tree canopy density

ChrisHChrisH Global Mapper UserPosts: 6
edited July 2014 in Technical Support
I'm attempting to run a view shed analysis of a building

The view shed runs fine, however where the building is located, it is heavily wooded. So we know for a fact that some of the areas the view shed says is "visible" is in fact not. Typically, there are no shapefiles for tree canopy in this region.

I began making my own generalized shapefile of tree canopy and setting an average elevation of the tree height at 40ft relative to ground. Works perfectly atop my DEMs of the region. However, about .0493 seconds into doing this I realized that I need to account for density and seasonal changes (ie no leaves in the winter).

I'm completely new to Global Mapper, so my knowledge is very limited with the program, and I don't know how to script. I was thinking about using the Density Grid, but again this area is very heavily wooded. I estimate there to be around 50,000 trees in this area covered by the viewshed, and I feel that would be far to time consuming and counter-productive clicking a point for each tree. I don't need the tree canopy/density to be 100% accurate but relatively close.

Would you suggest that's the best route? Any help would be appreciated. There is probably an easier solution that I'm just not seeing at the moment which is why I'm posting

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited July 2014
    For wooded areas you definitely want to create an area feature with a height relative to ground as you are doing. It is far too difficult to create a bunch of points for individual trees, although we do have tools for creating point features randomly distributed inside selected areas.

    I would suggest using the same coverage area, but with different heights for the different seasons. I think you can closely model the blockage of on-leaf and off-leaf conditions by using the full tree height for on-leaf, and subtracting maybe 25% from the height (just a guess, someone with more experience doing this operation might have a more scientific guess) for off-leaf conditions.

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • ChrisHChrisH Global Mapper User Posts: 6
    edited July 2014
    Didn't know about the randomly created point feature. That seems like the best course of action. Just shade in the wooded areas, create the points, remove the shaded in areas (leaving the points), and then run the view shed.

    I'll also try your suggestion of removing a certain percentage from the tree height for seasonal change

    Appreciate the help
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited July 2014
    Why do you want to leave the points rather than just using the shaded area? You can use areas with height as obstructions, so for a densely wooded area that seems to make sense more than random points that don't reflect actual tree locations. The tree canopy will also likely obstruct more than a single point location anyway so an area seems a better match to model the behavior.

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • ChrisHChrisH Global Mapper User Posts: 6
    edited July 2014
    I forgot to add in my previous post that I was going to add a buffer to each point to allow for the canopy coverage. I'm leaning back towards the shaded area though. I've been doing some test runs with the shaded areas I do have, and the view shed isn't reacting as well as I would have hoped

    Question: The points are at an elevation of 40ft relative to the ground, but don't extend down to the ground like my shapefile in the 3D view. Is the view shed treating them as points "in the sky" or like the shape file where you can see it extended to the ground?
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited July 2014
    The view shed will treat them as extending all the way to ground. If you want to view them that way in the 3D view, go to the Options dialog for the 3D view and check the box to extrude the areas.

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • BillBBillB Global Mapper User Posts: 119Trusted User
    edited July 2014
    Chris, apologies because I don't know anything about your project or its setting, but just a point of view from a landscape architect who has an interest in view sheds - I think you may be expecting too much of them.

    They are all models and have to be recognised as being theoretical. I would have thought it sensible to run one view shed bare-earth (no trees) and another with trees (as extruded shapes with a height) and state that the extent of visibility will be seasonally variable and is likely to range between bare-earth and full canopy. Trying to model tree canopies accurately - especially their density - could be misleading. The actual visual effects of seasonal change might be better demonstrated through techniques like photomontage.

    Regards
    Bill
  • ChrisHChrisH Global Mapper User Posts: 6
    edited July 2014
    Thanks for the insight and help global_mapper. I've finished the shapefile today (after working on other projects the past 2 days) and kept it as is with a 40ft relative to ground canopy without the "points as trees". To tie BillB into the convo, I realize the view shed wouldn't be 100% accurate and gives you a rough estimate. However I've physically been to the site numerous times, and the results I was getting were grossly "over viewing", if that make sense. The reasoning behind this post is that I'm new to Global Mapper and wanted to know if there was an easier way to do what I needed to do without going through and making a shapefile of my own

    After finishing the shapefile, I still wasn't happy, so I made a buffer around the building and added 50 trees randomly throughout and that seemed to have brought the view shed down to an acceptable level
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