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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.

Update: When answering the question "why do you have to use minus in this scenario " What I answered was basically

"after I have run the layer heights minus the layer water the watercourses will be lower than they really are in comparison to the surrounding surface. This happens because the pixels in the raster water i have the water values at 10 everything else at 0 (with reclassify). The height data will then get a value that's -10 lower than all the water courses. I'm doing this to get elevation in all the watercourses and in the modell the water will prefer to travel trough the water courses that's within the flow direction area. If I don't do this then some water courses may be without elevation and /or streams will be in the wrong places, even if I have a perfect height model, compared to where the actual streams are. So, I'm using this method to increase the chance that the watercourses are where they should be"

This was dead wrong. The answer I received that it will very well work with a perfect height model and that the height model should do just fine for analyzing. So the answer was wrong, that's not why I was using minus.

I'm totally lost here. No idea why of would use minus if that's not the answer. Any ideas?

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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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