GPS Navigation Software

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Jlawsen

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Re: GPS Navigation Software

JB said:
We have also overlooked the fact that "up to 1.5M error" can be anything from ZERO error to 1.5M, meaning that most of the time the error will be less, sometimes a lot less, than 1.5M. This would account for my own real world observations seeming to be a lot more accurate (or at least repeatable) than the worst case scenario all the geometric and atmospheric gymnastics suggest.
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Absolutely true.. If you let your GPS/WAAS reciever run in a stationary mode or if you're running on a semi level plane such as a lake, your altitude will eventually be dead on the money. Here's the problem, there's only so much storage space on the chips in the receiver and conventional computer memory is too slow to be usable. When the maximum amount of data has been stored, the units will clear themselves and you start all over again. It's what's considered a GPS warm start. A cold start drops everything and starts over and a hot start uses whatever data is present even if you've relocated. Ever noticed when you fire up your GPS in your back yard it sometimes shows you still at the lake. That's a hot start.
 
bruceb58

bruceb58

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Re: GPS Navigation Software

Jlawsen said:
Absolutely true.. If you let your GPS/WAAS reciever run in a stationary mode or if you're running on a semi level plane such as a lake, your altitude will eventually be dead on the money.
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This is absolutely not true. Unless the error has a mean value of zero this won't happen. But lets say for argument sake that it was. You spend a thousand hours mapping your lake. Of course you will need at least 1000 hours to get a good mean value. You now go on your lake to actually use the data you collected to prevent yourself rom running into something and you get the value that happens to have a 10 foot error. Sorry, but all that 1000 hours you spent collecting data is for naught. Same reason autolanding of aircraft can't use GPS either...too much error that you can't predict when its going to happen.
 
JB

JB

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Re: GPS Navigation Software

Augh. This is too much like discussing a headache with a neurosurgeon. I respectfully withdraw.
 
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Jlawsen

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Re: GPS Navigation Software

bruceb58 said:
This is absolutely not true. Unless the error has a mean value of zero this won't happen. But lets say for argument sake that it was. You spend a thousand hours mapping your lake. Of course you will need at least 1000 hours to get a good mean value. You now go on your lake to actually use the data you collected to prevent yourself rom running into something and you get the value that happens to have a 10 foot error. Sorry, but all that 1000 hours you spent collecting data is for naught. Same reason autolanding of aircraft can't use GPS either...too much error that you can't predict when its going to happen.
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Actually if we roll it back a bit and look at how I'm attacking it using both GPS and a Fathometer it then becomes reliable enough to redraw the map and give you a heads up.

What started all this for me was trying to get the exact lake elevation at the time a distributed map was made. I asked for it from Navionics figuring since they had the bottom contours surely someone recorded the elevation. No response. Next I looked at the USGS topos for the lake to see what I could come up with. The 1992 topos appear to use a WL of 5150 (total flood) and the latest 2012 use 5110. Ironically the highest its been since 1992 was in 1992 and it was around 5106. 5105.5 is optimal for water quality by the way.

So back to the Navionics maps and if I back step the contour lines using the 40' intervals as prescribe by the USGS topos I get really whacky results. I think it was at that point that I realized that Navionics was selling just so so maps. I'll stop short of calling it shoddy because they do provide to everyone but the most discriminite (like me). If you ask me to be honest and tell you that I really need this level of accuracy, the truth would be no.

So I got to thinking, I'm a programmer, why not put together a system that can be used with a set of easy to understand variables that could render the maps in different what if scenarios. Now we're were we are trying to figure out how to easily get an accurate elevation. As you can see, a 1.5 meter error after a cold start is unacceptable.

So I'm on my quest to figure this out I'm calling on a few old retired folks that I worked with in the past and have them scratching their heads. I know what I need but I'm not sure how to get it and apparently it's not a very easy question to answer. If it was, we wouldn't have the 1.5 meter error.

