Map & Compass issue

R.A.T.

R.A.T.

Randall's Adventure & Training
Joined
Feb 4, 2004
Messages
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I was teaching a map and compass class yesterday to a group of pretty sharp students and we ran into an issue. First of all the class is about a 3 to 4 hour classroom study, then we head to the woods and put everything we learned in the class into practice. The first course was already laid out by my co-instructor, Jimmy Dunn. We divided the class into two team of about 5 guys each and ran the 4-point course. Both teams hit the marks exactly and had zero problems. We re-grouped at the staging area and decided to let the students plot a new course themselves on the maps. My group plotted about a 2,500 meter triangular course and we set off into the woods. After reaching our second point (about 1500 meters in) we decided to double check our compass work with a GPS. Well, here's where the problem comes in. The West/East UTM was dead on what we had plotted and run with our compasses but the South/North said we were 240 meters north of where we actually were. We spent about 30 minutes verifying terrain (easy to do since we were in a steep draw with a creek at the bottom - nothing else like it on the map) and even triangulated from the draw and creek. We decided there was an issue with the GPS and stayed with what we knew to be accurate with our compass and map. In other words we said to hell with GPS and trusted our compass. Then walked our final azimuth out and hit exactly where we had plotted on the map when we made it back to the road. Once we got back to the staging area (an intersection of two small roads, also clearly shown on the map), we got my GPS out of the truck to verify where we were and also used the GPS that we had in the woods with us. Both GPS units said we were 240 yards north of where we were at the known staging area. Again, the West/East was dead on. What gives? I've never run into this problem with a GPS but it is clearly not accurate (North/South) according to the quadrangle we were using. I'm real gun shy of using GPS at this stage since we nailed every point using a compass, map and pace counting but the GPS said we were wrong. The only thing I can figure is the roads are marked in purple so they have been added by aerial survey since the last ground survey and never field checked. Is it possible the roads are not accurate on the map? With that said, the creek and draw we were at should have been accurate so even if the starting point of the road was 240 meters off on the drawn map, the second point should be accurate on the map and GPS. Anyway, I have done a LOT of map and compass and this is the first time I've had this happen. It proved to me that trusting a map and compass is far better than trusting an electronic device.

Jeff
 
I still use just a compass I just do not trust the electronic stuff. GPS is nice but in the end a compass is the way to go.
 
Older USGS quads were hand drawn. Your map could be over 100 years old depending on the region. Air photos have distortion and are stretched to match control points. If you look at how maps were done 50 years ago this would not have been a surprise. You can have error from the cartographer and from the photomapper. I remember doing a 1-250000 map and having to connect two ends of a major road that did not meet. 240 yards seems like a large error but alot depends on the scale of the map. Not unusual. What state are you in? What is the year of the map and has it been revised?
 
That's interesting, Jeff.

I know that there is some built in margin of error in GPS coordinates (or had been, in any case) for defense purposes in years past. I don't know what the margin is these days. (Though why it should only be N-S instead of E-W, I don't know.)

I wonder if there could be any kind of "interference" in the area that could have thrown off any readings.

Did you try reading the GPS in Lat/Long instead of UTM as an alternative to see if the margin of error was the same?
 
I remember Marty Simon telling a story of a group of Spec Ops guys he was teaching a survival course to in the Adirondacks, NY. Their GPS wouldn't work in that terrain. If I remember correctly he said they were lost without those GPS devices. Didn't know how to use a compass.
IMO, the compass is the best way to go.
Scott
 
Older maps were ground surveyed by control points which were done by old fashioned star and sun shots to determine longitude and latitude. They were very accurate. 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.
 
Well, it has me real gun shy of GPS right now. If it was just me saying it was off then I would figure it was my calculations that could be wrong but we had about 5 different folks check the UTM and the known location on the map and everyone came to the same conclusion. And this 240 meter difference was noted in two known locations on the quadrangle, with them being about 1500 meters apart from each other.

The reason this worries me is it's the same quadrangle that Jimmy Dunn teaches a combat tracking class. We're about to teach the class again and the next tracking class will have a base of operations and two tracking teams working at the same time. The teams will be radioing their location to the command center via UTM coordinates and most of these guys use GPS, so I'm wondering how this is going to go.
 
zman308 said:
Try to find a benchmark in the area of the map and see what your GPS says.
Click to expand...

Now that's good advice and I hadn't even thought about that. Before the next class I'll do that and let you know.
 
The benchmark idea is a good one.

Jeff, on the other forum you mentioned that you did switch to lat/long (as I inquired) and that it was accurate.

So, now I wonder if the UTM error is a mapmaker or GPS issue. Given that the GPS lat/long agreed with the map grid, it sounds like the UTM grid may have been overlaid on the map incorrectly.
 
oldman/Marty said:
Don't like em, don't trust em, won't use em. Give me a compass anytime.
Oldman/Marty Simon
Click to expand...


Can't disagree. I keep a very basic one that I got on the cheap in the pack for emergencies but don't use or rely on them otherwise.

I found them interesting to play around with a few times.
 
Blues,

I roughly figured Lat Long with my baseplate scale just as a backup. It appeared to be accurate with our location but I would not testify to that since I never precisely scaled it. At this stage I was 100 percent, positively, without a doubt, sure that where we said we were by compass was where we were. When we got back to the staging area it was getting late and everyone was tired so we just never double checked with Lat Long at that location.
 
oldman/Marty said:
Don't like em, don't trust em, won't use em. Give me a compass anytime.
Oldman/Marty Simon
Click to expand...

I have always used GPS units on most of my trips as a tool to verify my compass work. In fact, we use them a lot in the jungle to verify our compass courses. BUT...after yesterday I'm real gun shy. I'm sure they're a great tool but I will always trust my compass skills now over what the GPS may tell me.

Jeff
 
Ya it's map and compass for me as well. I feel between batteries, electronics, etc..., a good old map and compass is the way to go. I had a gps for a short time that I have since sold, as I just never trusted it. I remeber inside the front cover of the book that came with the gps, it said basically not to solely rely on this gps and to back it up with a compass. So I figure why not just go with the compass to begin with. I have read about problems with too many trees effecting gps (canopy) as well as too cold a temperature. I always carry two compasses.
 
R.A.T. said:
I have always used GPS units on most of my trips as a tool to verify my compass work. In fact, we use them a lot in the jungle to verify our compass courses. BUT...after yesterday I'm real gun shy. I'm sure they're a great tool but I will always trust my compass skills now over what the GPS may tell me.

Jeff
Click to expand...

Well, there's no question of their utility especially in an area where it's hard to triangulate or otherwise take bearings off known landmarks.
 
Blues said:
Well, there's no question of their utility especially in an area where it's hard to triangulate or otherwise take bearings off known landmarks.
Click to expand...

Exactly. In low jungle, you have no way of triangulating. What we usually do is find our known location from a river, then plot everything as we move. Either that or use a GPS...no other choice unless you start looking at celestial bodies.

Most topo maps of the Amazon jungle are just one big blob of green with a creek or river running through them...even if that.
 
R.A.T. said:
Exactly. In low jungle, you have no way of triangulating. What we usually do is find our known location from a river, then plot everything as we move. Either that or use a GPS...no other choice unless you start looking at celestial bodies.

Most topo maps of the Amazon jungle are just one big blob of green with a creek or river running through them...even if that.
Click to expand...

Yep. And I thought our rhododendron thickets (hells) were bad. :eek:
 
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:
 
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