GPS Accuracy

[Fishnwire]

I have a low budget GPS a friend gave me because he is too techno-idiotic to learn to use. I've seen it on sale at CTC for about a hundred bucks. I can mark a spot on it and later it will tell me I'm right on that same spot when sometimes I'm about 20-30 meters away.

 

It's just a cheap unit but I'm wondering how much the accuracy increases as the price point goes up. I can remember times crappie fishing that I (or my partner) would be getting fish steady out of two or three holes grouped together and 5 or 10 meters away lines would sit all day with barely a bump. We'd fish there for weeks and certain holes (regardless of the offering) would consistently outproduce holes very close by. I guess there was a sunken tree or weed edge or drop-off or something there they liked...the point is I'd like to be able to mark a spot and find it the next year within a couple meters or less...is this possible with better quality GPS units?

 

Thanks for your input.

[blarg]

I have a garmin, i think the number is a 76csx, the model is probably 4-5 years old, i can't say if its more accurate than average, but i seem to remember it has a setting that allows it to use more than just a few satellite in order to increase its accuracy, i have never found it to be out by much.

[mike rousseau]

I used to use a magelin cheap handheld... $80 from Canadian tire...

 

I marked a hot walleye bite... Went back the next day following the "go to" feature... Ended up stepping on my hole from the previous day...

 

It seem st the older units are more accurate then the new ones...

[irishfield]

How accurate are they... depends on what threat level the USA's homeland security is running on on any given day!

[crappieperchhunter]

I have a Garmin...lower end model. Just started really using it last year. I would say within 30-50 feet is pretty much average of how close it gets you. I got no problem with that.

[dannyboy]

With WAAS/EGNOS enabled, a good view of the sky and reasonable satellite geometry you get 2 to 3 meter accuracy laterally on current hand held units which track up to 12 satellites. On average you get 8 meter accuracy 95% of the time. Some high end units are centimeter accurate.

 

Selective availability hasn't been used since Clinton in 2000 to allow aircraft to use GPS for approaches amongst other uses, it wasn't even utilized during 9/11 and is no longer built into new GPS satellites. But when used it would degrade signal accuracy to 100 meters.

 

Dan

[bdox]

My handheld 60CSx will usually have about 3-5 meter accuracy.

[Billy Bob]

How accurate are they... depends on what threat level the USA's homeland security is running on on any given day!

 

Us U.S.A. boys have the secret code and with it we have pin point accuracy...LOL

 

The better units that have WAAS capability e.g. my Lowrance LMS 520c unit has WAAS that I can turn on and off depending on satellite signal strength....but even then you will not get 1000% dead on accuracy, unless you have the secret code....

[singingdog]

More expensive units offer greater accuracy in a few ways:

1. Being WAAS enabled, which allows the unit to read "extra" satellites that broadcast corrections to the regular satellite signals.

2. Better antennas that read lower quality signals (signals coming through cloud cover or tree cover), or reflected signals (signals bouncing off buildings or steep landscape features).

3. The ability to "average" (which 95% of GPS users don't use): averaging is when the GPS unit takes readings every few seconds for a certain time period and gradually "homes in" on the location. Averaging is one of the ways that super-accurate units (accurate within a few centimeters, and price tags in the several thousands) achieve the level of accuracy they do. BUT, it can take from several minutes up 15 minutes for those units to achieve that kind of accuracy.

 

If accuracy within a few meters is important to you, then look for a unit with the above features AND learn to use them.

 

I teach GPS use to a few groups every year. The students are typically using low-end units like the Garmin Etrex ($100). We consider accuracy withing 10 m to be acceptable with those units, but often have to settle for 15 m. Using a Garmin 62cs ($450), I usually get accuracy within 5 m under the same conditions.

[Garnet]

Try this, put your GPS on 5 mile scale and drop a gps pt. write the coordinates down now lower your scale to the minimum. You will find that your current position is not your GPS pt.

 

I have good GPS 798 bird and hds 8 before I install a gps pt. I reduce the scale to minimum first.

 

And when I'm returning to a pt. keep zooming in until you are at min. scale for best accuracy.

[Roy]

Very good information, Singingdog. Thank you.

[crappieperchhunter]

 

I teach GPS use to a few groups every year. The students are typically using low-end units like the Garmin Etrex ($100). We consider accuracy withing 10 m to be acceptable with those units, but often have to settle for 15 m.

 

 

I posted up farther on this thread and I have a Garmin Etrex. It seems my findings last year when I finally started using my handheld are the same as what you have experienced. Being new to this technology I didn't know if my unit was working at 100%. I now am certain it was. Thx for taking the time on your detailed reply. It has helped me to understand this technology better.

[COUNTRYBOY]

I WAS LEARNING TO USE MY GPS A COUPLE YRS BACK ON GB. I HAD A GOOD SPOT SO I MARKED IT. THE NEXT DAY I SELECT GO TO...WANTED TO SEE IF IT WAS ACURRATE. I'M WALKING ALONG AND IT SAYS..YOU HAVE REACHED YOUR DESTINATION AS I AM GOING THROUGH THE ICE. I PUT MY ARMS STRAIGHT OUT AND CAUGHT MYSELF. GOT MYSELF OUT...A LITTLE WET. SO BACK TO THE GPS...WAS IT THE EXACT SPOT OR DID IT MOVE ME 10 OR 20 FT FURTHER OUT ? GEZUS WATER WAS COLD

[Billy Bob]

I WAS LEARNING TO USE MY GPS A COUPLE YRS BACK ON GB. I HAD A GOOD SPOT SO I MARKED IT. THE NEXT DAY I SELECT GO TO...WANTED TO SEE IF IT WAS ACURRATE. I'M WALKING ALONG AND IT SAYS..YOU HAVE REACHED YOUR DESTINATION AS I AM GOING THROUGH THE ICE. I PUT MY ARMS STRAIGHT OUT AND CAUGHT MYSELF. GOT MYSELF OUT...A LITTLE WET. SO BACK TO THE GPS...WAS IT THE EXACT SPOT OR DID IT MOVE ME 10 OR 20 FT FURTHER OUT ? GEZUS WATER WAS COLD

 

Every GPS has setting that you can dialed in or out before it says "YOU HAVE REACHED YOUR DESTINATION".....I have mine dialed in the most it can go and rather not have it at all....

