Newbie Question Re: Sa & Waas

[PirateParrots]

Hi!

 

A few questions for all you experts out there!

 

I have a Lowrance AirMap 100 I used while flying. I'm not doing much flying anymore and am putting it to use geocaching. The GPSr that I have has an "expected horizontal postion error" reading on it. The manual says this is only accurate when used with a DGPS. With SA now turned off, can I now take this number at face value?

 

Also, is it a reflection of a GPSr quality if horiz. position error flucuates anywhere from 30 to 150 ft? What kind of accuracy should I REALISTICALLY expect from a handheld GPSr while geocaching?

 

With SA turned off, how does WAAS now fit into the picture? Does it still provide greater accuracy?

 

And, if it does, is there really a need to have a WAAS GPSr when the coordinates you are using may not have been determined using a WAAS receiver? i.e Garbage in-Garbage out.

 

I guess I'm a little frustrated because, I am ASSUMING newer GPSr's that everyone is using is gettting them within feet/meters of the cache. Whereas, mine is showing expected errors of 30-150 ft. Can I trust my GPSr when it says 5ft away from the cache, but giving me an expected error of 60 ft horizontal?

 

Also, in the air/car the unit updates very nicely. While on the ground, the position bounces around quite alot and doesn't seem to update very well. Is this normal for all units?

 

I really want this Airmap 100 to be a good unit and work well for geocaching. I'm developing doubts though...

 

Any help, insights or coaching will be greatly appreciated!

 

Thanks and FinsUp!

 

PirateParrots

[+Airmapper]

Never used a 100, but as you can see from my name, I'm an AirMap user too. I have an AirMap 500.

 

I know little about your unit, so I'll stick with what I do know, or both of us will get confused.

 

Also, is it a reflection of a GPSr quality if horiz. position error flucuates anywhere from 30 to 150 ft? What kind of accuracy should I REALISTICALLY expect from a handheld GPSr while geocaching?

 

On an older GPS, without SA it will be about as accurate as any other GPS without WAAS, but that depends on the antenna and number of satellites it can track. EPE of 30 to 150 feet seems normal for that unit (I assume, I see similar readings on new GPS's.)

 

With SA turned off, how does WAAS now fit into the picture? Does it still provide greater accuracy?

 

Not for your unit, it does not support WAAS. To answer your other question, n, WAAS is not everything, and a non WAAS unit will still work fine for caching.

 

It should work fine for Geocaching. But I do have some questions (Partly cause I'm curious.) Does it have a way to connect to a computer? In the distance readout, does it go to feet when under .10 miles, or does it go to zero?

 

Welcome to Geocaching and the forums!!!

[+CharlieP]

I am not familiar with your GPS, but I will take a shot at your questions. I was using an old marine GPS when SA was turned off and the unit somehow knew SA was off and the error/accuracy reading dropped from a typical 60 feet to 20 feet. So your error/accuracy reading probably reflects SA off conditions. But GPS makers have their own methods of computing estimated error, and therefore it is difficult to say what the number means. If you can, take the GPS to a known position that is in an open area, set the waypoint, stand at the position and see what the GPS tells you. In the open, with good reception conditions, the reported accuracy would get down below 20 feet with a typical Garmin reciever. If your GPS puts you right on the position, but reports 50 foot accuracy, then the accuracy computation is very conservative. If the GPS consistently puts you within 20 feet or so of the known position, it does not matter what the reported accuracy is, the GPS works.

 

You don't need WAAS for geocaching.

 

Other GPS units which don't automatically average, e.g. Garmins, will often display a "bouncing" position when reception is poor. The bounces may be one hundred feet or more. Out in the open with good reception the bounces are very small, like 20 feet.

[PirateParrots]

Hey Airmapper,

 

Thanks for the response! Glad to see another Lowrance user here. I haven't seen too many in my short time on the forum. Are you still flying? Helo's I assume from looking at you avatar.

 

A couple of things...

 

On an older GPS, without SA it will be about as accurate as any other GPS without WAAS, but that depends on the antenna and number of satellites it can track. EPE of 30 to 150 feet seems normal for that unit (I assume, I see similar readings on new GPS's.)

 

With SA now turned off, does WAAS provide a SIGNIFICANT amount of additional accuracy? Also, am I mistaken in assuming the additional accuracy is only as good as the accuracy of the initial coordinates?

 

It should work fine for Geocaching. But I do have some questions (Partly cause I'm curious.) Does it have a way to connect to a computer? In the distance readout, does it go to feet when under .10 miles, or does it go to zero?

 

The AM 100 can connect to a computer, and when it gets below .10 miles it goes to feet. Since my foray into GC'ing----I've been using EasyGPS and it seems to work great.

 

For aviation, I was quite impressed with the features. I didn't need anything IFR approved and it worked great! But, now am curious to know if I should convert it over to a "land based" GPS--i.e. installing MapCreate....so----

 

Here's a question for you, did you buy the Mapcreate software with yours? If so, do you think it is worth it? I am having diffculty locating a Mapcreate version 4 CD for my unit. They seem rather expensive and I am just wondering if it's worth it. Do most people that have GPSr's with maps use the map to actually navigate to the location? Or do most, (like I've been doing) start with mapquest to get to the local vicinity?

[PirateParrots]

I am not familiar with your GPS, but I will take a shot at your questions. I was using an old marine GPS when SA was turned off and the unit somehow knew SA was off and the error/accuracy reading dropped from a typical 60 feet to 20 feet. So your error/accuracy reading probably reflects SA off conditions. But GPS makers have their own methods of computing estimated error, and therefore it is difficult to say what the number means. If you can, take the GPS to a known position that is in an open area, set the waypoint, stand at the position and see what the GPS tells you. In the open, with good reception conditions, the reported accuracy would get down below 20 feet with a typical Garmin reciever. If your GPS puts you right on the position, but reports 50 foot accuracy, then the accuracy computation is very conservative. If the GPS consistently puts you within 20 feet or so of the known position, it does not matter what the reported accuracy is, the GPS works.

