GPS worthless

[Tallpine]

I was out trying to locate some property lines for a neighbor today.

I saved a waypoint at the house, and then another one at the NW corner stake.

Later, I noticed that the house latitude recorded was north of that recorded of the NW corner 

This, on an 80 acre parcel - the house is maybe 800-1000 feet SE diagonally from the corner.

I suppose the trees might have caused a problem with one of the waypoints.  At any rate... I sure hope none of you are planning to use GPS to find your way out of the woods

I think I'll just stick with a compass from now on.

[RevDisk]

I was out trying to locate some property lines for a neighbor today.

I saved a waypoint at the house, and then another one at the NW corner stake.

Later, I noticed that the house latitude recorded was north of that recorded of the NW corner 

This, on an 80 acre parcel - the house is maybe 800-1000 feet SE diagonally from the corner.

I suppose the trees might have caused a problem with one of the waypoints.  At any rate... I sure hope none of you are planning to use GPS to find your way out of the woods

I think I'll just stick with a compass from now on.

Were you using WAAS?  Was your GPS unit properly calibrated?  How many satellites were you hitting?

[Tallpine]

1 & 2) I dunno ... it's a fire dept GPS, supposed to be ready to use (in-service)

3) I didn't notice at the time

[RevDisk]

1 & 2) I dunno ... it's a fire dept GPS, supposed to be ready to use (in-service)

3) I didn't notice at the time

WAAS are ground stations that enhance GPS accuracy.  Allows for a worst case accuracy of within 7.6m.  Never, never buy a GPS unit without WAAS.  I don't care how great a deal it seems.

Calibration can also allow you to have higher accuracy.  Low end commercial GPS units have poor or no calibration settings.

And obviously, less satellites, worse or no accuracy.  Letting a GPS unit for a minute or two gives the best readings for a static position.

[mgdavis]

Mine has a setting where you can stay put for a while and "average" your position to get a more accurate reading.

[Tallpine]

Okay ... well maybe tomorrow I'll go up there with a sandwich and a coffee thermos and sit on that corner stake for a while 

Now I remember them talking about the WAAS at the GPS training last year.  I didn't buy a GPS - I borrowed this one from our VFD for a project and to do a little self training refresher and practice.  But I'm really pretty under-impressed at the usefulness of GPS 

Mmmm ... 7.6 meters - that's about 25 feet.  I guess that's close enough for a bomb

[mgdavis]

A mere minute or two on the stake should render a better fix on your corner  .
Thick cover will make it much harder to get a good signal.
What unit were you using?

[Harold Tuttle]

I guess that's close enough for a bomb

I gather the .mil GPS is a tad more precise

[Leatherneck]

I gather the .mil GPS is a tad more precise

Oh yeah. Much.

TC

[Fly320s]

Yep.  GPS for us peons is downgraded in accuracy.  Can't have the masses having pinpoint accuracy now, can we?

FYI, WAAS = Wide Area Augmentation System.  IIRC, it's a ground-based enhancement to GPS to improve the known inaccuracies in the system.  With a good GPS setup, you should be able to get to less than a 2 meter error.

[cordex]

The best claimed accuracy I've gotten on my RINO 120 is 8 feet.  Mine offers repeatable, accurate measurements to within around 15 feet or so consistently.

Not enough in and of itself for buried treasure or what-have-you, but it does what I need it to do.

[tyme]

Yep.  GPS for us peons is downgraded in accuracy.

Not since 2, if you mean it's intentionally degraded.  Selective Availability (the government euphemism for intentionally degrading c/a-code) is no longer active.

[Marnoot]

Not since 2, if you mean it's intentionally degraded.

They turned off Selective Availability, but don't military GPS receivers work off of a more accurate, powerful (and encrypted) signal/channel?

[Ben]

The military does use an enhanced signal, but civilian receivers can be just as, or more accurate and precise -- you just have to pay for it. Buy a Trimble Pro for $9K or so, collect data at times of the day when your Dilution of Precision is <4, post process your data, and you'll get accuracy and precision in the 5 centimeter range.

[Fly320s]

Whoa.[/Neo]

[Tallpine]

A mere minute or two on the stake should render a better fix on your corner  .
Thick cover will make it much harder to get a good signal.
What unit were you using?

Garmin 76S

Yeah, there were a lot of little trees around the corner post.

The thing that gets me is that the VFD bought these units for use in locating and mapping wildfires.  When actually responding to a fire, we are too darn busy to hardly turn these things on, let alone muck around with them to make sure we have good signal / no obstructions / steady reading ... etc.  Maybe it becomes more natural with practice, but mostly we just need to know our local area and how to find places and get there without using gadgets.

