I'm doing a lab and I'm trying to understand exactly why I'm using the tool "minus".

I have a raster which I'm calling "heights". I also have two rasters with water surfaces and watercourse. I have changed the values in the water rasters so that water pixels has the value 10 and noData has the value 0.

I have then used the tool "plus" to put these two rasters together into a joint raster which I'm calling "water" plain and simple.

Now, I am supposed to use the tool "Minus" to put in the water raster together with the raster heights, I can't understand why I need to do that.

My first thought was that the intention is to make sure that it's only the watersurfaces that shows up in my result later on and not land surfaces, but that doesn't seem right.

Why am I using minus in this scenario? It's to lower the watercourses I guess but why is that needed?

Update: The final goal of the lab is to find out from what area of the map a poison is coming from. To investigate this I have made an analysis of the watersurfaces in the area based on the water surfaces data and the flow direction.

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    What is the ultimate goal of the lab? I think we need more information on what is trying to be accomplished as a whole. Also putting in No Data as zero would be giving the no data value an actual value, where it should be null. – Pete Jan 11 at 21:58
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    This is called "burning in" the streams: see our threads about it here. It creates artificial channels to ensure the flow computed from the raster winds up in the water polylines (which is not guaranteed unless there is an exceptionally precise match between the two layers). – whuber Jan 11 at 22:47

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