Google map

[rup]

Using Google Map to compare satl & terrain images. Finding they don't really align at highest resolution.

Thought that I could see the AT cutting thru the woods in CV Maine, but alas, no.

Anyone else??

 

[Nessmuk]

Using Google Map to compare satl & terrain images. Finding they don't really align at highest resolution.

Thought that I could see the AT cutting thru the woods in CV Maine, but alas, no.

Anyone else??

I used Google Earth satellite images to plot a 678 waypoint route down the Yukon River for the Yukon1000 canoe race. Except for the areas where the river has obviously changed the landscape over the past couple of years, each waypoint was dead-on where it should have been when we actually arrived. Registration of lat/long with actual terrain features was highly accurate. Outside of the canyon areas where the river never changes, the 1950's era topographic maps (the most recent editions available) were hopelessly inaccurate and I didn't bother using topo maps at all. Even in some of those fixed locations there were inconsistencies in registration between topo and GE. I ended up correctly believing GE.

 

[Trail Bandit]

You will find that the positional accuracy of features on Google, and other aerial photo sources depends on the steepness of the terrain. All the photos are computer corrected using a digital elevation model. This leads to errors in steep areas, and around mountain tops, but is very good where the terrain is more gentle. If you plot a gps track over an orthorectified photo, believe the GPS track and question the photo.

 

[Nessmuk]

This leads to errors in steep areas, and around mountain tops, but is very good where the terrain is more gentle. If you plot a gps track over an orthorectified photo, believe the GPS track and question the photo.

Well, let me say that even in the mountainous regions of the Yukon River my GoogleEarth image derived waypoints were all still at river level. Every one of them proved accurate. If we had chosen to portage over a 5,000 foot mountain to shortcut an oxbow we might have seen the errors you mention.

 

[cushetunk]

You will find that the positional accuracy of features on Google, and other aerial photo sources depends on the steepness of the terrain. All the photos are computer corrected using a digital elevation model. This leads to errors in steep areas, and around mountain tops, but is very good where the terrain is more gentle. If you plot a gps track over an orthorectified photo, believe the GPS track and question the photo.

Do you have some source or other info on this? I'd be curious to learn about the relative error of GPS and the orthos.

My experience with the ortho/GPS combo in New England has left me pretty convinced that for anything short of a legal survey, the errors are negligible. It seems to me, given that there's still uncertainty with even the best handheld GPS, and that people on the ground don't navigate blind on the scale of 50 feet anyway, choosing to "believe" the ortho or the tracklog is rather academic.