How to use the Con Tool

[ShaunDillon]

I am trying to cut streams into my DEM. I have made my streams into rasters with a value of -1 (meters) with the rest of the raster having NoData value. I then want to make a conditional (or should I use Pick) to make a new Raster where the elevation is -1 where the streams are and otherwise have the value of the DEM. I have also tried to make a conditional expression in the Single Output Map Algebra without succes.
Part of my problem is that I simply do not know where you write the expressions (such as "Con(IsNull("Corrected_DEM"),???Original_DEM, "Corrected_DEM")". I use ArcGIS 9.3.1 but have mainly used the toolbox tool "Con".
Can someone explain to a newbie?
Regards Shaun

[MarkBoucher]

Part of my problem is that I simply do not know where you write the expressions

If you have spatial analyst, you can enter the command in to Raster Calculator. This is possibly and icon in the SA toolbar (I have not used 9.3 in a while), but i is also a tool in the SA Toolbox (spatial analyst tools.tbx\map algebra\raster calculator).

Have you tried Arc Hydro tools to "cut" streams into your DEM?

[ShaunDillon]

Ah. Ok now I have found it, and got it to work. Thank you for your help.