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Old 03-19-05, 12:21 PM   #1
Tweek
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Here is a track I made a long time ago, of one of the legs on the Wolf River/Shelby Farms trail in Germantown, TN
I just bought Street Atlas 2005 today and WOW what a difference it makes over the program that came with my Garmin GPS.

The trail is the black line, and its 3.89 miles long from entrance to exit of that leg of the trail.




and for something to compare it too, here is MAPSource.




*top screenshot is from my desktop and bottom is from my laptop*

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Old 03-19-05, 03:32 PM   #2
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GPS are the ducks nuts.
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Old 03-19-05, 03:46 PM   #3
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What's cool is to use the GPS to view the profile for the course. I think I'll take my GPS with me tomorrow.
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Old 03-19-05, 04:55 PM   #4
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Would you enlightend us luddites, noobies, heathens and hottentotts as to how these things work--or direct us to a good webiste that will explain all.
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Old 03-19-05, 06:16 PM   #5
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Very, very basically a GPS works similar to a sextant or RDF. Satellites send out signals that the GPS picks up. The GPS measures the angles of three or more satellites to give you a position. The more satellite signals your GPS can receive, say twelve the more accurate that position gets. These days accuracy is around 3 metres on a handheld GPS. On a panel mounted GPS with a differential aerial the accuracy gets down to about a metre.

Originally the US put in deliberate error so that the accuracy never got much more accurate than a few hundred metres. The main fear was that some terrorist could use GPS navigation to send a cruise missle into an attack.

However, the DGPS cancelled out the error. Many airports and sea ports have their own GPS aerials which give accuracy (I'm told) down to 10cm. The stronger signal from the fixed aerials is part of the reason the US eventually cancelled the input error. Also, people started relying on it and it became an essential service.
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Old 03-19-05, 07:31 PM   #6
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You should also give this a read.

http://www.garmin.com/aboutGPS/waas.html

It explains WAAS which is a series of ground stations that increase accuracy of GPS.
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Old 03-19-05, 07:52 PM   #7
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Quote:
Originally Posted by HaagenDas
Very, very basically a GPS works similar to a sextant or RDF. Satellites send out signals that the GPS picks up. The GPS measures the angles of three or more satellites to give you a position. The more satellite signals your GPS can receive, say twelve the more accurate that position gets.
Close but not quite there. The satelites transmit two pieces of information: their position and time. The receiver upon receiving the signal, calculates the time differential and by knowing the position which the satelite claims it's at can determine distance. When it receives the signal from another satelite with the same type of information, it can determine the distance from that satelite. And if it obtains position information from a third satelite, it can then perform a two-dimensional location calculation based on the intersection of three radii eminating from the three satelites. Add a fourth signal reference and it can give you the location in three dimensions. In short, the receiver only measures its distance from the satelite and not its angle. Everything else is derived.
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Old 03-19-05, 10:17 PM   #8
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Originally Posted by HaagenDas
Originally the US put in deliberate error so that the accuracy never got much more accurate than a few hundred metres. The main fear was that some terrorist could use GPS navigation to send a cruise missle into an attack.
Yep. We Yanks take the fun out of everything.

You don't want to be 100 m. off-target with that nuke. Someone might get hurt!
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Old 03-19-05, 10:23 PM   #9
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Handheld GPS's can be pretty darn accurate these days, I average about 12-17 feet in wooded areas, but if I'm on an open road i can get as accurate as 5-7 feet, pretty darn good if you ask me.
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Old 03-19-05, 11:27 PM   #10
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Originally Posted by eubi
Yep. We Yanks take the fun out of everything.

You don't want to be 100 m. off-target with that nuke. Someone might get hurt!
Yup, now they can squeeze that nuke in with surgical precision
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Old 03-19-05, 11:40 PM   #11
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USAPHOTOMAPS, at http://jdmcox.com/ is another super map making program, and it is free.
It is constantly improving , updated a couple of times a month .
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Old 03-19-05, 11:58 PM   #12
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Originally Posted by tsiya
USAPHOTOMAPS, at http://jdmcox.com/ is another super map making program, and it is free.
It is constantly improving , updated a couple of times a month .
DUDE!!! THAT IS AWESOME!!!!!!!!!!!!
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Old 03-20-05, 12:07 AM   #13
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Here we go, a screenshot of the trail on the satellite mapping program.

