Drift, inaccurate maps, or extremely steep country. When you are standing at the top of a cliff or the bottom of a cliff you can get the same Lat/Long or UTM from your gps. A S&R member in Wyoming first pointed this out to me years ago.
Yesterday I headed for the High Country to explore some new ground. I head out in the early morning darkness the Garmin Street Pilot c550 operating on the dash, and the Rino 530HCx operating in the center console of the pickup. The Rino was loaded with Topo US 2008 and the c550 with City Navigator.
I had about seventy miles to go on one of California's more rural State highways into canyon country, across ridges, and along a river in a steep canyon. The c550 on the dash was very visible and the depiction of my vehicle on the roadway was on the mark. At two or three pullouts I checked pointer and track created on the screen of the Rino. I was tracking on the highway.
Tracking on the screen of the c550 I could have “almost” steered by watching the screen. The view of the highway gave me clear warning of approaching hairpin curves.
The relationship between the unit maps and the Forest Service road I turned on to was way off. The road is a low maintains road and I suspect in has not been resurveyed since it was first lay-ed out. Most likely this road started out as a wildlife and Native American trail. Going from the ridge tops in this country to the river bottom is tough because of cliffs and steep slopes. Even the gold miners choose a different way into the area. It was not until the timber lobbyist convinced Washington that a route was found to the ridge top. It took a few switchbacks that were stacked on top of each other to get up on an East-West ridge. As I progressed up each switchback I was eating my own dust, which was flowing upslope. When I got to the top of the switchbacks I got out and enjoyed the view.
My next step was to check the Track, which had been created on the Rino. The track departed from the map in more then one location more then 100-feet. As I had proceeded up the road and hit a switch back I would check the screen. The switch back created using the Track tool was correct. The switchback on the map was incorrect.
The c550 using City Navigator was all over the place. At times it was on the mark and other times it was off. I proceeded out the East-West ridge to about the 7000-foot elevation and the c550 was good at displaying the Forest Service road numbers. But there were a number of minor logging spurs that did not show on the screen. I believe the spurs were upgrade from 4x4 trails to graveled logging roads in the 1980’s. Switching back and forth between City Navigator and US Topo 2008 on my computer showed the spurs on US Topo 2008. A few spurs did not show on City Navigator.
A review of my National Geographic California Series 4.0 shows the road coming in from the East and deadening on the ridge top and not dropping down in the series of switchbacks to the State highway.
The results very from the results which I collected on a National Forest 250 miles North of yesterdays locations. The bottom line is that I feel the mapmakers did a better job on the Northern Forest then those who created the maps on the Southern Forest. My conclusion is this is a mapping accuracy issue and not a gps issue. Satellite reception with both units was very good with the high sensitivity receivers.
I found a couple of certified BLM corners between a patented mine and National Forest land. When comparing a Forest Map, the survey markers, and the gps I feel that the gps was accurate. The only tool I wish that I had with me was my Rotary azimuth Plotter. I could have taken a bearing on a couple peaks and plotted my location on a map.
Is the lock on road option in Gamin's gps units a partial answer to mapping problems?
· A little side note:
A week ago a couple called 911 because they had become disorientated while out hiking. They had hiked from a trailhead upslope, crossing the Union Pacific tracks and continuing upslope to a high mountain lake.
On their return to the tracks they could not find the trail on the far side of the tracks. This was a very stressful time for the couple. I am sure the most basic gps unit in Track mode could have saved their marriage a lot of stress.
When I head for the woods my gps unit is in the track mode. It is simple enough to track back.
https://buy.garmin.com/shop/shop.do?cID=144&pID=8705#Magellan's basic gps unit is the Triton Series 200.
http://www.tigergps.com/magellantriton200.htmlA contact of mine that works in the area told me every few years somebody misses the trail on the far side because it is slightly offset.
I suspect the male half would have been happier to pull out a gps and navigate them back to the car, rather then taking the heat for getting them lost. Another downside is that because of the stress they suffered, he might have lost a hiking partner.
