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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

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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 4:47
  • @MichaelStimson, is there a link on the accuracy of those methods used by Google? – Graviton Jul 10 '18 at 5:17
  • 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 5:34
  • Possible duplicate – Fezter♦ Jul 13 '18 at 5:43
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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 5:52
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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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  • 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 8:21
  • 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 en.wikipedia.org/wiki/Lidar) – reducing activity Jul 13 '18 at 8:24
  • 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 8:38
  • 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 8:40
  • @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.

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