I've also been told that clouds can affect the GPS signal (but it was clear yesterday).  If it clears off today, I'm going to go out and mark waypoints on my own property corners just to see how they come out.  Theoretically, NW & NE corners should have (nearly) the same latitude figure, and NW & SW corners should have the same longitude figure, etc.

Given three known corners, one should be able to find the fourth corner just by using lat/lon.

BTW, this is rough country - lots of hills, deep coulees, and rimrocks.

[RevDisk]

Yep.  GPS for us peons is downgraded in accuracy.  Can't have the masses having pinpoint accuracy now, can we?

FYI, WAAS = Wide Area Augmentation System.  IIRC, it's a ground-based enhancement to GPS to improve the known inaccuracies in the system.  With a good GPS setup, you should be able to get to less than a 2 meter error.

Not anymore.  Technically, there is no intentional inaccuracy built into GPS anymore.  The military has advanced GPS capacilities, but it's due to much more expensive equipment.  There is a HMMWV mounted unit that's capable of accuracy roughly equal to the primer on a .50 cal.  Survey folks use 'em all the time.  Visit an DIVARTY and look at its survey team's gear for top notch mobile GPS kit. 

The issue is no longer the satellite signal.  It's the ability to pick it up and process the data.  More expensive the unit, generally more precise the measurements.

Garmin 76S

Yeah, there were a lot of little trees around the corner post.

The thing that gets me is that the VFD bought these units for use in locating and mapping wildfires.  When actually responding to a fire, we are too darn busy to hardly turn these things on, let alone muck around with them to make sure we have good signal / no obstructions / steady reading ... etc.  Maybe it becomes more natural with practice, but mostly we just need to know our local area and how to find places and get there without using gadgets.

I've also been told that clouds can affect the GPS signal (but it was clear yesterday).  If it clears off today, I'm going to go out and mark waypoints on my own property corners just to see how they come out.  Theoretically, NW & NE corners should have (nearly) the same latitude figure, and NW & SW corners should have the same longitude figure, etc.

Given three known corners, one should be able to find the fourth corner just by using lat/lon.

BTW, this is rough country - lots of hills, deep coulees, and rimrocks.

76S is a $249.99 MSRP consumer grade GPS device.  It's fine for hikers, not survey work.  It does its intended job, which is "close enough". 

Clouds do not affect GPS.  Thunderstorms can, but generally do not depending on the severity of the storm.   However, the composition of the terrain can.  Some areas have wonky signal.  I'd guess some sort of natural magnetic field, but I'm not a geologist. 

They turned off Selective Availability, but don't military GPS receivers work off of a more accurate, powerful (and encrypted) signal/channel?

The handheld units are called PLGR's.  Pronounced "Plug Er".  Yes, but no.  They have a Precise Positioning Service (PPS) signal, which used to be part of the intentional Selective Availability feature.  It's never been turned off, per se.  The intentional inaccuracy has simply been set to 0.  The band is called L1, and yes, civvies now have full access to it worldwide.  PPS doesn't really affect precision to any significant degree, but it does help with jamming.  So, not more accurate or powerful, just more jamming resistant. 

Theoretically, SA value on the satellites can be increased.  It won't happen.  If required militarily, we can use local jamming to increase inaccuracy to non-friendly forces.  Local meaning any portion of the globe we want, however large or small.


(I'm former US Army Signal Corps, 31U/25U.)

[Tallpine]

Well, I went out yesterday and saved waypoints on a bunch of corners.  The GPS gadget seems to be working okay now - at least as well as can be expected accuracy wise. IOW, about +/- 60' or 0.01 minutes of latitude (longitude of course not corresponding to a nautical mile at this latitude).

I still have the screwed up waypoint saved on the GPS though.  Viewing it on the "map" it is about 1200' SSE of where it should have been (or where I was actually standing when I saved it).  This one huge error still bugs me, though ... if it can do that once then it can happen again.  I suppose it is all right if you already know where you are, and can evaluate the waypoint for error 

I guess, based on my experience and the helpful posts here, that I still would stick to my original opinion that at least this level of consumer GPS is basically worthless.  I've never had much trouble finding my way around the woods or mountains, so it seems like $250 worth of junky gadget to use up batteries to tell you what you already know, or at least what you can figure out with a $30 compass.

IOW, if a person is so helpless that they need GPS, then they better just stay in town near a street sign

[Jamisjockey]

I've got a Garmin Etrex Vista CX.  Its been purty darn accurate, and I've geocached to within 10' of an object.  I've followed marked hiking trails and watched the "you are here" icon go right down the trail.  I've only had it bounce a few times, but never 1200' off......
Everyone should know how to use a map and compass, at least to back up the GPS.  Would I trust my life to my GPS?  Yep.

