[HN Gopher] Lidar reveals hundreds of long-lost Maya and Olmec c...
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       Lidar reveals hundreds of long-lost Maya and Olmec ceremonial
       centers
        
       Author : samizdis
       Score  : 58 points
       Date   : 2021-10-30 12:23 UTC (10 hours ago)
        
 (HTM) web link (arstechnica.com)
 (TXT) w3m dump (arstechnica.com)
        
       | 1024core wrote:
       | I'm more curious about the technology behind this survey. How do
       | they do it? What's the range? How high can you be and get a
       | decent result?
        
         | randomluck040 wrote:
         | Your questions depend on a lot of factors. Modern lidar systems
         | usually work with a "return" system (no official name). Imagine
         | you shoot with two lasers at the wall in front of you. One
         | laser travels to the wall and back to you. Another laser has to
         | pass a membrane (or a piece of paper - or leaves of a tree!) so
         | you get two signals back: one deflected by the wall right
         | behind the membrane and one of the membrane itself. Creating
         | the difference you get an overview of what the treetops look
         | like as well as what the ground underneath looks like. You can
         | have an arbitrary amount of return values, more make things
         | more complicated though.
         | 
         | The range depends on wavelength and wattage. In theory you can
         | go up to a few thousand metres, usually you work around
         | 200-1000 m when going airborne. Terrestrial you usually go for
         | less than a hundred meters to have a high point density as
         | well. This also kind of answers the last question: if you go
         | higher, your point density will be lower. In some cases it
         | might make sense, especially if you're looking at mainly flat
         | surfaces (still, depends on the use case) but in rough terrain
         | you want to digitise you might want to chose a higher point
         | density to replicate real world features more precisely.
        
         | twelvechairs wrote:
         | Lidar effectively turns to tradeoffs at different plane
         | heights. You may also be interested in Synthetic Apature Radar
         | which is a satellite technology that is very powerful but takes
         | some knowledge to test the right things and understand the
         | outputs
        
         | [deleted]
        
         | mainpeanut wrote:
         | Probably from an four seater airplane about 1 km to 1.5 km
         | above the ground
        
       | vesche wrote:
       | "The Lost City of the Monkey God" is an absolutely fantastic read
       | if you're interested in the usage of LiDAR for archeological
       | discovery.
        
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       (page generated 2021-10-30 23:01 UTC)