[COUNTRYBOY]

Every GPS has setting that you can dialed in or out before it says "YOU HAVE REACHED YOUR DESTINATION".....I have mine dialed in the most it can go and rather not have it at all....

 

 

I LEARN A LITTLE MORE EACH TIME I USE IT...ALSO BOUGHT A MUSTANG JACKET

[Dabluz]

I've been using handheld GPS devices for a very long time. Garmins, Magellans and now a Lowrance with a Freedom Map card that dates back to 1994.

 

All handheld GPS units have about the same accuracy....around 5 to 10 yards. I will actually go and say that the older units with WAAS are a tad more accurate because they are less sensitive. The more recent and sensitive units gather info from the satellites but also information that is sent by the satillites and bounced all over the terrain (hills, buildings etc) that induce more errors. A satellite signal that gets bounced off the mountain 1/4 mile away means that it's signal is 1/4 mile longer than it should be. However, the more sensitive units can also pick up more signals when the user is somewhere where a lot of the signals are blocked (canyon, heavy wood cover, trees, tall buildings etc). I had an old Garmin 45 with 8 sequential channels instead of the 8 (or more) paralllel channels that the units have today. Even though this GPS did not have a mapping, it showed me which side of the road I was driving on when I put the data through a program like Easy GPS. It was very accurate when I let it do it's job.

 

Surveyors use GPS units for extreme accuracy. Yes they are extremely accurate but they gather signals over a long period of time and the info is fed into computers. This is where "averaging" becomes important. Using the "averaging" option is of outmost importance if you want accuracy. Averaging is done by setting down your gps and letting it gather information according to it's update speed. Most of the time, update speed is about once per second. This means that it fixes a position in it's memory every second. After 30 seconds, it has 30 positions with about a 5 to 10 yard accuracy. But, if you average out the 30 positions, there is a position where the signal has passed the most often. You can see the wandering that a GPS does by immobilizing the GPS for a few seconds, entering a waypoint and then zooming in to maximum to see the tracks that were made around the registered waypoint.

 

When I use my old Lowrance Pro for registering a waypoint, I use the averaging option and let it work for a good minute before entering my waypoint. I do this when I want to find an ice fishing hole even though a few days have past and the wind has obliterated all traces of the hole. This is very handy where I live and the ice is over 3 feet thick. I can re-open holes with my ice chisel because the ice is not as thick in the old hole.

Edited December 15, 2011 by Dabluz

[Fishnwire]

Awesome info from everyone. Dubluz...you are quite the source on your own. Thanks to you and everyone else.

 

What I'm hearing is that using a GPS to be able to find an old hole that is frozen over and covered in snow is probably not going to happen, let alone help me drill a hole in that (nearly) exact same spot year after year.

 

Dubluz...one quick question if you don't mind. What you said about immobilizing the unit and averaging several waypoints taken in the same location makes perfect sense (not that I would have thought of it) but a buddy once told me that a GPS is most accurate when it is moving. Did he have no idea what he was talking about? It wouldn't be the first time with this guy.

[danc]

Just going from what my Garmin GPSmap 60CSx tells me, I've been within as close as 7 feet of accuracy in an open area outside. In my vehicle where I use it most often, I'm usually within about 15 feet or so. The only time I've used averaging was when I was hiding a cache, so that the seekers would have a more accurate waypoint. Bottom line. Close enough for me.

[irishfield]

6 mph trolling for 'skis on GBay.. don't trust any unit to put the rocks where you think they are!

[Dabluz]

Awesome info from everyone. Dubluz...you are quite the source on your own. Thanks to you and everyone else.

 

What I'm hearing is that using a GPS to be able to find an old hole that is frozen over and covered in snow is probably not going to happen, let alone help me drill a hole in that (nearly) exact same spot year after year.

 

Dubluz...one quick question if you don't mind. What you said about immobilizing the unit and averaging several waypoints taken in the same location makes perfect sense (not that I would have thought of it) but a buddy once told me that a GPS is most accurate when it is moving. Did he have no idea what he was talking about? It wouldn't be the first time with this guy.

 

 

In a way....he was right....for one thing. With the older handheld GPS units, you have to be moving to be able to use the compass. You can't stand still and just point the GPS to the north in order to go north. However, most of the newer units do have a magnetic compass built into the unit and those devices do not need to be moving to indicating where is north.

 

The "averaging" is a built-in option with all GPS units. When I decide to enter a waypoint on my Lowrance, I push on "enter" just once and then a menu pops up. In the menu, there is the option "averaging" which I choose from the menu and push on "enter" again. I give the GPS some time to gather data and push "enter". The GPS then counts the number of positions it is recording (according to the refresh rate) and when I think there are enough points, I push on "enter" again and the GPS's computer does the rest.

Edited December 16, 2011 by Dabluz