[GRASS-user] raster reclassification or map calculator to simulate past sea-level change

[Erik Victor Lundstrøm]

Hi everyone,

I've done some google searches related to my problem. Granted, by no 
means exhaustive, but unsuccessful nonetheless.
Thus, I hereby by contribute with my first post in the email group 
hoping to find some help.

I work in coastal Norway, and I'd like to produce different rasters 
based on estimates for when the sea-level was either higher, or lower, 
than today.
When I started doing this years ago, it was simply enough for me to use 
the raster calculator and then produce binary maps using the "<=" or 
">=" expression as I only wanted to visualise the difference.
However, today I'd like to continue to work with the rasters where the 
values on the other side of the threshold that I assign remain intact 
but
I find it way too tedious having to 1) produce a binary map, 2) convert 
the cells denoting the sea-level change into vector polygons, and then 
3) clipping the old raster into a new one.

I'll try and use an example for what I'd like to try and achieve 
instead.

Let's say I have an elevation raster with values ranging from -0.001 to 
1500m. Now, if I'd be interested in producing a raster where anything 
below 25m is submerged, then the method that I alluded
to above would produce the following raster = 0 & 1

What I'm interested in doing now instead, is to find some method where I 
still reset anything below 25m as "0", but where I still have all the 
remaining values intact.

I'm thinking that there should be a way, either through the raster 
calculator or the reclassify tool, where I could simply reset anything 
above or below a certain threshold as either "0", or perhaps even "no 
data" but at the same time keep all the other elevation values of the 
raster intact.

This message should hopefully be enough to indicate that I have 
relatively poor experience with both the reclassify or mapcalc-tool and 
thus any
brief guide on how I could solve my problem would be greatly 
appreciated!

If anything is unclear just let me know and I can try to reformulate.

Best,
Victor