Accuracy of topos and aerial photos

[+imajeep]

I've started using a GPS unit for hiking (it's what got me started in geocaching), in preparation for a hiking trip in the western US national parks in a few months. I'm plotting routes for some hikes we want to do, using the Garmin 1:24K National Parks topo set. I'm checking the routes I plot against aerial photos in ExpertGPS and Google Earth. And in cases where I can see the trail in an aerial photo, I have used the photo to plot a route.

 

I have been really surprised at some errors I am finding. The Garmin topo plots don't line up well at all with the aerial photos, and the aerial photos don't line up with each other. For example, take a look at the area around Mystic Falls in Yellowstone. And routes that I've created in ExpertGPS photos don't line up in Google Earth. Mystic Falls is good example of this problem, too. The errors are significant; running 80 to 100 feet or more.

 

I can understand that topos can be decades old, but I was really surprised that the photo plots didn't line up more closely. Is this level of error inherent in topos and aerial photos? How reliable are the aerial photos in ExpertGPS / Google Earth for plotting routes?

 

Thanks.

[+TotemLake]

These trails were originally surveyed by foot back in the days of hand drawn cartography. The mapped trail will be an aproximation of the actual trail as it was then.

 

Also, there is trail maintenance going on all the time. Sometimes there will be recovery efforts where the trail on the map will be abandoned and rerouted.

 

The cool thing is there are web sites out now where you can go to uploaded tracks of the area you might be interested in and see what the actual trail will be plus or minus the error position estimate at the time the track was created.

 

In Washington, the WSGA club did a project for a state park where the trails were gps tracked from border to border and then submitted to the ranger to update thier local maps.

[+imajeep]

The cool thing is there are web sites out now where you can go to uploaded tracks of the area you might be interested in and see what the actual trail will be plus or minus the error position estimate at the time the track was created.

I've been looking for those sites. Backpacker.com has some, and TrailRegistry.com has some, as well. Are there others?

 

I'm kind of surprised the US National Park Service (or someone else) hasn't collected GPS maps of the trails in national parks. What really surprises me is that nobody is doing for GPS hiking what geocaching.com does for geocaching.

[+WeightMan]

 

I have been really surprised at some errors I am finding. The Garmin topo plots don't line up well at all with the aerial photos, and the aerial photos don't line up with each other. For example, take a look at the area around Mystic Falls in Yellowstone. And routes that I've created in ExpertGPS photos don't line up in Google Earth. Mystic Falls is good example of this problem, too. The errors are significant; running 80 to 100 feet or more.

 

I can understand that topos can be decades old, but I was really surprised that the photo plots didn't line up more closely. Is this level of error inherent in topos and aerial photos? How reliable are the aerial photos in ExpertGPS / Google Earth for plotting routes?

 

Thanks.

While I am by no means an expert in this, but I might have a suggestion. I have not looked at any aerial photos of the area either. It is possible that some of the errors you are seeing are the result of corrections based on the angle of the photo. If you look at GE pics of some downtown areas, the shadows of the buildings do not always line up because they were taken at different times. Also you can see the sides of some buildings. That would not happen if the view were from directly above. I'm sure that some of the photos have had corrections based on the distance (or angle) away from the vertical. Those two items will cause some problems with the satellite photo. I don't know if it is still showing, but there was a very strange looking picture if you looked at the Stratoshere in Las Vegas. That was the result of several pictures having been pieced together near the building.

 

Just a thought.

[+TotemLake]

The cool thing is there are web sites out now where you can go to uploaded tracks of the area you might be interested in and see what the actual trail will be plus or minus the error position estimate at the time the track was created.

I've been looking for those sites. Backpacker.com has some, and TrailRegistry.com has some, as well. Are there others?

 

I'm kind of surprised the US National Park Service (or someone else) hasn't collected GPS maps of the trails in national parks. What really surprises me is that nobody is doing for GPS hiking what geocaching.com does for geocaching.

The idea of uploading gps tracks to update maps is still relatively a new idea. As well, you have to remember, a lot of folks do not have the technical skills needed to follow simple instructions when uploading tracks to a website. Couple that to folks who are willing to do so and the number of gps users doing this very thing is quite low.

Edited May 21, 2007 by TotemLake

[+imajeep]

The idea of uploading gps tracks to update maps is still relatively a new idea. As well, you have to remember, a lot of folks do not have the technical skills needed to follow simple instructions when uploading tracks to a website. Couple that to folks who are willing to do so and the number of gps users doing this very thing is quite low.

Darn--on the bleeding edge yet again!

[Tahosa and Sons]

Aerials are just a pretty picture and TOPO's are just a general reference. From my experience follow the trail, plot it using UTM's as you move and pay attention to the contour lines. I've tracked a lot of trails for the USFS and its amazing to see how much the maps are off. But my boots know where they are at.

If you get out this way look me up and I'll show you some backcountry trails.