Map & Compass issue

Swampdog said:
Jeff,
Double check your Map Datum on your GPS and your USGS Quad map to be sure they are both the same. Even though your map was up-dated in 1981 it may still be North American Datum of 1927 (NAD 27). I have had that problem before and when I switched my GPS Map Datum from NAD 83 to NAD 27 the problem was corrected.
Good luck! :thumbup:
Click to expand...

Good point.
 
As has been said.

The Datum on the GPS has to be set to the same datum that was used to make the map for the 2 to match up.

The GPS doesn't know which of the many datums you are using unless you tell it (set it).
 
Clovis said:
As has been said.

The Datum on the GPS has to be set to the same datum that was used to make the map for the 2 to match up.

The GPS doesn't know which of the many datums you are using unless you tell it (set it).
Click to expand...


Yes, this is most likely the issue. If you see a consistent shift in the location the GPS is reporting you to be at compared to where you know yourself to be, a datum mismatch is a very likely culprit, and a few hundred meters can be consistent with a datum correction from say NAD27 to NAD83. Depending on where you are, I've seen worse.

http://www.usgs.gov/faq/list_faq_by_category/get_answer.asp?id=378

Selective Availability (S/A), or the intentional degradation of the GPS signal was turned off a few years ago by DoD, so it wouldn't have been that, and that error would have been more inconsistent as he looked at the GPS.
 
I'll second or third that on the datum issue. It can be a real confusing pain the ass.
http://en.wikipedia.org/wiki/Datum
"Users of GPS are cautioned that they must always check the datum of the maps they are using. To correctly enter, display, and to store map related map coordinates, the datum of the map must be entered into the GPS map datum field."
 
I'm pretty sure that the issue is exactly that, not adjusting the GPS for the map datum.
I just went to the topo they used and looked up the difference from NAD27 to NAD83 and there was virtually no E-W difference but almost exactly the N-S variance that Jeff reported.

Seems that this mystery is now accounted for.
 
Jeff,
I hope you follow up and tell us more about this problem.

From your description, it sounds like you are not using the
GPS map feature at all. BUT, would a UTM reading implicitly
be using some map data? I wish I knew.

If you take only the UTM coordinates from the GPS,
and plot them on the USGS paper map, does it intersect
in the right place? As suggested earlier, what if you take
only the Long/Lat from the GPS and plot it on the USGS map?

zman308 said:
... If the road or landmark was added to a revised map it could have been added in with some "artist's license". Try to find a benchmark in the area of the map and see what your GPS says.
Click to expand...

Jeff and zman308,
I think this is the right idea, find out how general, and how widespread the
problem is.

Test more data points in an expanding distance from the problem.
Many "benchmarks" are also triangulation points but not all;
and you want triangulation points, which are specifically horizontal control.

On USGS maps, black script was better established than brown script.

Sometimes you can reproduce the problem in a more convenient place,
i.e. using an old map near your home, rather than traveling to the original
sight each time you test.
 
Right, what Swampdog said. All the USGS maps i have are nad27 and if i forget to switch the datum in the GPS, i'm off by 100m.
 
I think you guys have figured out my problem. Once again, by not studying up on a cool toy I have allowed it to bite me in the a$$. I have used map and compass all my life and just never really used GPS except in the jungle, and this is the first time this problem has cropped up. What we will do now is adjust all tracking participants GPS's to the right datum, but before we do we will verify what is correct. This problem isn't an issue with our map and compass class, obviously (unless you do what I did yesterday and say "Let's check ourselves with the GPS") but it is an issue with the LEOs that use only GPS.

Appreciate it!

Jeff
 
Well, I LOVE my GPS, though I'm completely comfortable using a map & compass.

My first thoughts are:

Make sure you have the correct datum selected to match your map (covered above)
Make sure you don't have the map set to "stick to road" or similar
 
While it was I who was the idiot on this whole deal, it truly reaffirms my belief that more is not always better when it comes to bushcraft and survival. There are way too many variables when it comes to electronic devices, and while they may be quick and neat to use, those variables can also cost you when we start relying on them and lose our tried and true basic skills. I'll be sticking with map and compass for my own navigation. It's something I know very well and something that just doesn't screw me up when it comes to the variables.
 
Guys,

Thanks for an awesome thread. My land nav & map & compass skills are adequate, but not great in my own opinion, and my GPS skills are about basic. I learned a lot from reading here. :thumbup:
 
As soon as i read the problem the first thing i thought of was the wrong date on the GPS. Im glade to hear you figured it out. Today the GPS is using the same info as the millitary does. Today i went geocaching and my GPS was off by only 5ft.

