PDA

View Full Version : Accuracy question



Old Philosopher
04-10-2012, 03:00 PM
I have a Garmin eTrex Venture HC.
Depending upon the atmospheric conditions, time of day, number of satellites accessed, etc., the "error" factor ranges from 11 ft, to as much as 46 ft. This has nothing to do with tree cover, or terrain.

I'm just wondering if this erratic behavior is the "nature of my beast", of if all GPS act this way, pretty much regardless of 'quality'?

Thanks for any, and all feedback.

ezra45
04-10-2012, 03:08 PM
Most civilian ones do...at least the four I've had did. Garmin and Magellan...

Regards,

ezra

Panzer
04-10-2012, 03:09 PM
Yes, not all GPS units are created equal. But all civilian GPS system are affected by atmospheric conditions.

From Garmin's website.

Newer Garmin GPS receivers with WAAS (Wide Area Augmentation System) capability can improve accuracy to less than three meters on average. No additional equipment or fees are required to take advantage of WAAS. Users can also get better accuracy with Differential GPS (DGPS), which corrects GPS signals to within an average of three to five meters. The U.S. Coast Guard operates the most common DGPS correction service. This system consists of a network of towers that receive GPS signals and transmit a corrected signal by beacon transmitters. In order to get the corrected signal, users must have a differential beacon receiver and beacon antenna in addition to their GPS.

KajunKat
04-10-2012, 03:14 PM
I have read that the military purposely decreases the accuracy of GPS for security reasons. They keep the good stuff for themselves. I can totally understand, don't want a garmin guiding a bomb now do we? I have always been totally happy with the accuracy of civilian models, they get you close enough.

flatlander88
04-10-2012, 03:43 PM
I have the Garmin GPSMAP 60CSx, and most of the time it gives me an accuracy reading within 15ft, so it says. There are times that I will be up to 30ft.

Sometimes that problem can be compounded by the fact the coordinates of the Geocache are taken with one GPS with an accuracy reading of +/-20ft and then you try to find it with another GPS of +/-20ft.

Hawkcreek
04-10-2012, 03:49 PM
My Garmin ETrex is the same way. I like out in the open county and it still takes it a while to get more accurate than say 20 feet. I think the new ones are a little better but by how much I couldn't say.
As a side note my grandpa has the same one and likes to mess around with them whenever we get a chance to hang out. We were at the beach one time and standing shoulder to shoulder his ETrex read 60 feet lower in elevation than mine. We tried several times over the weekend and it was always about 60 feet lower so even the same brands and models are "off".

riverjoe
04-10-2012, 04:19 PM
I have read that the military purposely decreases the accuracy of GPS for security reasons. They keep the good stuff for themselves. I can totally understand, don't want a garmin guiding a bomb now do we? I have always been totally happy with the accuracy of civilian models, they get you close enough.

Actually 46 feet would probabley be close enough for a 2000 pound smart bomb probabley the concussion could turn you to jelly .

KajunKat
04-10-2012, 04:25 PM
surely it would, thankfully we seem to be the only ones who have those and a method to deliver them. I think they are more worried about a Cessna packed with fertilizer. Of course, their "smart bombs" are piloted by "not so smart" folks so the point is probably moot.


Actually 46 feet would probabley be close enough for a 2000 pound smart bomb probabley the concussion could turn you to jelly .

Old Philosopher
04-10-2012, 04:33 PM
Thanks for the replies!
Yeah, I guess we should count ourselves lucky. There was a big stink about civilian accuracy, and it was changed to +/- 3 meters, down from +/- 100 meters several years ago.
When I was doing bombing stuff for the AF, we called it CEP (Circle Error Probability). I'm assuming the same criteria holds for GPS accuracy. So when it says "16 feet", it most likely means that your target is anywhere within an 8 foot radius from you.
My unit once told me the "error" was 30 feet, but it walked me up to within 3 feet of a cache.
I totally agree that with geocaching the error factor of your unit, coupled with the error factor of the cacher's unit, added to how well the cacher averaged their readings all boil down to a WAG of where it actually is! :D

Grizzly
04-10-2012, 04:38 PM
I would not say that it is totally a civilian GPS problem. If you were to get a Survey grade GPS it would be sub centimeter accurate, as good as anything in the military. But it would be pretty big to carry around and would cost you about $35K.

draco
04-10-2012, 04:39 PM
It does depend on the receiver and chipset. One of my GPS I can go to the exact coordinates of a geocache and look down and there it is. My older one you could be 30 feet off. I have also noticed that some people who hide the caches have better equipment than others. Some of these guys are never even close on their coordinates.

kankcamping
04-10-2012, 04:42 PM
Yeah, I think the ones real sensitive equipment in them are the ones you need for extremely accurate readings. Probably only necessary for people who like to climb huge mountains like Everest.

