[Bug] r.grow: failure in long-lat projects with small radius due to scientific notation

[neteler]

Describe the bug

When using r.grow in a long-lat project (EPSG:4326) with a small radius it fails due to an internal switch to scientific notation.

Invalid map <4e>
Parse error
ERROR: parse error
ERROR: An error occurred while running r.mapcalc with expression: one_buf =
       if(!isnull("one"),"one",if(r.grow.tmp.1081989.dist <
       4e-12,"r.grow.tmp.1081989.val",null()))

To reproduce

Store the following script as r_grow_bug.sh:

g.region res=1 n=90 s=-90 w=180 e=-180 -p
r.mapcalc expression="one = 1"
r.grow input=one output=one_buf radius=0.000002

Then run it:

grass --tmp-project EPSG:4326 --exec sh r_grow_bug.sh
WARNING: East (-180) is not larger than West (180)
projection: 3 (Latitude-Longitude)
zone:       0
datum:      wgs84
ellipsoid:  wgs84
north:      90N
south:      90S
west:       180W
east:       180E
nsres:      1
ewres:      1
rows:       180
cols:       360
cells:      64800
 100%
Reading raster map <one>...
 100%
Writing output raster maps...
 100%
Invalid map <4e>
Parse error
ERROR: parse error
ERROR: An error occurred while running r.mapcalc with expression: one_buf =
       if(!isnull("one"),"one",if(r.grow.tmp.1081989.dist <
       4e-12,"r.grow.tmp.1081989.val",null()))

Expected behavior

A code modification is needed to avoid this bug

System description

• Linux Fedora 40
• Affects all GRASS versions.