I have often found that terrain mode of Google Earth is pretty decent for approximating elevations and terrain contours.?ÿ However, yesterday I noticed that in my home town of Alexandria, LA, the new Google Earth terrain elevations are 20 feet too high.?ÿ I wonder what they are basing their datum on and wonder if it is a bad governmental DEM.?ÿ If so, this could affect planning and possibly even FAA aeronautical navigation charts, especially those on digital displays like the Garmin G1000 in aircraft.?ÿ It is easy to look at a USGS Quad at Alexandria, LA and compare the elevs to what Google Earth shows with terrain turned on.?ÿ Or maybe I am missing something stupid.
Is some of that subsidence after the USGS maps were made?
Was it a data entry error? Some years ago there was a 1-mile or so rectangle in an area I was interested in that GE 3-D view floated about 100 ft above surrounding terrain, looking like a magic carpet. That has been corrected now.
Frank,
The DEMs are available from the USGS.?ÿ last time I did it, it was kind of kludgy.?ÿ I can not remember the exact steps to get the data but you can definitely google it.
No subsidence here (0.02' over last 10 yrs.).?ÿ The levels Google Earth terrain shows are 20 feet higher.?ÿ ?ÿI use the USGS DEMs all the time, and find them pretty good, but the concern might be if they update them using same database Google Earth uses they could inject some bad data into the system.
Some time ago I ran tests comparing Google Earth elevations with real world topos in country settings with no trees. Google Earth was awful. I've also checked it to some control points around the city and it seems to be close in nice flat areas. Otherwise I would use with great caution. One example is a stream bed we were doing cross-sections for. If you tilted the Google image the stream appeared to be running along the side of the valley instead of along the bottom. I'm assuming that was the Google DEM data shifted from the real world. We just did a BFE for a house in a Zone A. Even the house's basement floor was a few feet above the BFE but if you use the Google elevation for the LAG, it would have been under the BFE instead of 7 feet above.?ÿ
They may be using SRTM (Space Shuttle Radar Topography) data, which is all there is in some areas of the world. Definitely not the accuracy we are accustomed to in the US.?ÿ
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https://www2.jpl.nasa.gov/srtm/index.html
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Frank,
You can try a USGS api that can return the ground elevation of most areas of the US.?ÿ Might be curious to see if it compares.
https://nationalmap.gov/epqs/pqs.php?x=-97.582889163&y=30.396694116&units=Meters&output=json
Just enter the above edited with your Long/Lat position in decimal degrees in your browser and hit enter. It should return a json txt string with the ground elevation.
Thanks,
Terry
Well....you know that Google is always right and all of your old surveys are now wrong by 20 feet.
You do realize that you will probably get a call like that dontcha!
We've only ever used Google Earth elevations as a general guideline when doing preliminary SWM modeling.?ÿ We usually reference the SWOOP ( https://www.arcgis.com/home/search.html?t=content&q=tags%3ASouthwestern%20Ontario&start=1&num=20 ) data when we want to tighten it up a bit.?ÿ But, as always, GIS just stands for "Get It Surveyed" when it comes to the brass tacks.
There is something messed up about google earth now, at least in this area. Previously I'd sometimes look as a rough gross check on things. Now it's way off. If you hit the "history" button and slightly toggle back to the past things change and become closer again to reality.?ÿ
I cannot figure out how to rotate the view anymore to sorta see the terrain relief. I know about the terrain exaggeration stuff but I used to be able to pan it so I could see the hills as I look at in an oblique sorta way and I can't seem to do it anymore.?ÿ
It works for me with Google Earth Pro version 7.3.2.5776 (32-bit) build Mar 5, 2019 on Windows 7.
In the navigation stuff that overlays the photos I click the ^ button just under the N and it tilts the view. If I grab the N symbol and drag it around the circle it rotates the view. Both motions can also be done with Ctrl and the arrow keys. If I click and hold the mouse button down on the eye symbol, I can drag the view up, down, and around.
I feel like a moron, I was using the wrong arrow.
That’s what blogs are for. Like forgetting a negative sign for geoid sep. and blog to the rescue. no such thing as a dumb question.
