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I am calculating various gridmetrics using the Lidar processing tools in FUSION. The dataset I am working with contains 154 flight lines of high density data, at a nominal pulse spacing (NPS) of 1.5 meters (full metadata available here). Specifically, I am using the grid metrics command in FUSION to calculate all returns above 2m (column 53). The attached figure shows the problem I have been encountering. You can see there are detectable grid values at the scan line overlap areas. There are even detectable values in the non-overlap scan line areas over lakes (observed as white blob in screenshot).

How should I be interpreting these apparently faulty results--is it a result of the Lidar dataset, or is it some type of processing issue with FUSION?

enter image description here

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    It looks like levelling hasn't been done, or at least not done well. Can you elaborate on returns above 2m, is that 2m from surface (lowest) or 2m above sea level? – Michael Stimson Jul 22 '14 at 21:38
  • Returns above 2m refers to 2m above surface. – Aaron Jul 22 '14 at 21:40
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    Is there something wrong with your surface at the ephemera of each strip? As the laser hits the extents ground points become less frequent in vegetated areas and more interpolation needs to be employed. How does it look with 1m above surface? – Michael Stimson Jul 22 '14 at 21:43
  • Many of the FUSION grid metrics based raster data look great (e.g. cubic mean elevation, 1st return cover above mean), although many look like the screenshot (e.g. all count above mode, all count, 1st count above mode...). – Aaron Jul 22 '14 at 21:50
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    Rotating mirrors tend to give better results at the edges as there is some aberration to do with oscillating mirrors accelerating and decelerating. It is a kindness to class the points captured with an oscillating mirror overlapping the adjacent strip as overlap points (class 12 ASPRS) and remove those from further processing, also don't forget High/Low noise classes - assuming of course your data is properly classified. – Michael Stimson Jul 22 '14 at 22:17
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The good folks at the Remote Sensing Applications Center (RSAC) noticed from the metadata that the Lidar data specs are insufficient to calculate many of these grid metrics. In particular:

  • NPS is 1.0 – 1.5 pulses/sq m
  • Side Lap (Minimum): 25%
  • Field of View (full): 40 degrees

These three parameters, especially when combined, will likely result in data gaps on the edge of flight lines. RSAC recommend the following specs when using FUSION grid metrics:

  • NPS (4-8 pulses/sq m),
  • A minimum 50% side lap,
  • A narrower FOV (30 degrees or less).

However, the data gaps may not be a problem depending upon how the data are used.

  • @Andre Good points all around--I appreciate your perspective. The AOI is primarily forested and the soil type is very rocky with a heavy clay component. – Aaron Jul 25 '14 at 22:28

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