Any advice on figuring out if a GPS is working correctly?

[bubbabean]

We just got started in geocaching. We picked up a inexpensive foretrex 101 to see if the family would like the adventures or not.

 

We've been using it for about a month with mixed results. We've found 2 or 3 caches but missed out of sevreal more. It may just be that the caches are well hidden, but I'm having a tough time with the GPS and not sure if it's "normal" or if something's not working correctly.

 

The problem is that I can get the GPS to lead me to the same rough location more than once. If I walk to a point and walk away and try again I'll end up way off from the previous spot(s). Sure we can then average and take into account the GPS accuaracy, but it's not working well.

 

If the unit has an accuacy of 30' (not good from what I know about GPS, should be able to get closer to 10') and I walk into a location 5 times I'll end up 200' away from where the other passes left me.

 

All the obvious stuff is understood, line of sight, trees, batteries, number of sats locked, triangulation, etc. I 've done orienteering and can understand how to use a compass for triangulation, but this GPS thing is killing me because I'd like to find a search location within a 10-20' diameter but currently I'm not having much luck with that. =(

 

I'm curious if the GPS isn't working correctly. Is there any way to check if it is or isn't working fine?

I guess I can use the NGS points, but any other advice? I'd really like to know if this thing isn't working correctly.

[CacheNCarryMA]

How about this method?:

 

- Using Google Earth, find an obvious landmark near your home, like the end of your driveway, and intersection, or something else visible on the map.

 

- Note the coordinates of the location using the software.

 

- Walk outside to the location and place your GPS on the ground next to the landmark. Let it sit for ten minutes or longer. Mark the spot (take a waypoint).

 

- Compare the coordinates saved on the GPS to the point plotted on the mapping software.

 

There may be some inaccuracy in the mapping software, but at least you'll know if you're not way off.

[+2Dee2Dee]

How about this method?:

 

- Using Google Earth, find an obvious landmark near your home, like the end of your driveway, and intersection, or something else visible on the map.

 

- Note the coordinates of the location using the software.

 

- Walk outside to the location and place your GPS on the ground next to the landmark. Let it sit for ten minutes or longer. Mark the spot (take a waypoint).

 

- Compare the coordinates saved on the GPS to the point plotted on the mapping software.

 

There may be some inaccuracy in the mapping software, but at least you'll know if you're not way off.

 

I see you understand SOME of the accuracy variables. So all things that can go bad aside why don't you just team up with a few others and test your GPS against theirs? Is your system quite old or fairly new. New with warranty or used in unknown internal condition? Does your accuracy in a clear area with clear sky improve considerably over the space of a few minutes? Check the page on your GPSr that shows how many satellites are in view AND their relative strength. Does it match what others see? You would like to see at least 6 though 4 works.

2Dee

[+imajeep]

How about this method?:

 

- Using Google Earth, find an obvious landmark near your home, like the end of your driveway, and intersection, or something else visible on the map.

 

- Note the coordinates of the location using the software.

 

- Walk outside to the location and place your GPS on the ground next to the landmark. Let it sit for ten minutes or longer. Mark the spot (take a waypoint).

 

- Compare the coordinates saved on the GPS to the point plotted on the mapping software.

 

There may be some inaccuracy in the mapping software, but at least you'll know if you're not way off.

 

 

Be careful using Google Earth to callibrate a GPSr--Its coordinates are actually less accurate than the receiver! 50-100 feet off is not at all unusual. The best way to determine the accuracy of your GPSr is to find a nearby benchmark. These things have known geo coordinates, which are very accurate. Mark a waypoint at the benchmark, and compare your GPSr waypoint coordinates to the known coordinates for the benchmark.

 

My experience has been that 20' accuracy is about the best I can count on, even when the GPSr claims to be more accurate than that. Apparently, that limitation is inherent in the GPS system. So, if you get within 20' of your benchmark, I'd say you're doing about as well as you can.

 

With some units, how you hold the GPSr has an effect on its accuracy. I started with an eTrex Vista that was pretty accurate if I held it horizontally. But if I held it vertically, I could be up to 200' off. I sold it pretty quickly and moved up to a Garmin 60 CSx, which is pretty accurate no matter how you hold it.

