# Buffer Polygon based on Ratser

**5**

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!

## Answers

422Trusted UserThis is not a high resolution DEM, so your raster should be more refined.

5Thank 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.

422Trusted User