Accuracy question

[Old Philosopher]

I’m 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 1”x2” 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.

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 didn’t 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]

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]

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]

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!