Cannot solely remove M value of Shapefiles

Question

I have a couple of Shapefiles which are of geometry type ZM (like PolygonZM, MultiLineStringZM, etc.), however my database (a PostGIS database) is of geometry type Z (like PolygonZ, MultiPointZ, etc.). When I import my shapefile into database, it will of course report an error. However, when I convert these Shapefiles, I find that M values cannot be removed without removing Z value.
Here are my efforts:
1. In ArcMap, use Feature Class to Feature Class in ArcToolbox, click "Environment" and disable M value there. However, output Shapefiles still have M values.
2. Using Export in QGIS, and choose PolygonZ or other geometry types with Z only in SHPT field. Again, output Shapefiles still have M values.
3. Use ogr2ogr. Here is the command line:

ogr2ogr -nlt polygonz new.shp old.shp

Again, output Shapefiles still have M values.
However, if I discard Z values as well:

ogr2ogr -nlt polygon new.shp old.shp

Output Shapefiles will have neither Z nor M values.
But when I try to add Z again, like:

ogr2ogr -nlt polygonz new1.shp new.shp

Both Z and M will be added, thus returning to PolygonZM.
Since these shapefiles have no actual Z values at all, I can safely discard Z values along with M values if I only need to deal with these files themselves. However, in my database, geometry columns are storing Z values, and they cannot be discarded. Can anyone tell me why can't I just discard M values and preserve Z values for these Shapefiles?

Answer 1

Shapefiles with Z contain also always M. See the technical specification of PointZ from https://www.esri.com/library/whitepapers/pdfs/shapefile.pdf. Same applies to other geometry types with Z.

A PointZ consists of a triplet of double-precision coordinates in the order X, Y, Z plus a measure.

PointZ
{
Double X  //X coordinate
Double Y  //Y coordinate
Double Z  //Z coordinate
Double M  //M measure
}

You can drop M when you convert data into PostGIS with ogr2ogr just as you tried to drop it from the shapefile with -nlt polygonz.