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We would like to collect a generic purpose topographic LiDAR dataset over a large region in Belgium. Our minimum requirement is to achieve at least 1 point/m² on the ground. Ideally, we would like to have images from spring or summer because it would then be possible to get some extra information about shrubs AND more chances to have good flight conditions, but this is not our priority (the density of soil return must be sufficient). Alternatively, we could work during winter in order to have less vegetation cover in crop fields and deciduous forest, but then we would have less information on understory shrubs. This decision is difficult to take without knowing how much "ground return" we would lose when the vegetation cover is at its maximum.

Therefore, I am wondering if there are some shared experience about the proportion of ground returns under deciduous forest with or without leaves. I guess that this proportion is also depends on the LiDAR footprint, but I could not find any comparative studies.

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    This should be widely discussed in the LiDAR literature and I doubt a good solution can be proposed here. – Spacedman May 18 '18 at 8:29
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    @spacedman I've looked in the litterature, but it is hard to find this kind of information because scientific studies usually focus on very specific conditions with the "best available" sensors. In my case I need a pragmatic solution to cover a large area with a limited budget. – radouxju May 18 '18 at 10:20
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    Your last paragraph is asking a lot, and most of the answers are going to depend on your specific circumstances. There's no way you can get a complete and correct answer in a text box on this site. Sure you might get some links and hints and tips, but this is not what this site is for. "We prefer questions that can be answered, not just discussed." – Spacedman May 18 '18 at 10:29
  • I see your point. I've edited my question to be more specific. – radouxju May 18 '18 at 11:25
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How much do leaves on/off affect the number of LiDAR ground returns?

It does affect significantly. Take a look at Wasser et al. (2013)'s figure 4 (adapted) and see how the frequency of returns are much more concentrated in the upper strata of canopy in leaf-on than leaf-off:

Adapted from Wesser et al. (2013), figure 4

During leaf-off season there will be more LiDAR returns hitting the ground due to less vegetation obstructing them (how much more will depend on many things such as the type of forest, density of trees, among others). Curiously, in a multi-layered forest there will also be new (not necessarily more) information about the understory due to the same reason. Generally, this means that in leaf-on season a higher sampling intensity will be needed to capture at least the same type of ground and understory information.

Then, it will resume to the project's budget*. If it can afford a satisfactory sampling intensity in leaf-on, fly it in leaf-on (vegetation data can be important for other projects/purposes as well). If it is not, then fly it in leaf-off season with the highest point density the budget allows to.

*I don't know how the weather works in leaf-on and leaf-off seasons in Belgium, but make sure there is no accumulated water or ice in the ground, so the laser is reflected instead of absorbed .

Side subject, perhaps you will find interesting taking a look at What to consider when hiring aerial LiDAR survey?.


Reference:

Wasser, L., Day, R., Chasmer, L., & Taylor, A. (2013). Influence of Vegetation Structure on Lidar-derived Canopy Height and Fractional Cover in Forested Riparian Buffers During Leaf-Off and Leaf-On Conditions. PLOS ONE, 8(1), 1–14. doi:10.1371/journal.pone.0054776

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