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Old 03-20-05, 12:28 AM   #14
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Top stuff, wish we had that facility over here.
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Old 03-20-05, 08:15 AM   #15
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Quote:
Originally Posted by HaagenDas
Very, very basically a GPS works similar to a sextant or RDF. Satellites send out signals that the GPS picks up. The GPS measures the angles of three or more satellites to give you a position. The more satellite signals your GPS can receive, say twelve the more accurate that position gets. These days accuracy is around 3 metres on a handheld GPS. On a panel mounted GPS with a differential aerial the accuracy gets down to about a metre.

Originally the US put in deliberate error so that the accuracy never got much more accurate than a few hundred metres. The main fear was that some terrorist could use GPS navigation to send a cruise missle into an attack.

However, the DGPS cancelled out the error. Many airports and sea ports have their own GPS aerials which give accuracy (I'm told) down to 10cm. The stronger signal from the fixed aerials is part of the reason the US eventually cancelled the input error. Also, people started relying on it and it became an essential service.
Yeah, got it. So like, what button do I push?
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Old 03-20-05, 10:15 AM   #16
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Quote:
Originally Posted by Tweek
DUDE!!! THAT IS AWESOME!!!!!!!!!!!!
It sure is! Doug writes great programs. Go back to His page every few days, He keeps adding more goodies. I am waiting for My asprin overdose to kick in, so I can hang My bike on the old Jeep, and drag My old tail around busting tree roots, someone has to map new trails. If I crash, at least I'll bleed out fast,LOL

http://photobucket.com/albums/v244/tsiya/
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Old 03-20-05, 10:39 AM   #17
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Actually, the best a non-WAAS GPS unit can manage is a position within a 10 m circle 95% of the time.
Claims of precision of a metre or below are bogus and have nothing to do with reality.
The rest of the time, it's worse. How much worse depends on how many satellites are in view.
In an uncluttered environment, with low horizons, you could have around 10 sats in view at the same time.
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Old 03-20-05, 10:57 AM   #18
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I agree that Mapsource is adequate and not great. It serves it's purpose and gets the job done but not flashy.

What I like about the GPS is your ability to navigate around cities like that group of subdivisions below in that picture. On that map, can you imagine trying to get to Massey Rd if you've never been there before? With the GPS, it's quite easy.

I was using my GPS yesterday to show me a safe route in that I could take to see the bike Expo in Edison NJ. The Exposition was held in the middle of sprawl with only fast highways leading to the center. While heading for the expo, I walked out of the house without a single map and basically used five waypoints that lead me to my destination without getting lost at all. There were lots of turns but it did not matter because the GPS was always pointing me in the right direction. That device is just as important to me as my extra inner tube!
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Old 03-20-05, 03:15 PM   #19
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Quote:
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Actually, the best a non-WAAS GPS unit can manage is a position within a 10 m circle 95% of the time.
Claims of precision of a metre or below are bogus and have nothing to do with reality.
The rest of the time, it's worse. How much worse depends on how many satellites are in view.
In an uncluttered environment, with low horizons, you could have around 10 sats in view at the same time.
Whilst I don't dispute your claim, I do believe I get well within a 10 metre error most of the time. I often start my GPS in the same spot and then take a mark. Most times the mark falls within 3 metres of my "home" waypoint which is the door to my garage. I would be happy to double that error as error which is probably still good enough for most of us.... It certainly is sufficient for me.
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Old 03-20-05, 04:02 PM   #20
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Yep, 10, or even 20 metres will be perfectly fine for almost all purposes.
But do be aware that the accuracy, at best, is around 10 m.
With a poor fix (3-4 sats), it could be several hundred metres off for some of the time.
Perhaps your "home" location allows a large number of sats in view at the same time. Then you could get down to just a few metres regularly.
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Old 03-20-05, 04:17 PM   #21
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Actually, the spot outside my garage rarely picks up more than six sats. I've been using GPSs since they first came out and found them to be a useful but inaccurate tool that always needed a map. Back in the olden days I used to take a fix every ten minutes when out bush. I used to amazed at the difference between fixes. Errors used to be in the order of 500 metres. I always used to triangulate off several fixes but found it easier to dead reckon as well.

