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About the Google Maps elevation API, which point of the elevation level is referring to? Whether to the actual ground level, or the elevation level that one can detect from the aerial (eg: at rainforest it is the height of the canopy tree)?

The document says it is the "surface" of the earth. But I am unsure whether it is really possible to know what is the surface of the earth if the land is covered with thick forest

Graviton
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  • I can assure you that gravity methods, LiDAR and RADAR all 'see' the ground through the trees, only photographic methods have difficulty. I can't see where they are sourcing the data from, it could be SRTM (the difference in tree height and ground is less than the stated accuracy so the point is moot) with a patchwork of better data where available (kind of like google earth imagery); perhaps someone knows where the metadata statement for this API is and how to discern the accuracy statement for the requested point. – Michael Stimson Jul 10 '18 at 04:47
  • @MichaelStimson, is there a link on the accuracy of those methods used by Google? – Graviton Jul 10 '18 at 05:17
  • I don't know. Perhaps someone who uses the API or has read more deeply into the usage notes will know. The data is likely to be a mixture of inaccuracies being comprised of a patchwork of sources. – Michael Stimson Jul 10 '18 at 05:34
  • Possible duplicate – Fezter Jul 13 '18 at 05:43
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    @Fezter I am not 100% sure, I would consider this question as asking about accuracy of elevation data. – reducing activity Jul 13 '18 at 05:52
  • @MateuszKonieczny, you're right, which is why I didn't close it. However, there is likely useful information in that post as it indicates that the data comes from a variety of sources and therefore various accuracies. Perhaps I should have said "possibly relevant". – Fezter Jul 13 '18 at 05:55
  • I've added an SFO airport diagram, and a screenshot showing google earth pro elevation at the Edge building on Genentech campus.SFO Airport Diagram Elevations are taken from apparent waterline.[![shoreline elevation by Edge building\]\[1\]](https://i.stack.imgur.com/wbkYw.jpg) – JACK MORRIS Jul 18 '22 at 22:24

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As documented on https://developers.google.com/maps/documentation/elevation/start

The Elevation API provides elevation data for all locations on the surface of the earth, including depth locations on the ocean floor (which return negative values).

https://developers.google.com/maps/documentation/elevation/intro clarifies that

Elevation values are expressed relative to local mean sea level (LMSL).

As with other parts of Google Maps data, this data comes from multiple sources with quality varying across globe.

Different sources may be affected by different types of errors. For example LIDAR will be extremely accurate (small paths in forest are visible in LIDAR elevation images), SRTM based data may have significant errors.

Research by Wang Y, Zou Y, Henrickson K, Wang Y, Tang J, Park B-J analysed some aspects of elevation accuracy. They also confirmed that "Google has been unwilling to release detailed information regarding the accuracy of the archive"

The same research notes that in some rare cases errors may be significant:

enter image description here

though overall

The precision of GE elevation data along roadways is satisfactory, and there is no evidence showing the accuracy of GE roadway elevation varies significantly between states or route types

note that this particular study limited analysis to road-related elevation data

But I am unsure whether it is really possible to know what is the surface of the earth if the land is covered with thick forest

For example LIDAR is capable of doing that.

A forest - aerial image (Orthophotomap from Polish government)

enter image description here

LIDAR image of ground of the same area, visible lines are forest paths. In this case LIDAR was configured to penetrate tree cover and to capture ground elevation data. (ISOK cień dataset by Polish government)

enter image description here

Though, one may configure LIDAR to collect treetop data.