I think the regular GPS's just use what the topographic info it was given and not a reading from an instrument inside the device, like a barometric altimeter.

46 foot errors is kind of buggy to me too. When I work my butt off to get to the top of a smaller mountain, I want my dang 46 feet of elevation, I worked for it so I want it logged on the trip calculator.

kankcamping
04-10-2012, 04:44 PM
oops, I thought you meant elevation, not your movement on the ground, Oh well.

riverjoe
04-10-2012, 06:49 PM
I've never gotten into Geo Cache but it seems a lot like what we used to call orienteering in the infantry . It was a difficult thing to do when you were in mountainous terrain , trying to judge contour lines , the azimuth of saddles , draws and distant hilltops .
So what skill is needed if your GPS tells you exactly where to go ? Im sure there is more to it then I think there is .

draco
04-10-2012, 07:03 PM
Ya still have to find it. LOL I have a few examples of Caches I spent way too much time on.

One was one of the rock key holders. Only the dang thing was on a huge pile of rocks. Like a needle in a haystack.

Another fun one was one based on the Da Vinci Code. I think I was the first person who was not working as a team to crack that one. Let's just say it took me 4 trips and some serious internet search time to find some of the answers.

One was in a spot that you did not know how you did not find it. Actually at one point I had my hand on it and did not know it. It was in a piece of wood that had a hidden secret compartment.

One was in a fake bird house hanging from a tree. I was looking everywhere but up.

The best thing about geocaching is I have found remote places close to me I never knew existed. One was at a old log cabin that is on city property but is not posted in any way so very few people knew about it. Another was at a old mill that was off the beaten path but had a nice county owned park that again very few people knew about. You have to pick and choose as some caches are boring but the ones that are cool are very cool.

riverjoe
04-10-2012, 07:34 PM
Now that sounds kind of fun unlike the infantry .

Old Philosopher
04-10-2012, 07:41 PM
My nemesis is a standard magnetic key holder hidden somewhere on/under an old RR caboose. I've been after it 5 times, and still DNF!!!

riverjoe
04-10-2012, 09:40 PM
My nemesis is a standard magnetic key holder hidden somewhere on/under an old RR caboose. I've been after it 5 times, and still DNF!!!
Too bad your so modern and don't carry an old military compass with you . It could help you find the magnet .

Old Philosopher
04-10-2012, 10:06 PM
Too bad your so modern and don't carry an old military compass with you . It could help you find the magnet .
And you've tried finding a 1" magnet on a 40' railroad car using a compass? :confused:

riverjoe
04-10-2012, 10:45 PM
Admittedly it's a theory . You might have to watch the needle really really close .

Old Philosopher
04-10-2012, 11:16 PM
Admittedly it's a theory . You might have to watch the needle really really close .
I'd test the theory with one of my little rare earth magnets, but I don't wanna screw up my faithful Silva. :3:

Nick107
04-11-2012, 08:43 PM
I have a Garmin eTrex Venture HC.
Depending upon the atmospheric conditions, time of day, number of satellites accessed, etc., the "error" factor ranges from 11 ft, to as much as 46 ft. This has nothing to do with tree cover, or terrain.

I'm just wondering if this erratic behavior is the "nature of my beast", of if all GPS act this way, pretty much regardless of 'quality'?

Thanks for any, and all feedback.

