Trail Origin

Want to know where the trails on the GPS Topo maps come from, why a trail you know of is not on these maps, read on.


Where do the trails come from:

All trails on these maps come from public GIS records of various federal, state or local agencies, occasionally from private collections related to specific recreational interests. The most common format is a shapefile, which is a multi-file collection of vector data defining the line and a database with line attributes. Private collections are most commonly in GPX format, which is a simple text file of vector coordinates with no trail attributes.

If a trail you're aware of is not on these maps, then that trail was not in any publicly available GIS data-set available for download or it was missed during extensive search sessions. If you're aware of a public trail data-set that should be included, by all means send an email with a link and your comments.

I regularly phone and/or email federal and state GIS offices seeking trail data, this is an example of one reply:


"We are currently working on our transportation plan and the estimated completion is approximately five years.  Be sure and check back with us then for a much more comprehensive dataset."


Every government entity defines their trail attributes differently and even branches of the same entity will uniquely define their attributes. Hence one national forest will differ from another in the available data presented on these maps. Rather than a consistent approach, all trails called "trail" with no name, number or attribute, Garmin's approach, these maps provide as much information as possible on an individual trail basis.

 

Why do the trail alignments sometimes vary so greatly from my GPS tracks:

To understand the answer you have to understand the source of most trail vectors in most GIS data-sets. Most trails, particularly in federal data-sets, are derived from USGS 24K Topo maps and are simply scanned from those maps and converted to vector format. The next step is to understand how those trail lines got unto the original 24K Topos. In 99% of the cases, one cannot see a trail in an aerial photo so the cartographer is left to align the trail based on field notes and his best guess, not very precise. Rarely are trails on 24K topos based on GPS tracks. I would estimated that 90% of all the trails on these GPS topos are based on scanned data and not GPS data. Additionally, it will probably take another fifty years for all trail data to switch from scanned to GPS.

The following screenshots will amplify the point:

   

The left image is an example of a scanned trail derived from a USGS topo map. Notice the soft switchbacks, relatively straight lines and alignment out of logic with the contours. The good news is that the start and end points are essentially the same as the correct trail alignment in the right panel. The right image was derived from a user submitted track file and that trail line replaced the original scanned line from the forest service file.

Here is the user submitted GPS track on top of the scanned original trail alignment:

  

The right panel shows that same user track incorporated into the New Mexico version 2 map. You can have your improved tracks show on the new maps as well, see here.