[+Miragee]

We just got started in geocaching. We picked up a inexpensive foretrex 101 to see if the family would like the adventures or not.

 

<snip>

Welcome to the Forums!

 

A friend uses one of those GPSrs and it works great. Plus, she is better at seeing the elusive cache than I am.

 

I would suggest you filter out the caches you look for in the beginning to only those that are "Regular" sized containers. Those should be fairly obvious to find, especially if the cache owner has the "Difficulty" rated less than '2'.

 

If you cannot find those . . . well, then maybe there is something wrong with the GPSr or the settings.

 

Welcome to the Addiction . . . er . . . Activity.

[+TrailGators]

We just got started in geocaching. We picked up a inexpensive foretrex 101 to see if the family would like the adventures or not.

 

We've been using it for about a month with mixed results. We've found 2 or 3 caches but missed out of sevreal more. It may just be that the caches are well hidden, but I'm having a tough time with the GPS and not sure if it's "normal" or if something's not working correctly.

 

The problem is that I can get the GPS to lead me to the same rough location more than once. If I walk to a point and walk away and try again I'll end up way off from the previous spot(s). Sure we can then average and take into account the GPS accuaracy, but it's not working well.

 

If the unit has an accuacy of 30' (not good from what I know about GPS, should be able to get closer to 10') and I walk into a location 5 times I'll end up 200' away from where the other passes left me.

 

All the obvious stuff is understood, line of sight, trees, batteries, number of sats locked, triangulation, etc. I 've done orienteering and can understand how to use a compass for triangulation, but this GPS thing is killing me because I'd like to find a search location within a 10-20' diameter but currently I'm not having much luck with that. =(

 

I'm curious if the GPS isn't working correctly. Is there any way to check if it is or isn't working fine?

I guess I can use the NGS points, but any other advice? I'd really like to know if this thing isn't working correctly.

The accuracy depends on how many satellitles are visible. There is a screen that will show you the current accuracy and it changes depending upon where you are. One thing you should do is let your GPS settle. Go to where you think the cache is and set the GPS down for a few minutes. Then pickup the GPS and check the coords. If you are still too far then walk in a straight line in one direction to get a new bearing on the cache. Once you get within the circle of accuracy then put the GPS in your pocket and look for likely hiding spots.

[bubbabean]

Thanks for the information folks.

 

Can I ask.... GPSrs (gps receivers?)

 

My foretrex is new, but I'm not sure it wasn't an opened box. It had batteries installed and the store guy opened it up to see if it was working. I don't know if they come like that or if this was used and returned. If it was returned, I'm suspicious as to why...

 

I may just return it and get another to see if another one works better, but I wanted to test the obvious stuff first.

 

 

Any suggestions on locations with known geo coordinates? Do those NGS

points offer anything that would be helpful?

[+Miragee]

Yes . . . GPSr is GPS Receiver.

 

I have read here about people going to certain benchmarks that have corrected coordinates, but I'm not up on that.

 

Although the Foretrex is cute, and very small, and my friend likes hers, I think I would have a hard time using it. If you are going to take it back, you might want to look at GPSrs that connect to a computer with USB, although I don't know what price range that puts you in compared to what the Foretrex cost . . .

[+Markwell]

I liked using a high accuracy order of local benchmark to find coordinates. They shoudl be spot on as the high accuracy ones would be verified at their coordinates. I will likely be anywhere from 3 to 30 feet away if it's acting OK.

[bubbabean]

Although the Foretrex is cute, and very small, and my friend likes hers, I think I would have a hard time using it. If you are going to take it back, you might want to look at GPSrs that connect to a computer with USB, although I don't know what price range that puts you in compared to what the Foretrex cost . . .

 

This thing does have a mini-phono serial port and connects to a computer, but it doesn't have any mapping support. Is that where you're heading with that comment? I just ask because talking to garmin they said that you really want a gps with a color screen because those are good for geocaching, have a geocache logo on it or something like that. And most of the color screen ones have maps.

 

 

I'll look for local benchmarks, but that's more new stuff to look into.

