|What is the precision of data (distance and elevation) in Google Earth.||yvandas||1/10/10 11:53 AM|
Distance measurements appear very accurate but elevation measurements appear to cary a huge error factor e.g. measuring the elevation of a 3 km long runway gives me 110 m at one end, then it gradually raises to 115 m in the center, then it gradually drops down to 104 m at the other end. I know that the elevation is based on the average terain elevation but the area used to compute the average elevation must then be very large in order to generate a 9 m error in the above example given that the closest building is 400 m away from the center line of the runway. Does anyone know: (a) the precision we can apply to distance measurements; (b) the area used in computing elevation data; and (c) the precision we can apply to elevation measurements.
|Re: What is the precision of data (distance and elevation) in Google Earth.||barryhunter||1/10/10 3:18 PM|
I beleive for many areas Google Earth uses SRTM data, which offers something like 30m vertical accuracy, at 90m horizontal grid spacing.
Other areas user high-resolution data from a verity of sources.
|Re: What is the precision of data (distance and elevation) in Google Earth.||yvandas||1/11/10 9:19 AM|
For the benefit of other readers, there is an excellent document on precision of SRTM data at the following link: http://dds.cr.usgs.gov/srtm/version2_1/Documentation/MIL-PDF-89020B.pdf. You are right in saying that we should look at elevation accuracy in Google Earth in the range of +/- 30 m. One interesting thing I noticed however is that Google Earth appears to apply an average of many sampled points in order to smooth the data in the horizontal plane (make the terrain look more flat).
|Re: What is the precision of data (distance and elevation) in Google Earth.||kumpanart||7/26/10 7:39 PM|
I found a big difference between distance measured in google earth and the UTM calculation. it might came from surface distance in UTM calculation (include earth curvature) and the direct line in google earth (chord under earth curvature). Does anyone know/confirm this assumption?
|Re: What is the precision of data (distance and elevation) in Google Earth.||kumpanart||7/26/10 9:29 PM|
It is a different zone. If limit to single zone, the difference is acceptable.
|Re: What is the precision of data (distance and elevation) in Google Earth.||weschrist||8/29/10 9:30 AM|
I am doing a research project in mountainous terrain near a highway. CalTrans has surveyed in ~100 reference stakes for road reconstruction, with elevation reported to 0.01 feet, or ~3mm.
The elevation I determined from total station surveys using the CalTrans stakes as reference points, as well as the elevation reported on the CalTrans stakes, differ from the elevation in GOOGLE EARTH by ~5m, or 16 feet.
|Re: What is the precision of data (distance and elevation) in Google Earth.||yvandas||8/29/10 2:34 PM|
Thank you for the additional information. This data is in line with the error margin I had noted while looking at a runway in the Ottawa area. However, I believe that we cannot conclude that 5m is the typical error margin when looking at Google Earth elevation data. I looked at a Colorado desert area (known to be very flat and without buildings) and the error margin there would be less than 1m. I also looked at the Cape Canaveral launch area which is located in a very flat terrain area. There again, the error is in the order of 1m and appears to be mainly contributed by the two tall buildings on the launch site. Same type of conclusion when looking at the runway cose to the launch pad. All that to say that the accuracy of the elevation data is very much a function of building structures and terrain irregularities surronding the area you are looking at i.e. if the surronding terrain is "smooth"; then the elevation accuracy could be in the order of 1m but could also be more than 5m if the surronding terrain is very irregular. The Google Earth elevation data appears to be an average of elevation data for an area centered over the point you are looking at. Unfortunately, I do not know the size of the area used to compute the average. If someone knows the answer to that question, I would very much like to know.
|Re: What is the precision of data (distance and elevation) in Google Earth.||CBXX001||8/30/10 2:17 PM|
I'm trying to calibrate my cycle computer by comparing it with google earth measurements of roads near my house in Florence, Italy. But then I ask, how accurate is google earth anyway?
So I go to the athletics track at Crystal Palace, London (lat 51.418694 lon -0.067387) and the distance from the 100m start line to the finish line appears to be around 0.25% below 100m.
Then I go to the athletics track in Barcelona's Olympic Stadium (lat 41.364774 lon 2.156008) and I can't see the track. But measuring the 18 yard line on the football pitch I find a 3.1% overestimation. Not sure that football pitches are marked out so precisely though!
Beijing's bird's nest stadium draw's a blank - the modern new fangled things don't let you see in. So much for progress.
So I go to Sweden's olympic stadium (lat 59.345655 lon 18.078848) and I get just about dead on 100m.
So I go to city of manchester stadium (lat 53.484000 lon -2.203923) and I get about 0.17% under 100m.
Finally to Don Valley in Sheffield (lat 53.395896 lon -1.425012) and again get about 0.17% under.
Seems like the underestimate increases proportionally with latitude, plotting these figures in excel! Thus in Florence I can add 0.47% to my cycle computer numbers and go faster and further!
But then I find an athletics track in Florence (43.779199, 11.280393), It gives me just 0.04% under over 100m. So I'm busted!
Back to the day job...
|Re: What is the precision of data (distance and elevation) in Google Earth.||yvandas||8/30/10 5:39 PM|
I did a lot of path measurements using Google Earth. The distances are generally very accurate but you must remember that Google Earth does not factor in differences in elevation while calculating distances between two points i.e. you could be climbing 1000 m then down 1000 m to reach a point 3000 m away in direct line and Google Earth will give you 3000 m between the two points, not something like the 3600 m you will actually be walking.
|Re: What is the precision of data (distance and elevation) in Google Earth.||jetsamoocow||3/25/11 6:53 AM|
I also am wanting to know over what area the altitude of terrain is presented. Barryhunter implied that spot measurements were by laser (with negligible horizontal averaging) - but that measurements were made on a horizontal grid of 90 m. I guess he's right, but I'd like to know the source. For satellite meteorology radiometers and weather radars have a beams of appreciable width, with a bell shaped pattern of antenna gain across the field of view - greatest in the middle and tapering off to zero a little way off centre. I suppose that this is how terrain altitude is smoothed in Google Earth.