A little background, I get to work / play with survey grade gps units for work. The variance in your fix is almost always the number of satellites your receiver is picking up. Even with our expensive equipment, if there are no satellites we are sol and pack it up for a few hours. I can offer a few tips if your interested

Old Philosopher
04-11-2012, 09:22 PM
A little background, I get to work / play with survey grade gps units for work. The variance in your fix is almost always the number of satellites your receiver is picking up. Even with our expensive equipment, if there are no satellites we are sol and pack it up for a few hours. I can offer a few tips if your interested
Sure, I'm all ears. I already have WAAS enabled.

draco
04-11-2012, 09:52 PM
A little background, I get to work / play with survey grade gps units for work. The variance in your fix is almost always the number of satellites your receiver is picking up. Even with our expensive equipment, if there are no satellites we are sol and pack it up for a few hours. I can offer a few tips if your interested

Please. Nice to hear what a pro has to say on the subject.

Nick107
04-11-2012, 10:09 PM
Sure, I'm all ears. I already have WAAS enabled.

This screen is your best friend
http://www.survival-gear-guide.com/image-files/satellite-screen.jpg

There are 31 US gps satellites, the vast majority orbit south of the united states. If you have an option, find a spot with open sky to the south. In the satellite screen, it shows a top down view of where the satellites are, and corresponding numbers in a bar graph below. Try using that to guide you to a stronger signal that can increase your positional fix. Sometimes you will just have bad timing and only have 4 or 5 satellites available, most of the time it's around noon in our neck of the woods (Montana included). Just remember a spot that doesn't work may be fine in a few hours (if it's open sky, of course).

Old Philosopher
04-12-2012, 01:42 AM
.... Try using that to guide you to a stronger signal that can increase your positional fix. ...
And from the acquisition screen, how does one go about doing that?

Horned Toad
04-12-2012, 01:39 PM
So what skill is needed if your GPS tells you exactly where to go ? Im sure there is more to it then I think there is .

None, unless someone takes it upon themselves to learn orienteering with a map and compass as a backup for when the GPS fails.

Panzer
04-12-2012, 01:52 PM
o what skill is needed if your GPS tells you exactly where to go ? Im sure there is more to it then I think there is .

There is more to it then that. A GPS is a tool and like any tool if you don't know how to use it you will be less efficient with it. There are technical nuances the operator has to be aware of and proficient with. You have to know how to accurately enter in coordinates, you have to know how to set way points, etc. The GPS will not tell you the best way to get there merely a path to follow, so you have to use better judgement.

santaman2000
04-12-2012, 03:18 PM
Yeah, I think the ones real sensitive equipment in them are the ones you need for extremely accurate readings. Probably only necessary for people who like to climb huge mountains like Everest.

I think the regular GPS's just use what the topographic info it was given and not a reading from an instrument inside the device, like a barometric altimeter...

Actually it doesn't use either. It triangulates your elevation much the same way it triangulates your position. It needs at least 3 visible satellites to triangulate altitude (elevation) and at least 4 visible to compute velocity.

these are minimums and in all cases, more is better.

santaman2000
04-12-2012, 03:27 PM
I've never gotten into Geo Cache but it seems a lot like what we used to call orienteering in the infantry . It was a difficult thing to do when you were in mountainous terrain , trying to judge contour lines , the azimuth of saddles , draws and distant hilltops .
So what skill is needed if your GPS tells you exactly where to go ? Im sure there is more to it then I think there is .

Yes there's a lot more to it. As someone already pointed out the GPS only gives you a diretion. You still have to figure out the best route to avoid obstacles if hiking (how to get around lakes, bogs, etc. or up/down a cliff. If your boating then remember the GPS doesn't tell you diddly jack about the currents, tides or winds that effect your course; likewise if you're flying.

Old Philosopher
04-12-2012, 05:36 PM
There is more to it then that. A GPS is a tool and like any tool if you don't know how to use it you will be less efficient with it. There are technical nuances the operator has to be aware of and proficient with. You have to know how to accurately enter in coordinates, you have to know how to set way points, etc. The GPS will not tell you the best way to get there merely a path to follow, so you have to use better judgement.


