Relative elevation

[MikeRoberts]

I have a line representing a river and a dem and contour of the surrounding land. I would like to create countours of relative height above the river i.e. lines representing where 20 m, 40m ... above the river are.
Any suggestions?
I am using an Arcview licence.
Thanks,
Mike.

[MattMead]

The easiest way I can think of off hand would be to use "TIN difference" to create polygons of each relative elevation and convert them to polylines.

1) find your river inverts: you said you had the stream centerline so just use a tool like EZ profiler (http://arcscripts.esri.com/details.asp?dbid=16031) to generate points along the centerline with your DEM elevations.

2) create steam cross sections at each profile point and assign elevations based on 20m, 40m above the invert elevations(basically saying that you have a 20m deep flood).  Make sure your cross sections are wide enough that they will fully contain whatever elevations you are working with.

3)use 3d analyst to convert the cross sections to a TIN for each elevation
3b) convert your DEM or contours to a TIN as well

4) use TIN difference in the 3danalyst toolbox for each of your "flood events" and it will yield a polygon representing the intersection of the terrain and elevation(your relative elevation)

These polygons can be converted to polylines using various tools(I use ET GeoWizards, but its a tool you have to pay for)

Also, if your stream reach is short enough and the stream inverts are fairly constant you could probably get away with using the contour tool in spatial analyst and just set the initial contour elevation to the stream invert.