how to quickly create barriers for view shed?

kwyateskwyates Global Mapper UserPosts: 15Trusted User
edited April 2015 in Technical Support
I am attempting to design and place walls in undulating terrain that the Global Mapper view shed function will "honor" as barriers. The solution offered by Josie (below) works well in flat terrain. However, the area that I need to work in has undulating terrain. What is desired is some quick method to create a 3D vertical extrusion from terrain that follows along a line feature. For instance, I would like to create a wall that is 8 feet tall that follows a line that I have imported or digitized ad hoc. In this way, the height or location of barriers could be quickly tested with the view shed tool until achieving the desired result. My attempts at creating an Elevation Grid with height-above-ground returned
some very, very odd results.
Any ideas?
Ken Yates
"You can use the Digitizer toolbar to create a narrow area feature to represent your wall. Select the area feature with the Digitizer tool, right click on the map and choose EDIT. I n the Modify Feature Info dialog click on the Vertices button and then click the button to Update Elevations from the Terrain . You can then edit the elevation of each vertex individually to the elevations that you want to the height of the wall. Click the OK button. Alternatively, you can add an attribute called 'Height' or 'Elevation', which will update all of the vertices at once.
With the area feature selected, right click on the map and then choose Advanced Feature Creation Options > TERRAIN – Create/Flatten Terrain from Selected Area Feature(s). In the Elevation Grid Creation Options dialog, check the boxes next to Use 3D Line Features as Constraints and Flatten 3D Area Features. Be sure to uncheck the box next to Heights Relative to Ground. You will now have a terrain grid that will act as a solid "wall" in your analysis. "


  • kwyateskwyates Global Mapper User Posts: 15Trusted User
    edited October 2014
    here is the 3D view of one of the sites where wall placement is indicated
    Attachment not found.
  • PShoreyPShorey Global Mapper Developer Posts: 184Moderator, Trusted User
    edited October 2014
    Hey Ken,

    I've been working with the data you sent me. I'll share my findings here for the benefit of other users or possibly trigger some better ideas. To summarize, you are trying to create an elevation grid from two narrow annular area features, one over the other, to represent a wall. While we are able to create a sensible TIN from those features, the code that generates an elevation grid from that TIN does not do well. The scattered red squares you asked about are the resulting elevation grid - not good. I'm not sure if we can fix that in the near term, but I'll look into it.

    However, another approach occurred to me. I think you can create the wall surface by combing 2 larger areas, which the elevation grid generator does very well with. First, create an area that has the same shape as the outer edge of the wall. Give this an elevation of your wall height, say 5 feet. Generate an elevation grid for that area. Next, create a second area with the inner edge of the wall. Give that an elevation of the fence height as well, and generate an elevation grid. Combine the two by subtracting the second from the first. That will give you an annular surface approximating a wall. Finally, add that to your terrain surface.
  • kentmankentman Global Mapper User Posts: 149Trusted User
    edited October 2014
    I think the way to achieve this is to import or draw your lines, buffer them to create thin polygons for each, add a height attribute and specify this in the 'get elevation from' just like Josie mentioned but instead of creating a surface from this there is an option in the view-shed dialog to take into account vector features (bottom right in the dialogue box) - this works well as I use it for trees and buildings.
  • kwyateskwyates Global Mapper User Posts: 15Trusted User
    edited April 2015
    The goal of this exercise is to create GM visualization of proposed walls both Viewshed and Line-of-Sight.
    The GM Viewshed solution works very well with honoring walls as barriers defined as vector features with heights.
    The Line-of-Sight tool does not "honor" the 3d vector barriers. I have found the line-of-sight tool likes to display elevation grid data and does that very well. My difficulty lies in creating an elevation grid surface that represents proposed (narrow) walls of various heights and placements. After creating the 2d location, walls in 3d design or CAD are generally extruded UPward from terrain to a height. I have attempted to reproduce this in GM using various approaches with no joy. Recent experience in GM has shown that the TIN solution seems to correctly process point data representing the wall as two parallel rows of closely spaced 3d points. Without the ability to control "maximum edge length" of the TIN however, large triangles edges are spanning concave regions and "holes" in the point data. Regardless, the resulting Elevation Grid goes bonkers with this type of point data and displays large colored squares that are not very representative of the selected smaller grid sizes or the original points' locations. I have repeatedly attempted Peter's suggestion using several variants but success eludes me. Any suggestions would be welcomed.
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