GPS Limitations: Last modified on: Sunday, July 25, 2010
There are many different types of GPS receivers out there with differring capabilities. I created all of my data in a way that they can be used on most basic or older units. The GPS does not have to be able to display a map to benefit from this information. The following list reflects my desire to allow widest usability of the trail data.
Waypoints ==> Locations that are manually marked
AKA "Points Of Interest" or POI
Limit length to 8 characters that most newer units handle
The first 2 to 4 letters are capitalized and are the same for all waypoints in a route to help group/identify them
These first 2 to 4 letters are used as the map key.
Waypoint comments are limited to 16 characters
There are many different waypoint symbol sets out there. These icon sets may vary even between versions of a GPS model. Mapping these symbols between units is problematic.
Garmin 76Csx symobls are used in files I provide
Use my GPXl8r application to translate them for other types of GPS units.
Lowrance units also have un-named points they refer to as "icons". You can't search for them, but they appear on the map.
Routes ==> A group of locations that you visit in a set order
Originally I limited to 30 points per route for older Magellan compatability. Most newer models handle 50 or more, but I now try to limit them just for documentation simplicity
Route names are limited to 13 characters in length
Routes can be traversed in either direction, although waypoint comments may not make sense if you do.
Tracks ==> Automatically recorded points that show where you've been or want to follow.
AKA "Bread Crumbs" or "Trails"
Track names are limited to 13 characters.
Limited to 250 points per track for earlier Garmin units. (Relaxed recently as most units now handle 500)
On Garmin units, the active tracklog can hold more points than saved tracks. The track is automatically simplified when saved.
I've manually tweaked these track points to represent the actual travel as closely as possible.
Because of the point limitations, they do not follow every twist & turn in the trail, especially on long stretches where you don't have any path options.
The number of tracks that can be saved on a GPS or viewed at one time varies per GPS.
GPS Accuracy ==> Varies
Depends on how well spread apart the "viewable" satellites are.
Virtually all units made since late 2005 are WAAS enabled, allowing as close as 10 foot accuracy.
WAAS is not as usable in the northern part of the US, I do not normally use WAAS while recording.
Typical estimated position error is in the range of 20-30 feet for most of the spots I mark.
Heavily wooded/steep areas typically have higher error potential.
Mapped Track Accuracy ==> Varies
As indicated above, it varies with the length of trail and how straight the trails are.
It is usually close enough to let you know which side of a street it was recorded on.
Larger discrepancies may show on trail sections where you don't have turn choices.
Trail Organization:
My Trails are split into several groups to help make working with them more manageable.
I initially split them based on their relationship to the confluence of the Platte River & Cherry Cr.
DS ==> Trails that are south of, or water drains ino the Platte River below the Cherry Cr confluence.
DN ==> Trails that are north of or water drains into the Platte River above the Cherry Cr confluence.
FN ==> Trails outside and north of metro area. Arbitrary cut: (Boulder/Brighton on to South Ft Collins/Greeley).
CR ==> Trails in Douglas county south of the metro area e.g. Franktown, Castle Rock, Larkspur
CS ==> Trails in El Paso county.
Trail Documentation:
I maintain both text and spreadsheet trail documentation for all of my mapped trails.
Documentation provides route statistics, waypoints, comments, notes and connecting trails.
GPS formats:
There are a lot of ways to store & transfer the data, I provide these:
GPS Exchange Format: .gpx
This is the defacto standard for GPS information exchange, and is a form of XML
Many software applications read/write this format, including some manufacturers (i.e.Garmin).
DeLorme Format: .an1, .anr & .pmo
Used with their Topo USA & 3D Quad software
.an1 stores waypoints, map symbols and track & trail items (also .an2 for Topo NA)
.anr stores route data
Since DeLorme started supporting .gpx, I no longer provide routes and tracks for this format.
I provide a draw layer for each zone of the tracks and waypoint files using PN20/40 symbols.
I now provide a map file that allows the entire trail network to be shown on PN20/40 by just copying it.
National Geographic: .tpo
Another major consumer mapping contender, very easy to use
Not as powerful as DeLorme, so it is much simpler to operate
Since they started supporting .gpx, I only provide a file with all tracks for zone in it.
Google Earth / Keyhole: .kml or .kmz (compressed)
Google Earth also reads .gpx files
You can use the free GPSbabel software to convert between these and many other formats.