Yes there's a lot more to it. As someone already pointed out the GPS only gives you a diretion. You still have to figure out the best route to avoid obstacles if hiking (how to get around lakes, bogs, etc. or up/down a cliff. If your boating then remember the GPS doesn't tell you diddly jack about the currents, tides or winds that effect your course; likewise if you're flying.
Ditto to all of the above. If I ask my GPS how to get to the Forest Service cabin 13 miles to the east of me, it draws a straight line on its display map showing me where the cabin is relative to my position. This is all well and good, if I had feathers! The "route" on the GPS takes me right over the top of a 7,000 foot mountain! :eek:

I can't speak for anyone else, but what I do in the above situation is sit down with my computer map software, plot out a defined route following the trails on the topo map, and then transfer it to my GPS. A route like this may have a couple hundred reference points ("way points"), and when I'm navigating with the GPS, it takes me from one point to the next. My trip to the cabin is now like a "connect the dots" game. ;)

riverjoe
04-12-2012, 06:22 PM
I hear what you're saying loud and clear . I actually broke one of my ironclad rules in the army and volunteered( never volunteer) to walk point because I was so sick of going to the bottom of the deepest valleys only to go up the steepest hills . At least the point man got input on the route and they would let you see the map and the compass .
Apparently our officers did not receive much training in orienteering . ( we were actually lost in Laos for about a week one time . )
Its like they could'nt recognize , saddles , fingers or read elevations .
Any way it would have been nice to have GPS but I'm afraid Charlie would have figured out a way to screw with it . I guess at the very least I would like to have a compass and a topographic map to double check my coordinates .

Nick107
04-12-2012, 07:10 PM
I can't speak for anyone else, but what I do in the above situation is sit down with my computer map software, plot out a defined route following the trails on the topo map, and then transfer it to my GPS. A route like this may have a couple hundred reference points ("way points"), and when I'm navigating with the GPS, it takes me from one point to the next. My trip to the cabin is now like a "connect the dots" game. ;)

If you go to the effort of setting your way points, the following link may be useful http://www.trimble.com/GNSSPlanningOnline/

you will have to customize it a bit.

Settings-
Select your lat / long position, height in meters, cutoff is important if your going to be in the woods, in my area I default at 15 degrees but sometimes if I know I'll be in thick cover I go as high as 30. Then select the time frame you plan on using your gps unit.

Satellite library-
deselect glonass (Russian sats) and Galileo (EU sats).

Number of satellites-
This gives you a bar graph of the number of satellites you can expect to have available, this is a best case scenario. Anything under 5 is pretty sketchy.

Old Philosopher
04-12-2012, 10:33 PM
...
Number of satellites-
This gives you a bar graph of the number of satellites you can expect to have available, this is a best case scenario. Anything under 5 is pretty sketchy.
Very interesting! Thanks again. Running a 48 hour range at my current location I'm averaging between 8 and 11 satellites.

Nick107
04-12-2012, 11:16 PM
This should help you get that cache at the RR caboose, you can get a few days or even a week planned out with the best satellite window to plan your next attempt. I may be able to help you tighten up a location depending on the coordinate datum they used to set it.

hog
04-13-2012, 03:34 AM
I have a Garmin ETREX H which is good for 10' most of the time but have known it to be miles out, literaly on the odd occasion.
I think most of the time atmospherics and cloud base are to blame after all its for civi use and pinpoint accuracy are not primary, does the job for me though as if a person is in the wilds and looking for their tent, if it cant be seen in a 10 foot area then perhaps its not wise to venture out..LOL.

Horned Toad
04-13-2012, 10:29 AM
Ditto to all of the above. If I ask my GPS how to get to the Forest Service cabin 13 miles to the east of me, it draws a straight line on its display map showing me where the cabin is relative to my position. This is all well and good, if I had feathers! The "route" on the GPS takes me right over the top of a 7,000 foot mountain! :eek:

I can't speak for anyone else, but what I do in the above situation is sit down with my computer map software, plot out a defined route following the trails on the topo map, and then transfer it to my GPS. A route like this may have a couple hundred reference points ("way points"), and when I'm navigating with the GPS, it takes me from one point to the next. My trip to the cabin is now like a "connect the dots" game. ;)

If you are going to do all that why donít you just get a map? My routine to do a similar hike especially on a trail would be.

Look at map at home, measure how far I want to go, using the old method of a piece of a paper and making tick marks.

See where I want to do my stops around here that is usually around places to get water.

Orient map at trail head, start walking, check map about once an hour when I take a break or at major trail intersections. Enjoy my hike, not worry about looking at a gadget and trust my own skill.