 

You don't need WAAS for geocaching.

 

Other GPS units which don't automatically average, e.g. Garmins, will often display a "bouncing" position when reception is poor. The bounces may be one hundred feet or more. Out in the open with good reception the bounces are very small, like 20 feet.

Thanks for the suggestion CharlieP. I will try to find a known position and following your instructions. It also sounds like I can put my WAAS concerns to bed.

[+CharlieP]

With SA now turned off, does WAAS provide a SIGNIFICANT amount of additional accuracy? Also, am I mistaken in assuming the additional accuracy is only as good as the accuracy of the initial coordinates?

That depends. In an open area, like on top of a grassy hill, with good reception, WAAS may drop accuracy from 15 feet to 9 for my Garmin GPS76. But in areas with poor reception, in the woods or in a valley, WAAS may actually cause problems and reduce accuracy. Some geocachers with WAAS capable GPSR's turn off WAAS for geocaching. I turn it off if I am in a poor reception situation and the GPS seems to be having problems computing a position.

 

And yes, the position is only as good as the initial geocache coords, which are often in error by 20 or 30 feet and sometimes by 100 feet or more.

 

Do a search on this board for threads about "averaging" to learn how you can improve position accuracy.

[+Renegade Knight]

Whe SA was turned on GPSs were accuate to 500'. When they turned it off, they became accurate to 20-50' and I'd say closer to the 20.

 

The main issue with older GPS is that they often only go to DDD MM.MM while newer ones are DDD MM.MMM that last M is worth up to 60' (Call it 30' on average)'. So they can only record coordinates to within 60' give or or take. Your GPS may or may not have that issue.

 

The workaround is to use UTM which is typically goes to the nerest meter (3').

 

For finding a cache your GPS is fine. For placing one your GPS is ok with a good clue unless you have DDD MM.MMM in which case you are as good as anyone else out there.

 

WAAS helps get better accuracy. Once it's up and working on a GPS the EPE (Estimated Position Error - Another name for what you have observed on your GPS). can be lower than 10'. I find that I keep it turned off on my GPS. It doesn't really impact my geocaching one way or the other.

Edited December 30, 2005 by Renegade Knight

[+Renegade Knight]

It's worth noting the reason I turn off WAAS is that is slows down my GPSs ability to recalculate it's position. Newer GPSs are fast enough this is a non issue. Plus on Magellans I'm led to believe turning it off doesn't make a difference. It does on my GPS V.

[+Markwell]

First off, just remember that SA has been turned off since May 2000. That's how geocaching got started.

 

From there, WAAS provides enhanced accuracy provided that A) the area that you're using the GPS has WAAS, and B) that your GPS can receive WAAS.

 

Back in our 2002 Chicago area picnic, WAAS was just coming into fashion on the units. We had a fun game where we had the coordinates for our picnic listed as a field marker (concrete, wouldn't move). When we set up the event cache page, we went back on multiple days and had 3 or 4 GPS units trying to find the best coordinates. One of our games at the event was for people to take those little yard flags (like the water company or electric company place in your yard before you dig) and mark where their GPS said "0 feet to target".

 

The people that day were all over the place:

 

But I will remember ONE comment from one of my buddies. He had WAAS turned off and placed his flag down, probably some 40-50 feet from the marker. Then he turned WAAS on, and got within 3 feet of the target.

 

Admittedly, it's anecdotal, but I always use WAAS now, and I'm lucky enough to live in an area where WAAS is pretty wide-spread.

[+Markwell]

You can actually test it yourself too. Find a benchmark in your area with high reliability of horizontal accuracy. Go to the spot, and see where your GPS says it is. Then try it with a GPS with WAAS on.

[+Airmapper]

Hey Airmapper,

 

Thanks for the response!  Glad to see another Lowrance user here.  I haven't seen too many in my short time on the forum.  Are you still flying?  Helo's I assume  from looking at you avatar.

There are several Lowrance users in here. They particularly hang out in the GPS Units and Software section of the forums. As for flying, off and on, I'm still a student. I fly a Cessna 150. (Got the helo avatar cause I thought it was cool. )

 

With SA now turned off, does WAAS provide a SIGNIFICANT amount of additional accuracy?  Also, am I mistaken in assuming the additional accuracy is only as good as the accuracy of the initial coordinates?

 

Depends, I think the others explained it better than I could.

 

Here's a question for you, did you buy the Mapcreate software with yours? If so, do you think it is worth it?  I am having difficulty locating a Mapcreate version 4 CD for my unit.  They seem rather expensive and I am just wondering if it's worth it.  Do most people that have GPSr's with maps use the map to actually navigate to the location?  Or do most, (like I've been doing) start with mapquest to get to the local vicinity?

 

Yes, my unit came with Mapcreate 6.2, and I later bought 6.3 TOPO for around $90. I am very satisfied with the newer Mapcreate. Especially the mapping detail. (It has taxiway diagrams for many airports, and with TOPO, it's almost as good as terrain warning.) Mapcreate 6.3 is a pain in the butt on an older computer though. I have to re-install it a lot. As for Version 4, I have no clue, it depends on the price. I think you can get Ver 4 at LEI Extras, the accessory company for Lowrance, but I'm not sure on the price.

 

As for using the maps to navigate, I do. Using Mapcreate I'll plan a route for long stretches of trips, but for cache to cache navigation, I depend heavily on the maps. Lowrance units, even the new ones, don't have auto-routing. This makes road navigation harder, but if you plan ahead, it's not to bad.