Aerial Imagery-GE Georeferencing

[Grasscatcher]

I've got a number of very accurately GPSd tracks that, when viewed in Expert GPS or in USA Photomaps, are "dead on" the visible trail in the image.

 

HOWEVER, when imported into and viewed in Google Earth, the very same tracks show to have a position "shift" to the North.....(amount in the range of 30-40 ft). Appears very visibly off/inaccurate.

 

Area is in the Colo Natl Mon West of Grand Junction. Datum is NAD83/WGS84. "Clamped to ground" in GE.

 

What implications does this have when georeferencing imagery in GE to put on a GPS? Or, do I have a setting wrong somewhere?

 

Any ideas?

[+StarBrand]

I know of several areas to the west of here that are a good 75 - 100 foot off on the GE maps. Verified with several GPS units.

 

It happens - anything you produce based off of those images will be off as well.

[+Team CowboyPapa]

My guess is that the Google Earth (is not GE a registered trade mark of ..... so I'll spell it out? ) is wonky.

 

Consider the relative probability of these two alternatives:

Both your handheld GPSr and the other maps are exactly off 75' @ xxx degrees from N while the Google Earth is unnoticeably perfect, or

 

Both your handheld GPSr and the other maps are unnoticeably perfect while the Google Earth is exactly off 75' @ xxx degrees from N.

 

My guess is that two things are close to perfect and only one noticeably in error.

[+dakboy]

My guess is that the Google Earth (is not GE a registered trade mark of ..... so I'll spell it out? ) is wonky.

Growing up in a "we bring good things to life" GE family, having a lot of family friends who did/still do work there, and even working there for a while myself, seeing Google Earth abbreviated as GE always throws me off.

[eaparks]

I'm buy no means an expert on using Google Earth so maybe someone else can elaborate more on this issue. When I import an overlay into GE if I don't first put the cursor on the very bottom center part of the eye in the navigation bar, I have displayed in the top right of my screen, and click on it, the aerial view is not straight down and the view is slanted just slightly thus making my overlay so that it will not overlay perfectly either in N to S or E to W depending on how the aerial image in GE is slanted.

 

So I always make sure my GE sat. image is being viewed directly straight down before importing any overlays.

Edited October 27, 2009 by eaparks

[Grasscatcher]

I'm buy no means an expert on using Google Earth so maybe someone else can elaborate more on this issue. When I import an overlay into GE if I don't first put the cursor on the very bottom center part of the eye in the navigation bar, I have displayed in the top right of my screen, and click on it, the aerial view is not straight down and the view is slanted just slightly thus making my overlay so that it will not overlay perfectly either in N to S or E to W depending on how the aerial image in GE is slanted.

 

So I always make sure my GE sat. image is being viewed directly straight down before importing any overlays.

 

EAP,

Thanks for the suggestions.

Just for grins, I deleted everything and re-imported my trail file again. This time, I was sure aerial view was looking "straight down" per your instructions.....no changes from what I had before.

I'm beginning to think that the georeferencing in Google Earth imagery is strictly for the "Oh looky, there's my house" crowd.......

 

Waypoints saved while on the track are shown "on the track" and have correct coordinates, but if you pick the visible point (a trail intersection) on the "picture" and get the coordinates for that point, then those are incorrect.

[XC_Tracker]

I know the free version of Google Earth is very inaccurate. It's good for getting an aerial image or checking a route, but as far as using it for pin-point accuracy, it's not really a reliable program. The georeferencing isn't aligned for the entire image, maybe just the center and a few other points.

[+user13371]

... the free version of Google Earth is very inaccurate...

This is too broad a statement.

 

1) In some placed the imagery is referenced very accurately, and in other places it isn't. Where I live in Portland Oregon, it lines up almost perfectly -- so well that the city offers a KML layer that does a really good job of showing property lines.

 

2) No matter how good the referencing is, it's not going to work if you're not using the program correctly. Turn off the terrain layer, and make sure you've not tilting the image.

Edited October 28, 2009 by lee_rimar

[+Searching_ut]

I've found google earth tends to have accuracy issues in areas with significant elevation variation. You'll often notice it quite significantly at the seams where it blends different images together. Straight line things like roads, will suddenly shift quite noticably at the seam area. Another area where I found significant referencing issues was in Alaska. The problems might have been because of the far northern location, or it might have been an elevation issue as you encounter both in that area.

[+eflyguy]

I've found google earth tends to have accuracy issues in areas with significant elevation variation.

 

Now THAT makes a ton of sense, as most images are not taken vertically!

..a

[+Redwoods Mtn Biker]

To get greater accuracy in terms of image registration, CO/DK/OR owners can try generating .kmz files using a third-party tool such as G-Raster or TopoFusion.

[Grasscatcher]

Just personal opinion, but I think Searching UT may have described what the problem is, and Redwoods Mtn Biker may be "on" to the solution..........

 

Georeferencing smaller "chunks" may correct the visible errors, at least on smaller maps.

 

I know that while hiking and mapping the actual trails, trail conditions and common sense have (at least so far) kept me from taking any (single) minus 500 ft elevation steps. Maps can only tell you so much.......