Horned Toad
04-13-2012, 10:46 AM
To me the fact that some of the information in this thread is considered new highlights the fact that not much skill is needed to use a GPS. I have been using a GPS on and off since the early 90s. At that time they were new in the military. For the GPS to function properly you had to go to the commo guys and get the correct info to plug into your GPS for it to be remotely accurate. We used the GPS but double checked everything by hand with a map and compass. When something was in doubt you trusted the map and compass.


We have GPS at work and I have a Vista H. The only time I use it is when I need to get coordinatesí for a report. Other than that I donít turn it on. There is a very clear dividing line in skill level based on whether my coworkers got here before GPS were issued and those that got here after.

So while they are necessary and useful for certain things I think itís sort of ironic that on a forum that preaches skill and knowledge over gadgets and gear, people get all hung up about a GPS.

Nick107
04-13-2012, 12:03 PM
I think some may have missed the point in the OP. but as to why use GPS when a map and compass has worked for x years... Because its a challenge with a huge payout. Being able to carry a device in your pocket with a topo map of the entire united states is nothing to sneeze at. Along with being able to download your exact route or points you found interesting. That said I always have a map and compass on a trip

KajunKat
04-13-2012, 02:33 PM
Yeah it is kind of like the argument of people using a sub MOA rifle with a $2K scope out of a box stand versus a barebow hunter stalking. They will both put meat in the freezer but one take more time, effort and skill.

santaman2000
04-13-2012, 04:55 PM
...We used the GPS but double checked everything by hand with a map and compass. When something was in doubt you trusted the map and compass...

...So while they are necessary and useful for certain things I think itís sort of ironic that on a forum that preaches skill and knowledge over gadgets and gear, people get all hung up about a GPS.

I've used a GPS (not especially good at it yet; at least not on foot) and maps and compasses. Frankly though to me they're ALL gadgets and gear. I usually do best just leaving them ALL at home and just paying attention to where I am and where I'm going.

Old Philosopher
04-13-2012, 10:12 PM
I think some may have missed the point in the OP. but as to why use GPS when a map and compass has worked for x years... Because its a challenge with a huge payout. Being able to carry a device in your pocket with a topo map of the entire united states is nothing to sneeze at. Along with being able to download your exact route or points you found interesting. That said I always have a map and compass on a trip
Thanks!
Funny how a thread about GPS accuracy degenerated into casting aspersions on the navigational skills of those who choose to use them. :confused:
I started orienteering in the 1960's. I worked in maps, charts and geodesy in the military. I used naval navigation charts extensively during my years of diving. I love cartography, and I keep a mental topo map in my head every time I venture into the field. My Silva compass is older than 40% of the membership here. That was not the issue.

Nick107
04-14-2012, 10:12 AM
Thanks!
Funny how a thread about GPS accuracy degenerated into casting aspersions on the navigational skills of those who choose to use them. :confused:
I started orienteering in the 1960's. I worked in maps, charts and geodesy in the military. I used naval navigation charts extensively during my years of diving. I love cartography, and I keep a mental topo map in my head every time I venture into the field. My Silva compass is older than 40% of the membership here. That was not the issue.

AMEN! Back on topic....

Now that we have a way to confirm the number of satellites, the next issue is signal noise. It doesn't have nearly the effect on accuracy that not having enough satellites will, but at times you will see a drop in accuracy because of it. The signal that is sent from SPACE is extremely weak and has to travel through two major atmospheric zones, the ionosphere and troposphere. Check out this page for a good tutorial http://www.trimble.com/gps/howgps-error.shtml

The error factor is expressed as DOP (dilution of precision) and it is one of the tabs on the site a shared a few days ago. A DOP over 8 means your accuracy will be negatively affected regardless of the number of satellites available. It should normaly be around 1 to 4.

Old Philosopher
04-14-2012, 10:52 AM
That is an extremely informative site. Thanks again.
I'm starting to believe our current level of solar activity isn't exactly helping, either.

Nick107
04-14-2012, 10:58 AM
Exactly, solar activity is a killer. Trimble is a pretty cool company, I think the best when it comes to gps equipment. Take a look around the site it's a great step by step walk through of how it all works. When it gets into differential and multi channel gps its talking about survey grade receivers, but the basics are all explained very well.

