Buffer Polygon based on Ratser

Dear Experts,

I'm using Global mapper 20.1.

I have a polygon shape file along a river stretch (represents maximum flooding area) and a DEM (30m x 30m raster) reperesnting the eelvation in and around the area.

Now what I need is to extend that boundary-polygon or buffer it towards outside (from the riverline on both sides/banks) such that the new boundary polgon is away from earlier boundary polygon by 3meter increase in elevation (based on teh udnerlying DEM) - which means in vertical cliff both boundary line will be same, in high slope the new boundary line will be little further away and in flat slope it will be much furtehr away.

I hope I could make my task clearer. Hoping for the hints from the expert on how to get this done?

Thanking you all!


  • JSLJSL Global Mapper User Posts: 434Trusted User
    You should be able to do this by using the Simulate Water RIse/Flooding feature.  This is how I would do it:
    Below is a polygon of a valley with a polygon showing a maximum flooding elevation at 1495m asl:
    You can then select the layer with the polygons and choose from the toolbar Analysis->Simulate Water Rise/Flooding and you'll get the below dialog box.  I chose 3m for the Water Level Increase Amount and selected the option to Increase from Selected Areas (i.e. Flood Plain, Water Body Rise)

    Then I clicked OK and changed the style of the resulting layer to solid blue and put it behind the original layer in the Overlay Control Center so you can see the contrast:

    Here's a close up at the top of the original polygon:

    This is not a high resolution DEM, so your raster should be more refined.
  • Thank you very much for the hint. Just one query to make it clearer. As per your example, the polygon is showing a maximum flooding elevation at 1495m asl. This is the case when the flooding is behind a dam whose top is at the 1495masl which is flooding teh area behind it upto teh level of the 1495 masl.

    But in my case the flooding area is below the dam due to the dambreak. I have a result of maximum flooding boundary polygon,  due to the breaking of a dam, extended about 100 km donwstream from the dam along the river. Therefore, at each river cross-section the maximum flooding level is different (not fixed as the 1495masl in the given hint/example by you).

    Then, I need to find out the new polygon which would represent the flooding area such that the flooding at each every locations of the river is 3meters higher than represented by the previous polygon.

    Do you think the same tool/approach can be used for solving this as well? How?

    Looking forward to your further tips.

  • JSLJSL Global Mapper User Posts: 434Trusted User
    I'm not quite sure if I understand you correctly, but if you already have a polygon or multiple polygons, try selecting them and then running the water rise function and see what it does when you select the option to Increase from Selected Areas.
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