GPS Advice

[PaddlingPitt]

I have a Garmin eTrex GPS purchased in 2000. I have rarely, if ever, used it for navigation until this past summer. But on our canoe trip in the NWT Kathleen and I were supposed to be picked up by float plane at a specific location. So I entered the coordinates as a waypoint, even though the GPS had never been there. I also entered coordinates for a fishing lodge that we wanted to visit.

On both situations the GPs indicated we were getting closer, and then changed its mind to saying we were getting farther away. We beached and I walked left, right, forward and back. Always getting farther away. Even when I eventually reached the desired location, GPS said I was still a long way away, like 600 m (2,000 feet). How can this be? Looking forward to your responses to alleviate my consternation!

 

[memaquay]

Ok, well I have used quite a few, and it hurts me to say it, but your 2000 etrex is a dinosaur in gps land. A modern cell phone would have probably worked better. My samsung, without any reception, still connects with satellites and shows me where I am.

However, assuming your gps is working, did you allow it time to initialize? Those old ones, when you go to a location more than 500 k from where you were before, needs to reinitialize its position. It can take a long time, sometimes.

You also need to know what datum series your initial coordinates were from, and what datum system is set on your gps. For instance, I routinely use NAD 27 because all my old topographic maps use that measurement.

Lots of coordinates use WGS 84 or NAD 84, and these will both create differences on a map if your gps is not set to the same datum. 600 meters would be quite a difference, but I have seen a 2 or 300 meter difference between NAD 27 and WGS 84. My guess is that this is your primary problem.

If you have a new-ish smart phone, download the earthmate program. It will show you where you are in real time on both a map, or with coordinates.

 

[PaddlingPitt]

Thanks, mem.

The default datum on my gps is WGS 84. I have never changed It. I don’t know what datum was used for the coordinates given to me By the fishing lodge or the float plane.

I have two questions.

1. Why would the gps tell me I was getting farther away no matter which direction I was going?

2. My manual says that waypoints are marked by actually being there. I entered waypoint coordinates from a long way away. Like 2,000 km. Is this possible, in terms of how waypoints work? I assumed it was. Just GOTO you dang GPS!

 

[Mason]

NAD 27 and WGS 84 are basically the same, I think with only a few meters difference. If you don't know what datum the pilot is using, you're potentially screwed. However, most pilots use WGS 84 as the standard. Were you using lat-long or UTM? While UTMs are easier to use, they also allow additional errors as they require Zone info, etc. Also, before using any GPS, it's worth watching the pickup of satellites and watching the estimated error--the estimated error should drop pretty dramatically. Current GPSs get within about 3m of accuracy, which is way better than your 2000 model--I've used GPSs since I had to use a US Military unit (PLGR) and had to send it in for re-encryption every year (and it was $2000, and we didn't own it!). You can't be too careful with someone knowing your position--it is wilderness, after all. If you're only picking up a few satellites, then there's a huge amount of error, and that sounds like what you experienced. With few satellites, you can sit in one place and your position wanders all over the place. In places, especially if you've traveled a long way before turning the GPS on, it can take a while before you get a good fix, sometimes several minutes.

You should be able to enter waypoints manually. Also, a basic current GPS is under $200 (I use a etrex 20, about the smallest and most basic of the lot).

 

[yknpdlr]

Right, failure to use proper Datum in the GPS versus the datum base used on the mapped coordinates is the likely culprit. However, WGS84 and NAD83 are the two that basically the same (within a couple of meters) in the USA and Canada. NAD27 is the one that can be dozens of meters diferent from those. USGS topo maps are typically on NAD27 datum, online maps are typicallly WGS84. The biggest navigation erroors typically occur when using lat/long, not UTM. Since lat/long cand be configured as decimal degrees, or decimal degrees plus deimal minutes, or decilmal degrees, decimal minutes and decimal seconds, unless you are very careful extreme confusion may occur. There is at leasst one famous Adirondack misplaced aircraft crash that had the wrong format send resuers miles away from the actual site. Get accustomed to using UTM. It being a decimal system base 10 is by far an easer and more accurate system than lat/long which is a base 60 meaurement system with distance meausrements that vary dependiing on latitude.

If you are near any major landforms, such as cliffs or deep ravines, the satellilte signal can give you bad coordinates due to multipath reflections of the signal. All the more reason to do a "sanity check" with terrain asssociation techniques from understanding of map reading in all cases with or without external electronic navigation aids.

 

[memaquay]

Without knowing what the datum from the given coordinates was, I resorted to google and found this article. Not sure if it answers your question, but adds an interesting dimension. Would have been interesting to know if other GPS units were working in the area.

https://www.allaboutcircuits.com/news/navigating-the-arctic-why-gps-might-fail-you/#:~:text=However%2C%20GPS%20is%20not%20quite%20as%20global%20as,also%20become%20a%20more%20common%20passageway%20for%20ships.

 

[yknpdlr]

When navigating on the Yukon River 1000 mile race, which spends some time and distance above the Arctic Circle, I had no problems with satellite reception making sense 99% of the time. A rare outlier position in my track sometimes occurred when we were opposite a cliff face, or were shadowed by high landscape to our south.

 

[Mason]

However, WGS84 and NAD83 are the two that basically the same (within a couple of meters) in the USA and Canada. NAD27 is the one that can be dozens of meters diferent from those. USGS topo maps are typically on NAD27 datum, online maps are typicallly WGS84.

I stand corrected--I knew it was one of those NADs.