Nick107
04-14-2012, 11:54 AM
In the end gps has a few variables that can mess with its accuracy, but with a little understanding and preparation I feel comfortable relying on it as a navigation tool. You wouldn't use a compass without setting and understanding declination, and you wouldn't go out with only one way to light a fire. Use the same common sense you apply to any tool and it can open up a new way of enjoying the outdoors.

Old Philosopher
04-14-2012, 03:54 PM
... You wouldn't use a compass without setting and understanding declination, and you wouldn't go out with only one way to light a fire. Use the same common sense you apply to any tool and it can open up a new way of enjoying the outdoors.
A bit off topic, but there are things I like about the GPS that are more complicated with other means, or require extra equipment.


Walking speed (recalculated every second)
Time to destination (based upon walking speed)
Distance to destination (based upon preplanned route)
Dedicated geocaching apps and log
Stop watch
Calendar with best fishing/hunting dates and times based upon geographic location
Sun and Moon programmable calculator
Scientific and 'normal' calculator
Of course, date and time


All in a unit a little bigger than a pack o' cigarettes. :p

riverjoe
04-15-2012, 09:02 PM
I think some may have missed the point in the OP. but as to why use GPS when a map and compass has worked for x years... Because its a challenge with a huge payout. Being able to carry a device in your pocket with a topo map of the entire united states is nothing to sneeze at. Along with being able to download your exact route or points you found interesting. That said I always have a map and compass on a trip

See I didn't think about the topo map on screen . The last topo I bought cost me $9.00 .
I might have to look into one of these .
Ok whats the best GPS for a cheapskate . ( maybe this should be on Iz's what s the best underwear thread )

Old Philosopher
04-15-2012, 09:46 PM
See I didn't think about the topo map on screen . The last topo I bought cost me $9.00 .
I might have to look into one of these .
Ok whats the best GPS for a cheapskate . ( maybe this should be on Iz's what s the best underwear thread )
My Garmin eTrex Venture HC can be had now for about $100. I had to download and install 3rd Party topo maps, but now it does everything I expected, and more. A little studying of the manual, and it's really easy to use. It's perfect for someone like me who doesn't want to go back to school to learn what all the buttons do.
The open source topo maps I got were built by a bunch of cartographers, and comes with annotations for fire lookouts, campgrounds, historical sites (locations of old towns no longer on other maps), old mines, etc. I'm a TH'er, and these maps are great for finding prospective places to use my metal detector.

Horned Toad
04-15-2012, 10:52 PM
Well since I have been away I am just going to shotgun a bunch of these in one post.


Being able to carry a device in your pocket with a topo map of the entire United States is nothing to sneeze at.

True as long as it works, but if it breaks though, or if the batteries go dead. With solo chargers though, the battery issue is becoming less of a concern


Thanks!
Funny how a thread about GPS accuracy degenerated into casting aspersions on the navigational skills of those who choose to use them. :confused:
I started orienteering in the 1960's. I worked in maps, charts and geodesy in the military. I used naval navigation charts extensively during my years of diving. I love cartography, and I keep a mental topo map in my head every time I venture into the field. My Silva compass is older than 40% of the membership here. That was not the issue.

Iím not sure how you come up with those assumptions but whatever. There are enough clues in your post for someone to realize you know how to use a map. As far as practical accuracy, I think maybe you have an unrealistic expectation. You want to find a 1Ēx2Ē square on a 40 ft train car.


A bit off topic, but there are things I like about the GPS that are more complicated with other means, or require extra equipment.


a Walking speed (recalculated every second)
b Time to destination (based upon walking speed)
c Distance to destination (based upon preplanned route)
d Dedicated geocaching apps and log
e Stop watch
f Calendar with best fishing/hunting dates and times based upon geographic location
g Sun and Moon programmable calculator
h Scientific and 'normal' calculator
i Of course, date and time


All in a unit a little bigger than a pack o' cigarettes. :p

A,b, and c nice but something I refuse to worry about when I am in the woods.
I check moon phase, sunrise and sunset before I go, but itís just a general thing I do out of habit. The one you didnít list which is about my only fancy gadget on my watch is an altimeter. If I was still in Florida an altimeter would be pretty pointless but in places with real mountains they are nice to check against the topo.