Go to cliffs you know well and observe that in GE you get a steep descent rather than a cliff. Thus I reckon that the effective beamwidth of the GE altimeter translates to something like 30 m at the ground.
|Re: What is the precision of data (distance and elevation) in Google Earth.||yvandas||3/25/11 11:01 AM|
Thank you for the information. However, it looks like the beamwidth at the ground is much more than 30 m. If it was 30 m, then looking at a surface larger than 30 m in diameter should give us a fairly accurate reading of the elevation at the center point but this does not appear to be the case. For instance, the main terminal building at the Mirabel airport ( 45°40'59.46"N, 74° 1'43.81"W) has a flat roof 90 m X 350 m and is at least 20 m above ground level. Yet, the elevation at the center of the roof is 74 m, the same as that of the surronding ground. If someone has an explanation for this I would like to know.
|Re: What is the precision of data (distance and elevation) in Google Earth.||barryhunter||3/25/11 11:15 AM|
Have you read up on the SRTM data, eg:
Note that Google doesnt use exclusivly SRTM data, at least in places the augment it with higher-resolution datasets. I dont know the exact sources, and areas covered.
|Re: What is the precision of data (distance and elevation) in Google Earth.||yvandas||3/25/11 1:38 PM|
Thanks barryhunter for the link. If I read the info correctly, SRTM elevation data in Montreal would be accurate to +/- 90 m which looks like a lot. Actual ground elevation data for the Mirabel runways (known to be at 82 m MSL - see following link: http://www.ourairports.com/airports/CYMX/pilot-info.html) appears to be within +/- 12 m of what GE gives us. What is really strange is that bridges and buildings appear to be ignored by GE (as if everything is laid flat on the ground) whereas top of mountains elevation data appears to be very close to reality. For example, Pic Champlain ( 48°19'56.86"N, 68°49'18.64"W) elevation data provided by GE is very close to elevation countour lines you would find on high accuracy topographic maps. Could it be that GE "flattens" all man-made structures?
|Re: What is the precision of data (distance and elevation) in Google Earth.||jetsamoocow||3/25/11 2:13 PM|
Hi Barry and Yvandas! Great to get your posts. Shuttle Radar Topography Mission - I was wondering! Isn't Mirabel a fairly new Montreal airport? The answer might be that main terminal wasn't built at the time of measurements (smile). If that explanation won't do, then the "beamwidth" might be bigger - or artificially enlarged for security reasons. ...
I'm struggling to place photos for Panoramio/GE taken among mountains of Yunnan, SW China. When I type in 28.0618 N, 99.8763 E, or some such place GE whizzes to the spot and hovers over it looking down at maybe eye altitude 5.6 km. I then read the altitude (4224 m). Descent to ground level changes the altitude only very little; I expected it to get refined as I approached. Investigation here is consistent with a represented "beamwidth" of ~30 m. I am today having trouble with photos taken in Gezancun - Giza village; high topography along the valley eastwards from Geza isn't comparing very well with my photos! Authoritative explanations of "beamwidths" and fields of view would be good.
|Re: What is the precision of data (distance and elevation) in Google Earth.||yvandas||3/25/11 7:05 PM|
FYI, Mirabel was in operation long before GE got on-line.
While we are trying to find answers on our own, wouldn't it be nice to have this info readily available to the GE users community within the GE site environment? Just a tought.
|Re: What is the precision of data (distance and elevation) in Google Earth.||listiyofitri||4/25/11 6:27 AM|
thank you for your discussion, I could learn more about Google Earth Elevation, and fortuitously i need terrain elevation and would to use GE elevation. But if GE using SRTM (Shuttle Radar Topographic Mapping) is not a terrain elevation but surface elevation where the point elevation is not in the terrain (ground) but maybe in the vegetation, or building, or other structure (natural or non natural structure). And that's give explanation if the accuracy of GE elevation is not same for other places, especially in flat area or in hilly area.
I think, GE elevation is Ok if it is just use for management, but not in engineering purpose.
|Re: What is the precision of data (distance and elevation) in Google Earth.||snick.k||8/5/11 10:28 PM|
I believe the individual points are fairly accurate in most cases where the terrain is fairly flat. In mountainous terrain I think the elevation data is less accurate because the point you are evaluating is not directly on a measured sample point and GE is interpolating between measured points to estimate the elevation of the point you are looking for. If you are measuring a road that is in a canyon with steep walls, you can have some variations from point to point.
This becomes apparent when you look at the calculated summary statistics (which are garbage) for a profile of a mountain road. I have seen instances where the summary statistics where GE claimed roads had maximum gradients on the order of 45% and total elevation gains on the order of double what you would get from a hand calculation (large accumulated elevation errors).
|Re: What is the precision of data (distance and elevation) in Google Earth.||tomthehaggis||7/20/12 1:37 AM|
Thank you for your useful post. I just wanted to try to confirm for myself what you state in this post.
Assuming you walked up a 45 degree slope, from Pythagoras that slope (the hypotenuse) would 1.4 times the length of the horizontal, would the measurement in GE be the horizontal distance or the hypotenuse?