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nhinze
12-14-2009, 23:02
Dear all,

I just completed my first draft of an interactive AT map using the ATC GIS data. I also calculated the elevation in order to provide an elevation profile.

Check it out at http://www.trailheadfinder.com/long_distance_trail/appalachian_trail and let me know what you think!

Nick,

BrianLe
12-14-2009, 23:24
Seems pretty similar to what Postholer has for the major trails (http://postholer.com/gmap/gmap.php).
Perhaps there's particular functionality in one that the other lacks? Looks like the same underlying (Google) technology.

One issue that I know that Postholer has is having the mileage for a particular point --- as calculated --- line up with the official mileage for a particular trail. Did you by chance implement a lookup table or other solution to make this more accurate ?

I don't mean any aspersions by these questions, no doubt this was an interesting project to put together!

Compass
12-15-2009, 00:07
Here is another map version http://fivemillionsteps.com/trailmap.php with three trails(AT, PCT, CDT)
The first two posts and this one each have strengths and menu adjustments to customize the page so zoom in and try the different options.

Manwich
12-15-2009, 00:29
My site, TheTrailWiki.org (http://www.TheTrailWiki.org), too.

nhinze
12-15-2009, 06:29
Getting the mileage to align is a problem. My mileage is only 2132 miles and that is due to the following I believe:

- Differences between GPS track and the way the distance was officially measured (possibly using a wheel ?)
- The distance in my calculations do not take in account the elevation. It is only a great circle distance between each GPS point. I may try in the future to incorporate elevation in the distance calculations to see how much difference that makes.

Postholer.Com is a great website. I came across it a week ago while I was already working on my project. I couldn't find something similar before!

Bidwell
12-15-2009, 21:32
It's good, but it needs way more advertisements. Sign up for another Google Adsense account.

Manwich
12-15-2009, 23:56
Getting the mileage to align is a problem. My mileage is only 2132 miles and that is due to the following I believe:

- Differences between GPS track and the way the distance was officially measured (possibly using a wheel ?)
- The distance in my calculations do not take in account the elevation. It is only a great circle distance between each GPS point. I may try in the future to incorporate elevation in the distance calculations to see how much difference that makes.


Myself, Mr. Parkay and others already beat ourselves up over getting the math right. The conclusion is that the difference is pretty negligible. TheTrailWiki, Postholer, and other websites will never be more than a loose planning tool... (and most of the ESRI data is already +/- something like 25-40ft in accuracy anyway)

The AT was never measured in one fell swoop. It's a culmination of recorded distances across hundreds of landmarks.

The digital data will never rival anecdotal data. Your mileage is still 98.02298850574713% correct. So long as your users know that there is a <2% margin of error, and that they should never rely on the technical data, I think you'll survive.

Plus, I think the ATC's data is already a decade old, no?

nhinze
12-16-2009, 06:34
I recalculated the distance using the elevation difference and it added 20 miles to the total distance. I'm still short at about 2152 miles. The distance added about 0.8 % to the distance. I will use this distance in the next update.

The ATC data was last updated 5/21/09 and the GPS tracks are anywhere between 2002 and 2005. There are plenty of issues in the GIS data and I had to clean it up with ArcGIS (parallel tracks, tracks not on AT, etc...). The ATC also does not provide elevation and I had to use ArcGIS to estimate it.

I will add a disclaimer that the data NOT official.

Thanks for all the comments!

postholer.com
12-16-2009, 21:19
The AT was never measured in one fell swoop. It's a culmination of recorded distances across hundreds of landmarks.

The digital data will never rival anecdotal data. Your mileage is still 98.02298850574713% correct. So long as your users know that there is a <2% margin of error, and that they should never rely on the technical data, I think you'll survive.Nor will the anecdotal data rival itself. If you wheeled the trail twice, I'd bet the ranch you'd end up with different lengths. :-?

I've scoured the postholer (http://postholer.com/gmap/gmap.php) PCT data much more than the AT data as I'm very familiar with the PCT. Making the calculated length match the published trail length is easy. That goes for any trail.

Even if you find a large error of +/- 5% over a 100 mile segment of trail, at 20 mpd we're talking +/- 1 mpd. That's nothing for an error that large.

The technique used at postholer does not use waypoints to 'adjust' the data. It calculates it outright. For the PCT, 1700 miles into the trail the anecdotal/calculated mileage are off by 6 miles. At 2,655 miles they are of course exact, with varying degrees of lesser or greater error along the trail.

If I have 150 miles between resupplies and use calculated mileages with an error of < 2%, there's nothing 'loose' about that. Not only is it an excellent planning tool, it's far, far easier than extracting from written text. Also, you can get any 2 points along the trail, you are not limited by what the author chose.

-postholer

The Weasel
12-16-2009, 21:55
I recalculated the distance using the elevation difference and it added 20 miles to the total distance. I'm still short at about 2152 miles. The distance added about 0.8 % to the distance. I will use this distance in the next update.

The ATC data was last updated 5/21/09 and the GPS tracks are anywhere between 2002 and 2005. There are plenty of issues in the GIS data and I had to clean it up with ArcGIS (parallel tracks, tracks not on AT, etc...). The ATC also does not provide elevation and I had to use ArcGIS to estimate it.

I will add a disclaimer that the data NOT official.

Thanks for all the comments!

I suspect that your trivila "off" amount is due to MUDS. While there may be a net elevation between points, unless total elevation change is included, which is extraordinarily difficult for small changes (say, 50' up or down) the total distance will be off.

This is an excellent example of the truism that "The map is not the territory." Until there is a calibrated 'on the ground' measurement, there won't be precision. Even then, in the matter of 2,000 miles or so, tiny features such as inside and outside radii of curves and so on can make a difference that adds up over a long trail.

Still, it would be nice if there was a fairly well calibrated measurement to serve as the closest possible. Who will offer ATC the dough for this?

TW

postholer.com
12-17-2009, 01:32
This is an excellent example of the truism that "The map is not the territory." Until there is a calibrated 'on the ground' measurement, there won't be precision.

At < 2% error there is precision. If a 100 mile segment of trail differs from the published distance by 2 miles, then that is certainly precision.

Over the years I've heard discussions of 'exactism' regarding walked vs gps distances as if there is some perfect obtainable distance that all others are compared. As stated earlier, if you wheel a trail segment 'on the ground' on 2 different occasions, you will get 2 different mileages. Exact is a misnomer.

Also, I was looking at some of the AT google maps. fivemillionsteps, trailwiki, trailfinder, ATConservancy and they all omit the detail provided by the gpx data. At zoom levels at or below 7.5 minute maps you're left with a boxy inaccurate trace. All these maps do it the same way, like they all stole the code from one another. Trailwiki doesn't let you zoom all the way down and doesn't even have a scale. Weird.

The postholer map (http://postholer.com/gmap/gmap.php?trail=at) is the only one that maps the complete detail available from the gpx files and displays the mileages/elevations.

I was wondering why that is?

-postholer

nhinze
12-22-2009, 22:39
Postholer: My map uses the GPX data. You have to zoom in to see all the points. I decided when creating my map that a maximum number of points should be displayed at once to avoid slow downs in the browser. To see all the points on my map you have to zoom in to near max using the street map (terrain doesn't go far enough).

I just added unofficial milemarkers for easier planning: http://www.trailheadfinder.com/long_distance_trail/appalachian_trail