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drunken pirate
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Discussion Starter · #1 ·
I am thinking about compiling a complete mtb trail guide for Pisgah and was wondering what, if any gps data I need to include. I plan on riding all the trails and noting the mileage of all important features as well as barometric altitude info and was wondering if I should also include gps way points? I know nothing about gps but figure if I'm going to ride all those trails to collect data it would be foolish not include a gps if that info might be useful in anyway. Anyone following me here? Suggestions?
 

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since 4/10/2009
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34,375 Posts
Here's what I suggest:

GPS accuracy will vary both by the day and from minute to minute on your ride from a number of factors. A single GPS track might come out with strange errors in it. I was working on a similar project once and I found it much easier to get multiple tracks for a trail (I preferred at least 3 but sometimes that was not possible) so I could see if they were "tending" towards the same location. I'd then hand-draw a final file for the trail system map. I used GIS software, but you could use Google Earth just as well. It is also a GIS program, but one more intended for the general consumer.

Waypoints for important features are a good idea. However, you don't need to make particular note of the elevation from your barometric altimeter. You can get those elevations within a CONSISTENT margin of error from a topo map. Problem with a barometric altimeter is that its accuracy varies over time unless it's calibrated OFTEN with known elevation points.

You will want to use a handheld GPS for this project that gets the absolute best reception you can get. An Edge is more than fine for the typical fitness ride or finding out where you are in the woods, but it's limited when it comes to actually creating maps. I really like the touch screen interface on the Oregon 450 (since you'll be making many waypoints, text entry ease may be important to you), but it doesn't have the best antenna of the bunch. I'd recommend a GPSMap 62s, and buy an external antenna for it (you can put the antenna on top of your helmet or backpack for optimal reception). You could also do well with an older 60CSx with an external antenna if you're more budget-minded.
 

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drunken pirate
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Discussion Starter · #3 ·
Thanks for the response. The accuracy of gps units is one thing I was really wondering about. I'm not trying to map this area, a good Nat. Geo map is in print, but instead comprehensively compile trail data - mileage between intersections and landmarks, as well as altitude, primarily. If there is a need for gps data I might as well get that as well but riding each trail three separate times to get it is unrealistic. I think it would be fun to know the exact coordinates of say the thirteen river crossings on the South Mills River Trail but doubt something that uses an antenna that I can afford is going to work down there...

Still I might as well carry a gps as long as I'm systematically riding every trail in its entirety. It couldn't hurt anything and would give me a comparison for my bike computer and altimeter (which I can easily accurately calibrate before every outing). Price is a factor. I don't think I need anything fancy but might as well get something that I could use for other applications as well. I'm going to be carrying a notebook and voice recorder as well as I do this project so I really don't need to be able to make notes on the gps device. Or do I? I know nothing about these things..
 

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since 4/10/2009
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34,375 Posts
driftwood said:
Thanks for the response. The accuracy of gps units is one thing I was really wondering about. I'm not trying to map this area, a good Nat. Geo map is in print, but instead comprehensively compile trail data - mileage between intersections and landmarks, as well as altitude, primarily. If there is a need for gps data I might as well get that as well but riding each trail three separate times to get it is unrealistic. I think it would be fun to know the exact coordinates of say the thirteen river crossings on the South Mills River Trail but doubt something that uses an antenna that I can afford is going to work down there...

Still I might as well carry a gps as long as I'm systematically riding every trail in its entirety. It couldn't hurt anything and would give me a comparison for my bike computer and altimeter (which I can easily accurately calibrate before every outing). Price is a factor. I don't think I need anything fancy but might as well get something that I could use for other applications as well. I'm going to be carrying a notebook and voice recorder as well as I do this project so I really don't need to be able to make notes on the gps device. Or do I? I know nothing about these things..
Sometimes, it's helpful to have the notes actually part of the GPS file you're working with, but if you'll be putting things into a notebook, that will work, too. The external antenna that is compatible with Garmin's handhelds is not outrageously expensive. Maybe $50 or so. In an area with difficult conditions, it can mean the difference between a signal or nothing. When you get a signal, it can really improve the quality of things for the sort of project you're doing. If you needed absolutely accurate measurements, yeah, it still wouldn't be enough. And yes, the hardware that can achieve that kind of accuracy is obnoxiously expensive, but for your purposes, it would be pretty good. One thing that will help with waypoint collection is that most handheld GPS receivers have a waypoint averaging feature that collects coordinates while you stand still over a period of time, and will calculate your actual position based on an average of all those points. The better your signal, the less time that will take. You'd be surprised with GPS reception. You'll probably have a signal more often than not even in the challenging conditions of the South Mills River Trail (I have not ridden that trail specifically, but I've ridden on others nearby). But, with you wanting measurements, you'll want some reasonable accuracy and you will want to take some steps to get the best you can.

With regards to the barometric altimeter, it will drift over the course of hours within a day, so while your measurements shortly after calibrating it will be spot on, the accuracy will degrade over the course of the day. Again, if you're using it for that, you'll want to calibrate it several times over the course of the day. It would be easier for your purposes to get elevations from a map after the fact. There's some error inherent with that, but that error will be consistent from point to point.

The problem with getting mileages between intersections and whatnot is that you're going to need tracks. A single track is going to have a range of error associated with it, and you just won't know what it is. The more tracks you get, the easier you'll be able to find the error and account for it. When I was working on my project, I was not doing all of the data collection myself. I was having other people provide me with GPS tracks from their rides, also. That was the only way I was able to get multiples for a trail, and would be the way you'd have to work, also.
 

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drunken pirate
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Discussion Starter · #5 ·
That is a lot of good info and a lot to think about. Thanks :)

I realize my individual results aren't going to be 100% accurate and I do plan on tapping others for their gps tracks so I can try and get an average for the important splits. And the altitude is really not that important at all.

Anyway I think I'll go for the 60CSx with an external antenna as that other one is out of my price range... Thanks for the suggestion, anything else I should know or get?

Next time you are in the area you should ride all of South Mills River :eekster:
 

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since 4/10/2009
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34,375 Posts
driftwood said:
I realize my individual results aren't going to be 100% accurate and I do plan on tapping others for their gps tracks so I can try and get an average for the important splits. And the altitude is really not that important at all.

Anyway I think I'll go for the 60CSx with an external antenna as that other one is out of my price range... Thanks for the suggestion, anything else I should know or get?

Next time you are in the area you should ride all of South Mills River :eekster:
You could probably do well to download tracks you find on various sharing sites, too. Might save you a lot of field time.

IMO, you should invest in decent software for your computer. You didn't say if you're on Windows or Mac. If on Windows, Topofusion is a nice program that has a "network" tool that will average out the tracks for you. Krein (who posts here on occasion) just put an update to that network algorithm in his latest update that addresses some concerns I brought to him about that tool awhile ago.

I would have ridden it on my last visit (several years ago), except for the fact that my weeklong trip got cut short on the Pisgah/Dupont leg because of neverending rain. I did a route called the Clawhammer route (as described in a guidebook I got for the area) that was awesome. I did that after a couple days of rain, and things just got worse after that.
 
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