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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
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:
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)
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)
Though, one may configure LIDAR to collect treetop data.
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)
Jul 13 '18 at 8:24
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.
