Q: How do you specify the UTM zone in mapshaper? I have a GeoJSON file containing boundary data for Norwegian administrative units called fylker. The original source of the data (GeoNorge) identifies the coordinate system as EUREF89 UTM zone 33, 2d.  I need it in WGS84.
I'm trying to use mapshaper to do the conversion.  I load up the file, open the console and execute the command -proj from=utm wgs84. It appears to work; the shapes displayed get slightly squashed, but I think that's normal.  After that I simplify the shapes a bit and export a fresh GeoJSON.
But when I open the new GeoJSON file in geojson.io, the shapes are offset quite a bit -- hovering in the ocean about half way between Norway and Iceland.  I'm not a GIS expert by any means, but it looks like it's interpreting the source data as if it were in UTM zone 30 or 31, instead of UTM zone 33.
I tried -proj from=utm33 wgs84, but it failed because it didn't recognize utm33 as a valid projection. The documentation doesn't mention this.
So how do I tell it that this data is in UTM zone 33?  Or is there something else I'm doing incorrectly?

A: The mapshaper docs specify using a Proj.4 string, EPSG code or alias. "utm33" is none of those, I don't know where you got that from.
Try (just going from the docs here, I can't actually test):
-proj from=25833 crs=4326

Or:
-proj from=EPSG:25833 crs=EPSG:4326

25833 and 4326 are the EPSG codes for ETRS89 / UTM zone 33N and WGS84
