, I am un-impressed too.

Sometimes I think we should go back to pencil and paper as it seems like we are creating technology that makes tasks take longer, more difficult, and unreliable. Other times I wonder how some people just make it through a day.
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Old 10-21-2014, 07:02 PM   #13
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Originally Posted by Soldier311 View Post
I've read the link and it still doesn't say anything about Lat/Lon being based on any sort of datum; nor do any Google results discuss it.
I work with this kind of stuff every day in my real job as a meteorologist and atmospheric dispersion modeler, so I'm not totally uneducated on the subject of cartography and geography.

The reason I've used UTM's in the past is twofold: 1) it's what I use at my work, so I'm familiar with it, and 2) I can draw lines across my USGS topos maps which allows me to more easily pinpoint locations since I can use a grid tool such as those available from maptools.com.
But if I'm always forgetting which map datum I've got up in my GPS unit, relying on UTM's is a recipe for confusion.

It is my understanding that Lat/Lon is the ground truth and that the UTM coordinate system is based on a map projection (transverse Mercator) of the earth. I'm not trying to pick a fight over this, nor am I from Missouri, but I'd like for someone to "show me" where Lat/Lon coordinates are discussed as being based on any sort of datum.
Here is a simple experiment:

Use your GPS and get your current lat/lon. Write it down. Now don't move. Go into your settings and change the Datum, but still use lat/lon. Now get your current lat/long and compare the two. You could even plot them on a map. Try doing this with as many different datums as you want. Depending on which datums you compare, the differences could be minor or significant.

Let us know how this turns out.
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Old 10-21-2014, 07:38 PM   #14
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the differences could be minor or significant.
What units do you measure "minor" and "significant" in?
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Old 10-21-2014, 07:42 PM   #15
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It is my understanding that Lat/Lon is the ground truth and that the UTM coordinate system is based on a map projection (transverse Mercator) of the earth. I'm not trying to pick a fight over this, nor am I from Missouri, but I'd like for someone to "show me" where Lat/Lon coordinates are discussed as being based on any sort of datum.
Let's try again. A Datum is the mathematical description of the Earths surface. That model is used to calculate a mathematical set of numbers representing a N/S, E/W position on the surface that are called Coordinates. Coordinates can be derived from Datum in many Formats. Latitude & Longitude is one such Format and the oldest format used for defining of a position on the Earths surface. That doesn't mean that Lat/Long are the most accurate or most appropriate Format.

There are also many Datum that have been developed over the years as "standards". But, there is only one Datum that is generally recognized around the world as the "International Standard" and that is WGS84 (World Geodetic Survey 1984). When you want to calculate a position on the Earth you must first choose a Datum with which you can calculate your coordinate reference system.

Whether you choose Lat/Long, UTM or one of the 70+ other Formats, the position you derive will be very close to the same depending upon your mathematical error in deriving the coordinates. UTM is actually easier to derive an accurate position (using grid tools as you noted) on a map because it is a "square grid" system and that is why UTM is used more often when paper maps are needed in the field.

The US has used Lat/Long as a "standard" format and in general because the US FAA mandates Lat/Long so goes the rest of the world (except our Military which uses a grid system derived from UTM: MGRS.
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