MiTS 2 has a function called Google Earth Elevation Import, that allows you to read the elevation points directly from Google Earth, without relying on Surveyor’s input.
Naturally, the question is, how accurate is Google Earth Elevation import, and given the range of validity, what can we use it for?
Before we tackle this issue, we need to understand what are the different ways one can obtain elevation levels.
A. SRTM Data
SRTM stands for Shuttle Radar Topography Mission. In 2009, SRTM consisted of a specially modified radar system that flew on board the Space Shuttle Endeavour during the 11-day STS-99 mission. Two radar antennas were outfitted in order to capture the topographic data. The elevation datasets are affected by mountain and desert no-data areas, thus the results could be inaccurate if the voids are large or the voids are completely covering summit or ridge area. However, Google Earth improved the results by using (in house) interpolation algorithms to mix and mesh SRTM data with other data in order to derive accurate results for these area.
The accuracy of SRTM Data can range from 5 to 10 meter, depending on where you are. In Malaysia, the vertical accuracy ( both absolute and relative) stands at between 6-9 m, as shown below:
There is also a study on the accuracy of different global topographic data in Borneo, readers can check out the report here.
It should be noted that when it comes to SRTM, forest covers may not matter so much as compared to the method accuracy.
SRTM utilizes satellite data, where as Light Detection and Ranging (LiDAR) is an optical remote sensing technology used to collect a wide range of topographic data.
The vertical accuracy of LiDAR stands in between 5 and 25cm. So it’s a much better improvement over SRTM data.
It should be noted that LiDAR has no problem seeing through treetops. Compare the two images below, with LiDAR configured to penetrate tree cover and another to capture ground elevation data.
C: Surveyor data
Without a doubt, surveyor data is the MOST ACCURATE of all, simply because the surveyors are there on the ground ( pun intended) to take the reading. The vertical accuracy can be in mm, depending on where you put the machinery.
It’s the first choice of all.
So, where does Google Elevation stand?
We don’t quite know.
All we know is that Google uses multiple data sources, ranging from LiDAR to SRTM to some other unknown sources. We do know that Google doesn’t use surveyor data.
But it should be noted that Google starts to use LiDAR since some 8 years ago, at some places, which gives it a vertical accuracy of 5~25cm error. Unfortunately Google doesn’t want to disclose at where. 8 years have passed, so now the data accuracy can only improved.
For the places that Google doesn’t use LiDAR, SRTM ( or the interpolations) steps in. The accuracy would be significantly less.
Then, when to use/ not use the Google Elevation Import function?
- It is recommended that you don’t use Google Elevation Import if you have the surveyor data.
- For neighbouring sites that don’t have the surveyor data, use your judgement whether you want to use Google Elevation data, bear in mind that you may or may not have LiDAR ( we don’t know, Google doesn’t disclose) level vertical accuracy.
- However, if you just want to do a preliminary study of the site, in order to propose preliminary roads and drainage layouts, and you don’t have surveyor data ( because it is still in a very early stage), and all you need is just relative contour lines ( not absolute contour values) and topological features, then Google Elevation Import maybe a good choice because Topographic maps are already sufficient for this use.
In other words Google Elevation import is good for qualitative study such as hydrological and geological modelling, planning and construction, land use planning, global change research, telecommunication and natural resource management. But for quantitative studies, consultants should engage other more accurate methods.
Know of any elevation level data source that improves over Google Elevation? Let us know in the comments!