[AJ Dual]

The orbits of the GPS constellation are such that three or more are supposed to be over the horizon at any one time, however, a lot of the time one, two or even all three satellites are near the horizon, rising, setting, or even just skimming along the horizon. Sometimes it'll be several minutes until you get a good clear read on two or more. Also, IIRC, WAAS works better the further south you are. I recall reading that in my Garmin manual.

The other good advice is to leave your GPS at rest when you first turn it on and try to be in a high, or flat clear area. That helps a lot too. The better a fix it can get when it starts, the better it'll do when it has limited signal later.

Unless you were in a really deep valley, or had very high hills or bluffs, I'd say something is either wrong with that GPS unit, or it was the perfect confluence of bad-luck factors, the satellites were low in the sky, the trees and the hills etc. My cheapie Garmin eTrex Vista for $150 seems to do consistently better than 20 feet, at least judging from known landmarks like highway intersections in the map.

[Tallpine]

The orbits of the GPS constellation are such that three or more are supposed to be over the horizon at any one time, however, a lot of the time one, two or even all three satellites are near the horizon, rising, setting, or even just skimming along the horizon. Sometimes it'll be several minutes until you get a good clear read on two or more. Also, IIRC, WAAS works better the further south you are. I recall reading that in my Garmin manual.

The other good advice is to leave your GPS at rest when you first turn it on and try to be in a high, or flat clear area. That helps a lot too. The better a fix it can get when it starts, the better it'll do when it has limited signal later.

Unless you were in a really deep valley, or had very high hills or bluffs, I'd say something is either wrong with that GPS unit, or it was the perfect confluence of bad-luck factors, the satellites were low in the sky, the trees and the hills etc. My cheapie Garmin eTrex Vista for $150 seems to do consistently better than 20 feet, at least judging from known landmarks like highway intersections in the map.

Funny thing is that shortly before marking the errant waypoint, I marked one at the neighbor's house (out in the drive, not in the house) and a later check proved that one to be just about right.  Then I hiked NW up on the ridge to the NW corner stake and marked the errant waypoint.  Later examination of the GPS unit "map" shows me walking SW from the house a couple hundred feet (maybe?) and the waywardpoint SW of the house.  Clear day, right on top of a ridge, and scrubby trees around (10-20' tall) and the thing loses its mind

You can sit still and just watch this GPS unit randomly vary as much 0.01 minute.

[AJ Dual]

Can a firmware update be done?

Obviously the F.D. would have to approve, or have their quartermaster, I.T. guy, or whoever is responsible for the GPS units to do it. But that might be worth looking into.

If it's intended for use in brush and forest fires, that could get someone killed. 

[Tallpine]

Can a firmware update be done?

Obviously the F.D. would have to approve, or have their quartermaster, I.T. guy, or whoever is responsible for the GPS units to do it. But that might be worth looking into.

If it's intended for use in brush and forest fires, that could get someone killed. 

Not if I just take the batteries out

[RevDisk]

Can a firmware update be done?

Obviously the F.D. would have to approve, or have their quartermaster, I.T. guy, or whoever is responsible for the GPS units to do it. But that might be worth looking into.

If it's intended for use in brush and forest fires, that could get someone killed. 

Even if inaccurate by say 500 m, I'm trying to imagine how a faulty handheld GPS unit could get anyone killed. 

The last brush fire I marched through (grumblegrumble), you could see it from quite a distance off.  Not to meantion, fires do move on their own time to time.  If the reading is inaccurate by a couple hundred feet, just wait 15 minutes.   

[Tallpine]

Even if inaccurate by say 500 m, I'm trying to imagine how a faulty handheld GPS unit could get anyone killed. 

Well, I suppose if one called in a slurry drop X distance from Y spot, and you got it dumped on your head instead ... might not kill you, but that stuff can beat you up pretty bad coming in at 150mph.

Or if your safety zone was supposed to be at Z/z coordinates, and you were depending on the GPS to guide you there (though usually a safety zone is or nearly is within sight, or else it is too far to get there in a hurry).

The main plan was supposed to be:

1) locating a "smoke" by triangulation (compass bearings from two "known" points).  With GPS theoretically any viewpoint can be a known point because with the GPS you "know" your position 

2) directing other resources to a located fire, by transmitting your coordinates over the radio.  Of course in this rough plateau country, you can't just follow a GPS course, because you will likely end up across a steep coulee from the fire and have to drive five miles to get around it.

3) mapping the extent of a fire after the fact - for records, and getting an "accurate" figure of acreage burned.  Not much of a life threat there.


I just don't have much faith in GPS anymore, and there's not that much that you couldn't do just as well without it.  Just one more toy to distract you from your mission.