In QGIS, I have a number of LIDAR scans of the same geographic location over a number of years. The actual values in the .asc files differ markedly (see image). Do the different value ranges for the different scans mean anything? As some are the same ranges, some are not. As its the same geographic location I would have expected the value ranges to be identical.

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closed as unclear what you're asking by Andre Silva, LaughU, whyzar, Vince, Jochen Schwarze Nov 12 '18 at 16:18

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    Are these really DSMs? How do you define a DSM? What those values represent? Elevation? In what unit of measurement? Something indeed looks strange, but we need much more details. Post “lasinfo” (header and vlr info) results from the point cloud. – Andre Silva Nov 12 '18 at 11:42
  • I acquired them from Digimap digimap.edina.ac.uk/lidar - I cannot use lasinfo as the tool always seems to crash. They are ,asc files. – IanGS Nov 12 '18 at 13:41
  • Please ask only one question per Question. – Vince Nov 12 '18 at 14:50
  • Unless you have a subscription to Digimap, you cannot access their files. The files are 'pucker' as they are produced by the UK Environment Agency. I don't know what else to say. The core question is "why does the values on the greyscales differ?" – IanGS Nov 14 '18 at 17:04
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    That is why I asked if you could show the full filenames. Could have some hints. The answer to your core question could be “it depends”. It could be they aren’t exaclty from same location (same extent); it could be surveys accomplished in different seasons (leaf-on/off) assuming elevation is in milimeters which also you did not say to us; it could be different methods to produce DSMs, for example using different set of returns and interpolation methods; the DSM resolution (a small one) could produce pixels where DSM equal DEM (could explain the negative values); among other possibilities. – Andre Silva Nov 14 '18 at 20:10

I don't know your dataset, but LIDAR DSM are made of point observation in a given field of view, which are resampled in a grid in your case. A few cm deviation between two date could make a small high feature visible (or not). A LIDAR also has several returns, so maybe your DSM correspond to the first, second and third returns, hence another difference in height.

  • Its actually the same grid recorded at different years over the past 6 years. Do those values actually mean anything? Or is it simply a numeral and its all about the range/relativity of those values in any given scan? – IanGS Nov 12 '18 at 16:52

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