Reality will likely take hold when I get up to the lake on Saturday and get some depth readings. I'll probably just get the current elevation from the marker at the ramp and say, that's what the depth was when Navionics made the map, this is what it is now so the elevation at the time the map was made is this. I'll only need one depth reading and I can create the model.
 
bruceb58

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Re: GPS Navigation Software

What lake is this and how big an area are we talking about? Can your fathometer give you data? Seems to me it would be way easier to just figure out the lake elevation from the depth at a dam if there is one. Then you don't even need to worry about GPS elevation.
 
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Jlawsen

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Re: GPS Navigation Software

bruceb58 said:
What lake is this and how big an area are we talking about? Can your fathometer give you data? Seems to me it would be way easier to just figure out the lake elevation from the depth at a dam if there is one. Then you don't even need to worry about GPS elevation.
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Eagle Lake is the lake, it's the second largest natural lake in California and the first and last lake in my 2 month fishing trip that I begin on Saturday. There is no dam, that would make it way to easy, LOL.. Therein lies the problem, nobody knows for sure what the true surface area of the lake is. There have been some minor surveys of the lake but no real hydrolgic efforts. Bing Maps has the best shots of the lake from last fall. The Google maps are from 2009 and unfortunately the key hazards are masked by wind noise.
 
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Jlawsen

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Re: GPS Navigation Software

Got my NEMA 2000 to USB/Serial converter yesterday. I did a comparison between the Satellite sentences that I am recieving and found no difference in any of the four receivers. I expected that would be the case but was hoping that more definitive data might be found from a high end receiver. For instance the altituted is sent only to the tenth of a meter. Correcting altitude is impossible down to beter than a couple of feet when my error rate is 1.4m which is what I'm finding to be consistent when using WAAS.

All hope is not lost though. On lakes where there is an accurate monument I'll just use it. I guess I know now why Navionics can't do bottom contours using remotely acquired altitude data.
 
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Jlawsen

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Re: GPS Navigation Software

Just found out I can change a few firmware switches in my Global Sat reciever and get a few more decimal points in the data. I doubt it will end up being any better but I'll run it and see.
 
bruceb58

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Re: GPS Navigation Software

Jlawsen said:
Correcting altitude is impossible down to beter than a couple of feet when my error rate is 1.4m which is what I'm finding to be consistent when using WAAS.
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You should be happy with the 1.4M if that's really all it is! Looks like you are around 300 miles from the WAAS ground reference station at Fremont,CA so you are at least in the 500 mile radius of it.
 
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Jlawsen

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Re: GPS Navigation Software

bruceb58 said:
You should be happy with the 1.4M if that's really all it is! Looks like you are around 300 miles from the WAAS ground reference station at Fremont,CA so you are at least in the 500 mile radius of it.
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Yeah, it's hard to tell if that's all it is for sure. It was consitent accross 8 satellites that I was locked on when I dumped the data out. One thing I found was that while the raw data was giving an altitude of 185', Google Earth was giving me 196' running in real time GPS mode. There isn't an option to use WAAS or VTG in Google Earth so I don't know if was doing it behind the scenes or not. My guess is not. Google Earth seems a little behind the times now. I wonder what the Micorsoft/Nokia partnership will do to Bing Maps. I would have to believe that they will make some big improvements. Their imaging is definitely more current with the exception of the street level.
 
bruceb58

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Re: GPS Navigation Software

Google earth is not very accurate in elevation at all and it really doesn't need to be.
 
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Jlawsen

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Re: GPS Navigation Software

bruceb58 said:
Google earth is not very accurate in elevation at all and it really doesn't need to be.
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I agree with that. I just loaded up Micrsofts Street Maps 2011 to see how it compared and it does use the GPS position and elevation correctly. Well as correct as it is anyway. I haven't had time to do much with it but there's no doubt that the aerials are more up to date for the areas that I'm concerned with. I'll get more familiar with it during my vacation and see what happens. I hit the road in the early a.m. tomorrow.. Yahoooo!
 
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