See I didn't think about the topo map on screen . The last topo I bought cost me $9.00 .
I might have to look into one of these .
Ok whats the best GPS for a cheapskate . ( maybe this should be on Iz's what s the best underwear thread )

Topos are good, I just hate the small screen on the gps. One of the main reasons I got an ipad is because I have topos on it on a screen I can see easily. I mainly use it for work though. I have not taken an ipad out camping.

Old Philosopher
04-16-2012, 02:23 AM
Iím not sure how you come up with those assumptions but whatever. There are enough clues in your post for someone to realize you know how to use a map. As far as practical accuracy, I think maybe you have an unrealistic expectation. You want to find a 1Ēx2Ē square on a 40 ft train car.
When geocaching, a GPS get you in the vicinity. Obviously you have to use your eyes to find the cache. Sometimes even that isn't easy, when you're talking about trick rocks, and logs. :rolleyes:


A,b, and c nice but something I refuse to worry about when I am in the woods. Different strokes for different folks. ;)

.... The one you didnít list which is about my only fancy gadget on my watch is an altimeter. If I was still in Florida an altimeter would be pretty pointless but in places with real mountains they are nice to check against the topo.The elevation (altitude) derived by the GPS multiple triangulation is just as accurate as a barometric altimeter which can be affected by changes in the weather. I've used several barometric altimeters, and one thing a person learns is to check it before bedding down, and reset it in the morning, if the barometer has risen, or fallen during the night.
The two main pieces of information a GPS gives you are your geographic coordinates, AND the elevation at your location. I didn't mention the altimeter function, or the coordinates function because that's just basic info.

knochelbiter
04-16-2012, 02:40 AM
Exactly, solar activity is a killer. Trimble is a pretty cool company, I think the best when it comes to gps equipment. Take a look around the site it's a great step by step walk through of how it all works. When it gets into differential and multi channel gps its talking about survey grade receivers, but the basics are all explained very well.

After using a big survey-grade Trimble last year I have to admit I have mixed feelings about them now... when it worked it really worked (accuracy down to centimeters) but when it didn't work it might pop up to 25m or more accuracy while our handheld units were still getting 6-8m. Did you ever experience this? It was widespread across the project so I know it wasn't just our unit.

This got worse later in the afternoon, but this at a latitude nearing 60 degrees north. I'm guessing the high-accuracy satellites used by the Trimble are fewer, far between and maybe closer to the equator than the ones used by more common commercial GPS units?

Nick107
04-16-2012, 08:03 PM
After using a big survey-grade Trimble last year I have to admit I have mixed feelings about them now... when it worked it really worked (accuracy down to centimeters) but when it didn't work it might pop up to 25m or more accuracy while our handheld units were still getting 6-8m. Did you ever experience this? It was widespread across the project so I know it wasn't just our unit.

This got worse later in the afternoon, but this at a latitude nearing 60 degrees north. I'm guessing the high-accuracy satellites used by the Trimble are fewer, far between and maybe closer to the equator than the ones used by more common commercial GPS units?

Achieving centimeter level accuracy requires very solid corrections from another unit set up at a known location, and some way for the two to communicate reliably. Im guessing the issue your team had was in keeping a consistent connection to a base station. It's usually done by transmitting corrections on a radio frequency. If it's a big area being covered, holes can happen and when it does your drop in accuracy is substantial, and it sucks... I've done a lot of work lately with cellular networks to log into a reference network instead of having to set up a radio base station. The technology is really becoming solid, and with the future of cell network coverage, it's very possible that even recreational gps users could receive corrections in a few years.

Also, right now there are only three satellite networks up and running. The American satellites we all use, and the Russian (named Glonass) and European union (named Galileo) networks. Pretty soon recreational gps units will be able to use Glonass and Galileo satellites, which will help fill in the gaps we have right now, especially the farther north you go.

knochelbiter
04-16-2012, 11:58 PM
Five years ago I was used to working with a compass, map and hip-line and today we can get centimeter-level accuracy from space. Pretty amazing.

But the real problem with technology is that you can get used to it so quickly. Give me a GPS with sub-meter accuracy and I'm all happy, but take it away the next day and suddenly the whole project is in disarray, things don't line up, etc. Whereas 5 years ago I'd be *happy* to get the results we get on bad days now :)

Thanks for the info!