Then I bought my second GPS and was impressed how it was able to give altitude and a better fix on position. With the altitude accurate to a few hundred feet I was able to better fix my position in mountainous terrain.

This latest GPS has got a fair background map, accurate to about 500 metres. The beauty of this model is that it give you an accurate indication of accuracy based on sats and signal. For it's pitfalls, I still love using my GPS.
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Old 03-20-05, 06:14 PM   #22
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Quote:
Originally Posted by CdCf
Actually, the best a non-WAAS GPS unit can manage is a position within a 10 m circle 95% of the time.
Claims of precision of a metre or below are bogus and have nothing to do with reality.
The rest of the time, it's worse. How much worse depends on how many satellites are in view.
In an uncluttered environment, with low horizons, you could have around 10 sats in view at the same time.
I do a lot better than that, with, or without WAAS. All You need to do is go slow, and let the readings averaqe out. The test I use for accuracy is to pick out a landmark, such as a driveway, visible in a satellite photo, from USAPHOTOMAPS, download the coords into My Garmin 72, go to the spot, and see what I get. I am usually within 3 meters. I don't live in a cluttered environment, of course, why would I want to? If You have accuracy issues You are likely expecting instant results, just slow down and give the GPS time to compute, or move out of the clutter. Concrete canyons suck!
I geocache, when I am not riding around looking for critters, and many times I have been able to get close enough, using My GPS 72, to find a cache consisting of a 35mm film can. If I drop a lens cap from one of My cameras, I can use the GPS to retrace My track, and expect to be close enough to spot the lens cap. No way is this bogus! Some of You are just in too much of a hurry. Hurry is stupid, because in the end, You are gonna die, go slow, and enjoy! I will catch up with You at the next traffic light, anyway!
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Old 03-20-05, 06:34 PM   #23
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I know that speed is an issue with cheaper models. I know a bloke who bought a cheap hand held and then brought it into a plane and it bogged itself at 100 knots. Certainly in my experience, accuracy does seem to improve with movement. I left a camera at a resting spot and was able to get back and recover it.

I just wish I could afford the one my mate has with a colour screen, hypsometric tints and a damned accurate background map.
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Old 03-20-05, 06:41 PM   #24
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Quote:
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I know that speed is an issue with cheaper models. I know a bloke who bought a cheap hand held and then brought it into a plane and it bogged itself at 100 knots.
Many commercial handheld GPS receivers were speed-limited to prevent people from buying them and using them for aviation navaid purposes. Some of the motivation for this was marketting driven and others were liability/legal driven. The manufacturers didn't test those units and approve them for aviation thus they didn't want to be held liable for an incident culminating from someone using a GPS receivers in an unapproved fashion. They also want to be able to command top-dollar for their aviation units which had undergone more rigourous testing in a flight environment.
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Old 03-20-05, 07:23 PM   #25
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Quote:
Originally Posted by tsiya
If You have accuracy issues You are likely expecting instant results, just slow down and give the GPS time to compute, or move out of the clutter.
Sorry if you got the impression that I own a GPS unit.
I never have and I very much doubt I will.
Frankly, outside of marine and aviation applications, there is very little need for GPS in any form.
A good map for a few bucks, along with a compass, is all you need to navigate by foot.
Save your hundreds or thousands of dollars for bike parts instead!

I do, however, like to keep up with new stuff, so I've read up on the subject.
I've read a fair number of long-term stationary tests over the years, and they all give pretty much the same picture. An accuracy of roughly 10 metres for +95% of the time.
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