 

[PaddlingPitt]

I think I need to be more specific. My main concern is not that my actual location on the map differs from the gps location. It’s that the gps can not find the coordinates that it seeks. It leads me to being close. But when I actually get close, according to the gps, it now says I am getting farther away from the coordinates it seeks. Farther away no matter which direction I go. Left, right, forward, or backwards. Always farther away. That is one very confused gps. It can not find the very coordinates that it seeks. It wanders around in circles.

 

[yknpdlr]

i repeat. Possible multipath confused signals could be your problem. Confirming your location with map reading is most always the best solution to confused electronics.

 

[PaddlingPitt]

I agree that map reading is the best. This is the first time in 32 years of wilderness paddling that I used a gps. I’m thinking that the problem was not multi-path confused signals. We were paddling on the barren grounds, with very flat uniform terrain.

 

[memaquay]

How does your GPS perform around your house? Enter the waypoint for your house and see what happens.

 

[yknpdlr]

Read any of several googled online articles on GPS accuracy

GPS Accuracy and GPS Drift

GPS Accuracy and GPS Drift - Should you rely on it anymore? - Mosaic51

Discover how GPS accuracy and GPS drift can affect various industries who depend on pin point accuracy and see if they can do any better.

www.mosaic51.com

​

 

[Glenn MacGrady]

I'm binge watching the entire five seasons of Stargate Atlantis, and the odd phenomenon exhibited by the PP et ux. GPS closely resembles the behavior induced by a deadly Wraith force field.

If that's not the explanation, my uneducated guess is that that old GPS is on the blink. Testing it at other waypoints makes sense. Or test it against a new device by ordering a good GPS from Amazon that is returnable for free within 30 days.

 

[Mason]

I agree that map reading is the best. This is the first time in 32 years of wilderness paddling that I used a gps. I’m thinking that the problem was not multi-path confused signals. We were paddling on the barren grounds, with very flat uniform terrain.

And if you're not getting the satellites, you're not getting the location. I believe satellite configuration up north is reduced from down south (might have something to do with lack of users up there??). If you have a full array of satellites visible by the GPS (up to 8 or more) and you're still getting a bad location, then it's likely your GPS. Electronics capacities have changed a bit in 22 years.

Regarding entering a location into your GPS from a map, if the map is NAD 27, and your GPS is WGS84, then you're adding a significant error right there. To add a NAD27 map location into your GPS, you need to change your GPS to NAD27, then enter the coordinate. Changing it back to WGS84 will convert the NAD27 coordinate to WGS84, and you're good to go.

 

[yknpdlr]

A more mathematical analysis of GPS errors, if you care to follow through and process the math, can be found here:

Error analysis for the Global Positioning System - Wikipedia

en.wikipedia.org

https://www.ucalgary.ca/engo_webdocs/MEC/02.20162.MOlynik.pdf

Consumer level (mainly non-mathematical) analyses)

https://www.sbsc.in/pdf/resources/1586018466_GPS_INTRO.pdf

GPS Errors and Biases Part 1

GPS pseudorange and carrier-phase measurements are both affected by several types of random errors and biases (systematic errors). These errors may be classified as those originating at the satellites, those originating at the receiver, and those that are due to […]

what-when-how.com

GPS Errors and Biases Part 2

Receiver measurement noise The receiver measurement noise results from the limitations of the receiver’s electronics. A good GPS system should have a minimum noise level. Generally, a GPS receiver performs a self-test when the user turns it on. However, for […]

what-when-how.com

Among others

Pick your error source among many

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[PaddlingPitt]

How does your GPS perform around your house? Enter the waypoint for your house and see what happens.

I did exactly that when I got home. GPS was dead on. What inferences do you make?

 

[PaddlingPitt]

Thanks, yknpdlr. Very interesting links. I probably won’t be using my gps for navigation anymore, not because of my experiences this summer, however. First, this was the first time I used it in 22 years, only because I was given specific coordinates to be at. Second, this was likely our last remote trip. Age seems to have caught up with us!

 

[memaquay]

Well, sounds like you experienced some kind of anomaly if your gps was working at home. Could be a good beginning for a book with a title like, I dunno, "Lost in the Barrens" or something like that.

 

[yknpdlr]

I probably won’t be using my gps for navigation anymore,

i am not in general anti-GPS. I have long (>45 years) taught navigation. Frst from the air in USAF, then on land for licensed guides, BSA and SAR, as professionally employed to train for state homeland security to L.E. and other professionals. At first, of course, beginning in the woods with my father, GPS did not exist, at least for civilians at the time, so I learned how to navigate by classical traditional methods from Dad. Even in my USAF days as a flight Nav instructor and evaluator, it was old school technology, no GPS yet commonly used in the early '70's. Then I finally applied the skills I knew on land with BSA, licensed guide, and SAR students, my philosophy is for students to well learn the basics first before touching or trusting the electronics. I teach a separate GPS skills course for students who are ready.

However, being a SAR crew boss and team trainer, knowledge and skilled use of GPS is indispensable to accomplish the incident rescue mission. My other main use of GPS is when canoe racing, especially for multi-channel routing in new or changeable channel waters (such as on the Yukon River races and others). Also, in general during on water race training and during racing, I use GPS just mainly for monitoring my speed when not necessary for map route navigation on otherwise familiar water routes.

Finally, I am of the old school opinion and skill long ago taught to me by my father and I never forgot. If I find myself a "might bit confused" in the woods as to exactly where I am, I do what my father taught me as a youngster. Have a seat on a log or a rock, Pull out a sandwich and the map and compass. Do some deep thinking about where I have recently been in the landscape, while I enjoy my lunch, Figure out where I am, and most importantly what to do next with the information I have most recently learned. It has never failed me. When I read the weekly forest ranger SAR reports, I fear that thoughtul skill seems to have been very much lost in recent years.