Map issues

I've read all the stickies and still am not able to figure out a map issue that I think I'm having. Just purchased a 60CSx. Also bought and installed Topo 08' and City Navigator NT. GPS plots on Topo 08' are at least 100 ft off of the given track at times (sometimes they are on track, but not often). For reference sake my location while at the house is actually showing as I'm across the street on the map. City Navigator seems to be dead on and very accurate. Is this to be expected? I'm confused, as I really want to use Topo 08' to navigate some off road stuff, but can't imagine this is as accurate as it gets. Sorry if it has already been covered, and appreciate the help in advance.

Peace

I have the same issue with Topo 08 and City Navigator when I view them on Mapsource. It doesn't look like Topo is off from City navigator in the same direction or distance all the time. it seems to vary. i am also interested if anyone has any info about this.

Only thought I have is to look into what the map Datum the map set is based on and how the GPS is set, the default is "WGS 84'. I think the datums have to do with the assumed shape (it's not flat nor a perfect sphere) of the earth. In the perfect world the GPS datum setting should match the map data.

Not sure how you determine the map datum used for the Garmin maps but all technical maps list the Datum somewhere.

Another thing to remember is that the Garmin Topo maps are based on some very old USGS (1:100k I think) topo maps from the 50's, 60's & 70's. The 1:100K paper topo that I have at hand is "1927 North American Datum" (NAD27 on the GPS) and has on it "To place on the predicted North American Datum 1983 (NAD83 on the GPS) move the projection line 19 meters North and 87 meters East". That is not to say they aren't useful for geographic details as they rarely change. As a road map the Topos suck.

I think for geography locations (Summits, etc) if your GPS datum settings match those of the map from which the data was derived you would find them quite accurate.

Bruce

I have the old US Topo and it's off a lot of times too.

You make it sound like you are flying an airplane on instruments and worried about hitting a building in the fog. You follow the road or trail with your eyes open not with a hood on your head only viewing your GPS.

I have found lots of map sets that did not know that the state has moved the road or built a new interchange with ramps on the oposite side from the old ones.

Try riding the same road several times and see how far apart your own recorded tracks are. Keep your zoom level at 0.2 and you will never know the difference.

Thanks for the helpful replies guys. Looks like my belief is confirmed with your observations as well. My ultimate concern was only due to the "differences" between the roads on Topo being somewhat inaccurate as reference points compared to the roads in City NT being pretty close to dead nuts. Unfortunate to me, the only real reference point I've had in my limited use thus far have been roads. Knowing that now, I can certainly apply the best judgement when utilizing topo for offroad exploration. The Zoom level advice definitely helps out as Jerry stated.

I've been transcribing TAT west waypoints and other routes in AZ, UT, NV, and CA, for Mobius III, and have found the same thing when I cross-reference the coordinates with GoogleEarth. Since I'm routing point-to-point (as opposed to auto-routing) for most of these sections, what I've started doing is using the CN data whenever there's a road visible there, when there isn't, I use the Topo. When the trail Sam has you on is not on either database, I find it on GoogleEarth by zooming in close, and then mark the location, cut and paste the coordinates into a waypoint on RoadTrip, and link it into the route. As a last step, I go back and recheck all the via points, superimposing all the points onto GoogleEarth images, and then go back and adjust anything that's off. (Usually Topo-only roads/trails, but not always).

In general, when the Topo is off, it's a little off -- a few hundred feet at most. CN is mostly spot-on, however, when CN is off... it's REALLY off -- like, there isn't anything that even resembles a road within a mile -- it's simply a line on the map that corresponds to nothing on the ground.

I worry less about the accuracy in the open flat sections where the sight-lines are longer, and more so in the hilly curvy areas where there are more side-roads and dead-ends. After you've done this awhile, you learn where to plant your via points so that it's (mostly) clear when you're riding it where you're supposed to go.

Dr. Rock that makes complete sense, and I appreciate the insight. I did exactly as you described today to acclimate to it, and now am able to better create my waypoints. Both Topo and NT will definitely be useful to me now that I understand the strengths and weakness.

I went by a very large, and mature lake today using Topo. The map showed it as a small river:huh. That certainly speaks to the sometimes out of date POI's on the map.