Sasha
 
When I went through USAF Survival School in the Truckee area of N. CA. , we climbed the same hill three times because of a local compass disturbance. We gave up on the compass and just used the map (in the dark). As I remember, only a few of us made it to the objective camp site that night.
 
For all those of you who dislike GPS's, I'll point out that when Doug Ritter, of equipped.org fame, was asked for his top 10 most important items for survival, the first item listed was "a mapping GPS w/ spare batteries, or secondarily a map & compass, AND the knowledge to use them together".

Realistically, most average folks who venture outside or drive through remote areas don't have sufficient skills in use of a map & compass to protect themselves. On the other hand, more and more average housewives (and house husbands, and kids) have become quite comfortable using mapping GPS's with navigation capability. Had James Kim had a mapping GPS he could have been provided with a much better idea of where is was and how close he was to nearby points of interest. Stories abound of people who went for hikes and simply couldn't find their cars or campsites as darkness or weather approached.

The most basic of GPS units can prove to be lifesavers for those who are navigationally-challenged IF basic practices are followed (setting waypoints frequently, bringing extra batteries). Realistically, GPS units are remarkably reliable and its easy to carry extra batteries. Heck, even magnetic compasses have been known to fail every now and again.
 
sasha said:
As soon as i read the problem the first thing i thought of was the wrong date on the GPS. Im glade to hear you figured it out. Today the GPS is using the same info as the millitary does. Today i went geocaching and my GPS was off by only 5ft.

Sasha
Click to expand...

Not exactly. The military has a correction frequency that civilian GPS's don't. If SA is turned on again it will affect your GPS but the correction information will be used by the military GPS systems to take SA error out keeping them as accurate as yours is now with WAAS.

KR
 
kenk said:
For all those of you who dislike GPS's, I'll point out that when Doug Ritter, of equipped.org fame, was asked for his top 10 most important items for survival, the first item listed was "a mapping GPS w/ spare batteries, or secondarily a map & compass, AND the knowledge to use them together".
Click to expand...

Kenk, I disagree with Doug on that. First priority is learn map & compass alone effectively, second, learn primitive navigation skills alone effectively, thrid, learn GPS effectively.

Then learn how to use them all together or in any combination.

Electronic devices often run out of power or get rendered inoperable at the critical moment in the real world. Murphy's Law. Reliance on knowledge first aided by devices second is the rule.
 
kr1 said:
Not exactly. The military has a correction frequency that civilian GPS's don't. If SA is turned on again it will affect your GPS but the correction information will be used by the military GPS systems to take SA error out keeping them as accurate as yours is now with WAAS.
Click to expand...
KR1 is correct; today, your civilian GPS unit is as accurate as it can possibly be (since about 2000). However, the DoD can easily introduce errors without notification, and can increase or decrease the error as needed.

It's hard to say if they ever will do that again; didn't seem to protect the average US citizen any, and today would screw up millions of drivers and airline pilots and mariners who depend upon GPS.

I'm sorry I was late to the dance, but as soon as I read RAT's error, the first thing I thought of was datum. That's about exactly the error you'd expect from that: on average, about 200m. Not sure why that isn't stressed more in GPS usage; it's something I personally make a point about for just such occasions. Pilots tend to rely on WGS 84 for airline use, and often forget to switch from one datum to another when taking their portable GPS system with them.
 
Brian Jones said:
Kenk, I disagree with Doug on that. First priority is learn map & compass alone effectively, second, learn primitive navigation skills alone effectively, thrid, learn GPS effectively.
Click to expand...
I agree with this, but would put primitive last after GPS, only because the average person encounters GPS more often now, with car nav systems, cell phones, and so on, and primitive approaches are more fuzzy and local.

But regardless, each of these systems work...and each of them has the capacity for easy failure. Each is also subject to the most basic error. I'm both proud and NOT proud of having made an error in each of these three methods, and learned from it...fortunately, all in non-life-threatening situations.

All three should be studied thoroughly. My only concern with teaching GPS first is it becomes an easy crutch, people get comfy easy with it and never get around to compass navigation.

In my own mind, I've found that I appreciate GPS navigation a whole lot more by knowing compass usage. Not that GPS is so much easier to use, but you learn to look at a map differently when knowing GPS, and you tend to look at a GPS map differently by knowing how to use a compass. If you know both of these methods, you probably know what I mean.
 
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