Global Mapper v24.0

Complex terrain grading issue - approach

Treebeard
Treebeard Global Mapper UserTrusted User

I need to reduce a terrain area using a sloping polygon that has an irregular shape. I need the irregular polygon to follow a slope shown in the profile. I am really struggling with a workflow to approach this. Help would be appreciated!


Answers

  • HI Treebeard.

    Currently I don't think you can do what you are attempting to do in GM directly as it is rather complex...

    GM can create "flattened" pads no problem under Advanced Feature Creation Options/Create Flattened Site Plan Grid from Selected Area(s), but that is only for flat i.e. zero grade areas.

    It looks like you have an area that slopes NW to SE (I couldn't make out your elevations due to low resolution of image) and I don't think you can do this directly in GM. You could try a workaround where you project out at 90 degrees from your central profile line to where it intersects the area boundary and apply that elevation to the vertex on the boundary and repeat multiple times. However this will only give you an approximate answer.

    Interested to see if anyone else has a suggested workaround.

    Cheers,

    /al

  • Treebeard
    Treebeard Global Mapper User Trusted User

    Here is a workflow that was shared with me by the Global Mapper staff.

    Thank you for writing in. The following workflow should help with your goal of reducing a terrain area to a specific slope, using a sloping area feature that has an irregular shape. See the attached diagram for reference when following the steps listed below. 

    1) Using the path profile tool, create a path profile that runs along the side of the irregular polygon, but not inside it. We will use this in the following step to create an area feature that will enclose the irregular polygon. Next, in the path profile viewer, create a line feature whos end vertices' elevations will be used as a reference for the elevation difference that will result in the desired slope. This can be done in the Path Profile viewer by using the Measurement tool to create a line with the desired slope. With the measurement tool selected, please left click at the point where you want to begin the line segment, and left click to end the line segment. To save this line feature, right click in the path profile and select Create Line Feature From Sub-path



    Once the feature has been created, please use the feature info tool to select the line, click the vertices button and write down the elevation values of the two start and end vertexes. 

    2) Next, please create a rectangular area feature using the digitizer encompassing the irregular polygon area (see reference diagram above), with one side created on top of the line feature generated in the previous step. Now we will have to manually adjust the elevations of all four corner vertices so that the sides of the rectangle have the desired slope value. We can use the elevations of the line feature created with the Measurement tool, along with the side lengths to calculate these values. Editing these vertices' elevation values can be done by selecting the area feature with the feature info tool>vertices>select each vertex individually> Add Elevs. To save the changes in elevation click OK


    3) The next step will be to convert this vector feature into a raster grid. Select Create Elevation Grid, and use the created rectangular feature as the source for this grid to generation. In the Grid Bounds tab, select Use Layer bounds and choose the irregular polygon layer.

    4) Now that an elevation grid with the desired slope has been created, please use Analysis>Combine/Compare Terrain Layers to merge the new grid into the original grid layer.

    5) The grid region inside the irregular shaped area feature will now have the requested slope, but how well it matches up with the terrain outside the polygon will vary. You can adjust this by first selecting the irregular shaped area feature, navigating to control center, right clicking on the combined grid layer>choose the feathering tab. In the feathering tab, please select Feather Within Currently Selected Polygons, and select a value for Size of Feathering Area.


  • Hi,

    I would first create a large rectangular area that can fit your polygon inside and give its vertices an elevation that generates the slope you want, then create a grid using only the rectangle and then select the polygon and get its vertices elevation from the rectangle grid. You can then move it up and down using the shift operation and bring it to the desired elevation.

    Then you can use it with "create flattened grid" with the option "flatten to vertices elevation".

    Best regards

  • Good workaround Michele. I like it and of course much simpler!

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