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reducing activity
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  • It seems that your image doesn't directly answer the question "But I am unsure whether it is really possible to know what is the surface of the earth if the land is covered with thick forest" – Graviton Jul 13 '18 at 08:21
  • I thought that illustrates well that LIDAR can see through tree cover. Do you think that explicit "Air borne lidar digital elevation models can see through the canopy of forest cover" would be useful? (from Chiu, Cheng-Lung; Fei, Li-Yuan; Liu, Jin-King; Wu, Ming-Chee. "National Airborne Lidar Mapping and Examples for applications in deep seated landslides in Taiwan". Geoscience and Remote Sensing Symposium (IGARSS), 2015 IEEE International. ISSN 2153-7003. via https://en.wikipedia.org/wiki/Lidar) – reducing activity Jul 13 '18 at 08:24
  • I'm not too sure what are you saying here. There are plenty of tree cover that can be seen, so can LiDAR penetrate those or not? – Graviton Jul 13 '18 at 08:38
  • First image is a colorful aerial photo of a forest area (with trees blocking everything), second image is a greyscale LIDAR image of a forest area (with ground shape visible, as LIDAR penetrated through tree cover). – reducing activity Jul 13 '18 at 08:40
  • @Graviton I tweaked the answer, image description should be now better. – reducing activity Jul 13 '18 at 11:14
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As mentioned by @Mateusz Konieczny, an important thing about Google Earth is that is uses different sources of different quality. It tries to provide ground elevation above the local mean sea level everywhere.

As far as I know, for global DEM data, most information comes from SRTM. SRTM is C-band RADAR. With this wavelength, the signal can penetrate through open or low canopy, but it is not accurate in dense forests. According to this study, there is a bias of approximately 6 m in densely vegetated areas (that is, SRTM is 6 m above the soil on average).

Alternatives to the SRTM include :

  • airborne LIDAR: with high density LIDAR you can see through the canopy. Several archaeological discoveries have been done thanks to this ability. Unfortunately, LIDAR data are quite expensive and only a few countries in the world have a full and open coverage.

  • digitalized topographic maps based on ground survey information (not always easy to tracck the primary source to evaluate the quality of those products)

  • spaceborne LIDAR (only one satellite as far as I know, with 1 km spacing)

  • photogrammetry (in countries with deciduous forest, you can see the ground in winter)

On top of that, the heigth of those DEM is sometimes based on different referentials (the mean sea level is not the same everywhere in the World), so it is tricky to combine many sources.

radouxju
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I was looking for answers as to how accurate the Mean Sea Level reading is in the San Francisco Bay Area using Google Earth Pro (the numbers are similar, just in meters, on Google Earth), since I was calculating building and ground elevations for a drone shoot at Genentech in SFO airspace for FAA authorization. What I found was that in Google Earth Pro, the reading at the zone between the high water and low water marks on shores surrounding the location, what I would think was MSL, was around -10ft. That raised my curiosity; was gravity and the spinning of the earth making MSL 10ft less in our zone? Or is Google Earth Pro just off? My son reminded me that the earth is around 21 miles fatter at the equator - because of spin and gravity - and it's getting fatter as the glaciers and ice caps melt. Could this be because of MSL being higher at the equator? Not sure, but for purposes of my survey and to get this project done I am making what I think are appropriate adjustments to the Google Earth readings.

To see what I'm talking about, If you go to the Genentech building called "The Edge" at 37deg,39'28.52"N 122deg22'39.89", just North of that is the San Francisco Bay Trail which reads at between -1 and 0 ft. elevation, which it is not as I've stood there and I'd say it was at least 6-8ft above the high water mark. Then if you go to what should be the mean water level at the shore a few feet away (I looked for signs of high and low water marks to find the median), the reading is -10 to -11ft. Looking at the N.E. end of SFO runway 19L or R, Google Earth Pro has similar readings at the waterline as -11ft and the runway surface at anywhere from -3 to -1ft. SFO is NOT underwater... yet. The SFO airport plan says the runway has an elevation of 11ft. So at least here in the Bay Area, I am adding 10-11ft to my Genentech elevations based on the airport diagram, because I trust that is more accurate since airplanes have been successfully landing here for a while (with the notable exception of an Asiana Airlines failure of a few years ago-which came in too low; maybe they looked at Google Earth Pro before their flight plan???).

Regarding elevations of trees: At least around the Genentech property,the elevations appear to follow the contours of the bushes and trees, and definitely the buildings and structures. Any conclusive info or deeper insight will be appreciated and helpful for my planning, but for now I'm making what I think are good adjustments to the elevations.

Vince
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Jack Morris
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