 

I did take the unit out today and I've taken pictures of the sat screen as it was pretty odd. The think is suppose to try to find up to 20 sats. I never saw more than 4 with good bars. As it was moved around I saw sats with strong singals drop out to be replaced with other weaker ones. Slight changes in orientation would cause the thing to lose and gain sats. I don't think that's normal. I did read some comments on this thing having a problem keeping track of locked sats. I'm not sure if that was a problem with a specific unit or if it was a product problem. I think I'll try another one to figure out if it's the unit or a generic foretrex problem.

Edited July 1, 2007 by bubbabean

[+Miragee]

I believe there are twelve satellites. In an open area, you should have a good signal from most of them. With my Vista C, I often get all of the satellite bars and an accuracy of 8'.

 

A color unit will be able to accept maps and even auto-route you. That is why I upgraded from my monochrome Vista to my Vista C almost two years ago. The Vista connected with the Serial port, but the USB connection of my Vista C is a lot faster.

[bubbabean]

Yup, GPS shows 12 satellites. Only 11 were found for these pictures...

 

I set the thing on the ground outside for like 20 minutes and then started taking pictures.

I started to move around trying different directions and orientations of the GPSr...

The satellite strengths change from shot to shot. I gave the unit a little time to adjust to the movement, but really is that needed? I don't think locks satellites should be falling off like this. The most I moved was maybe 10'.

 

Slightly overcast day, unit was in the open with pretty good view of a lot of the sky.

 

This is the straight untouched unit:

 

The rest of these are all after picking the unit up and moving it around...

 

 

Does this look normal? I have a tough time believing satellites drop off so quick and with that frequency.

[Cape Cod Cacher]

Use a benchmark or another GPS (no need to put an"r" after it, geocaching is the only place I have ever heard/seen it. I'll change when I meet someone using a GPSt (transmitter), besides, we don't pronounce the last 'aah' around heah).

[bubbabean]

Ah another MA person... sure I'll skip the arrhs

 

 

I found a local benchmark PID AF8303, I have no idea about the altitude adjustment, but the values the GPS was showing is different than the values on the benchmark page.

 

Values from the database:

N 35° 54.050

W 078° 39.150

 

My values were

N 35° 54.044-.045

W 078° 39.167-.169

 

There were between 6 and 7 satellites locked in the readings. I walked alway and back a few times to see how the unit would react on different readings.

 

I didn't set a waypoint (is that important, or does a standard reading work fine?)

I didn't have WAAS enabled.

 

I'll enable WAAS and set a waypoint when I go back at another time.

Edited July 2, 2007 by bubbabean

[+EScout]

The benchmarks are listed in a different format: ddd mm ss.sssss, which is degrees, minutes, seconds and decimal seconds. We typically use: ddd mm.mmm.

 

I bet that your GPSr is working fine. If you want to compare it to a super accurate benchmark, go here: NGS benchmarks by County, in the USA

 

Choose your state, county and GPS Sites only. What you want is an Adjusted or GPS Observed mark. You want a mark on the ground in an accessible location.

[bubbabean]

EScout, thanks for the information.

 

I was using the geocaching benchmark page to get my data. It appeared the location was in the ddd mm.mmm format, was I wrong? And isn't this information the same as the NGS database? Or is the geocaching benchmark data user entered?

 

If I'm reading the data in the page you linked the benchmark I used isn't even listed. Guess I need to find another one. I liked the gecocache search because of the user comments allowed me to find something I know I can locate. =)

Edited July 2, 2007 by bubbabean

[+EScout]

Bubbabean:

Yes, this site's benchmarks are in the correct format. The ones listed at the NGS are different. This site lists some of the select "super accurate" ones along with most that are not very accurate. Those typical BMs have coordinates that can be up to hundreds of feet off their listed coords.

 

The super accurate, GPS sites BMs will be easy to find, because your GPSr will bring you right to them. Some however, will be inaccessible due to being at airports, on buildings, etc. Just look for the ones on roads and sidewalks. Remember to convert from ddd mm ss.sssss to ddd mm.mmm. Converting and rounding will give you a point within a 3 foot radius of the true coords.

Edited July 2, 2007 by EScout

[+StarBrand]

One of my Garmin units used to act as described - particularly if the battery on the weak side. Once I upgraded the firmware - it was much better.

 

Also have heard about a few folks that needed to send thier unit into Garmin - got back a much better working unit.